Vol. 64, No. 18 (2015)

Dynamical modeling and analysis of buck converter operating in pseudo-continuous conduction mode
Li Zhen-Hua, Zhou Guo-Hua, Liu Xiao-Tian, Leng Min-Rui
2015, 64 (18): 180501. doi: 10.7498/aps.64.180501
Abstract +
Taking buck converter operating in pseudo-continuous conduction mode (PCCM) for example, through a detailed description of the switch state of the switching converter, its accurate discrete-time model is established in this paper. On the basis of the model, bifurcation diagrams of the PCCM buck converter with the variations of circuit parameters are obtained, including load resistance, equivalent series resistance (ESR) of inductor, inductance, capacitance, reference current, and input voltage. And the complex dynamical behaviors existing in PCCM buck converter, such as subharmonic oscillation, period-double bifurcation and chaos, are revealed. Under different load resistances, time-domain simulation waveforms and phase portraits of PCCM buck converter are obtained by Runge-Kutta algorithm based on the piecewise smooth switch model. The working states of PCCM buck converter, reflected by the time-domain waveforms and phase portraits, are consistent well with those described by the bifurcation diagrams. It is shown that the time-domain simulation results verify the validation of the discrete-time model.#br#From theoretical analysis and simulation results, some conclusions can be obtained below. 1) When the load resistance gradually decreases, PCCM buck converter has a unique bifurcation route, i. e. , from PCCM period-1 state, PCCM multi-period oscillation via period-double bifurcation, chaos, CCM-PCCM multi-period oscillation, to CCM period-1 state via inverse period-double bifurcation. What is more, the bifurcation analysis with the load resistance serving as parameter indicates that the PCCM buck converter is more suitable for light load conditions, and its stable state will be lost and operation mode can be shifted (from PCCM to CCM) with increasing the load. 2) The ESR of inductor is closely related to the power loss and will affect the stability of the PCCM converter. The larger the ESR, the more the power loss will be. However, the PCCM converter is more stable if the ESR is larger. 3) Period-double bifurcation or inverse period-double bifurcation exists in the PCCM buck converter with the other circuit parameters varied in a wide range except for the load resistance, and there are three working states of buck operating in PCCM, i.e., stable period-1 state, multi-period sub-harmonic oscillation, and chaos. The research results in this paper are useful for designing and controlling PCCM switching converter.
self-similarity of Rydberg hydrogen atom in parallel electric and magnetic fields
Li Hong-Yun, Yin Yan-Yan, Wang Qing, Wang Li-Fei
2015, 64 (18): 180502. doi: 10.7498/aps.64.180502
Abstract +
Using the semi-classical methods, the self-similarity structure of Rydberg hydrogen atom in parallel electric and magnetic fields is analysed in this paper. Based on the Hamiltonian canonical equations, all the escape orbits are found, and the escape time and the initial launch angle of every escape orbit can be derived. The self-similarity structure of escape time plot is found by studying the relationship between the escape time and the initial launch angle of electron in parallel electric and magnetic fields. The relationship between the self-similarity structure and escape orbits is also established through the study of the escape orbits in the escape time plot. The regularity of escape orbits in self-similarity structure is found and the corresponding escape orbits in self-similarity structure plots meet the law of (-o)k. According to this rule, the self-similarity structure can be easily found, and the rule is applicable to other research system. Moreover, the influences of scaled energy and scaled magnetic field are analyzed in detail. It is presented that the dynamic behavior of the Rydberg hydrogen atom is sensitively controlled by scaled energy and scaled magnetic field. Different scaled energies or scaled magnetic fields can lead to different escape behaviors of electron. It is also found that the self-similarity structure is present not in all cases. When scaled energy or scaled magnetic field is small, the escape time plot is simple, and no self-similarity structure is observed. When scaled energy or scaled magnetic field increases, self-similarity structure appears accordingly and the system becomes complicated. When scaled energy or scaled magnetic field changes, the self-similarity region also changes. For a given scaled magnetic field, with the increase of scaled energy, self-similarity region shifts toward the bigger initial launch angle, while self-similarity region shifts toward the smaller initial launch angle with the increase of scaled magnetic field for a given scaled energy.


Incoherent broadband cavity enhanced absorption spectroscopy for measurements of atmospheric HONO
Duan Jun, Qin Min, Fang Wu, Ling Liu-Yi, Hu Ren-Zhi, Lu Xue, Shen Lan-Lan, Wang Dan, Xie Pin-Hua, Liu Jian-Guo, Liu Wen-Qing
2015, 64 (18): 180701. doi: 10.7498/aps.64.180701
Abstract +
We report the development of an incoherent broadband cavity enhanced absorption spectroscopy (IBBCEAS) based on an ultraviolet light emitting diode (UV-LED), and the IBBCEAS instrument is used for simultaneously measuring of the atmospheric HONO and NO2. The cavity-enhanced method is characterized by high sensitivity and spatial resolution. The incoherent broadband light is focused into a high-finesse optical cavity, two highly reflecting mirrors form the ends of the cavity, and the light is then trapped between the two highly reflecting mirrors, resulting in long photon residence time and long optical path length. The effects of the Rayleigh scattering of the gases in the cavity and stability of the UV-LED light source were discussed in this paper. The reflectivity of the highly reflecting mirror was calibrated by the difference of Rayleigh scattering of He and N2, and the optimum averaging time of the IBBCEAS instrument was confirmed to be 320 s by the Allan variance analysis. Detection limits (1) of 0.22 ppb for HONO and 0.45 ppb for NO2 were achieved with an optimum acquisition time of 320 s. In order to test the accuracy of measured results by the IBBCEAS instrument, concentrations of HONO and NO2 were recorded during about continuous three days by the IBBCEAS instrument and compared with the results obtained by a different optical absorption spectroscopy (DOAS) instrument. The results of HONO show a linear correction factor (R2) of 0.917, in a slope of 0.897 with an offset of 0.13 ppb; NO2 concentration measured by the IBBCEAS instrument accords well with the result obtained by the DOAS instrument, with a linear correlation of R2 = 0.937, in a slope of 0.914 with an offset of-0.17 ppb.
Wavelength-dependent perpendicular-harmonics efficiency from oriented CO2 molecule
Li Yan-Peng, Yu Shu-Juan, Chen Yan-Jun
2015, 64 (18): 183102. doi: 10.7498/aps.64.183102
Abstract +
In this paper, we numerically study the efficiencies of high-order harmonic generation (HHG) from CO2 molecule exposed to strong laser fields with different laser wavelengths and different orientation angles. Through calculating the HHG spectra in the directions parallel and perpendicular to the laser polarization, we show that the efficiency of perpendicular harmonics can be higher than or comparable to the parallel ones at the relatively small and intermediate orientation angles in some wavelength cases. At larger angles, the efficiency of perpendicular harmonics is generally lower than the parallel one. Further analyses show that the structure of the CO2 molecule plays an important role in the HHG efficiency and this role is also related to the laser wavelength. Specifically, we show that the relative yields of perpendicular harmonic versus parallel harmonic are closely associated with the parallel and perpendicular dipoles of the molecule. Due to the effect of two-center interference, the parallel or perpendicular dipoles of the molecule show some deep hollows in some energy regions, which depend on the molecular orientation, and so do the corresponding parallel and perpendicular harmonics. As the parallel harmonics are suppressed due to the interference effect strongly in some energy regions, the yields of the perpendicular harmonics, which are not subjected to the interference effect in the corresponding energy regions, can be higher than the parallel one. As a result, the integrated harmonic yield (i.e., the harmonic efficiency) in the perpendicular case can be higher than the parallel one, especially for the cases with short laser wavelengths and small orientation angles. In these cases, the interference effect induces the suppression of parallel harmonics in the whole HHG plateau. We therefore expect that the interference effect plays an important role in the HHG efficiency in these cases. For the case of long laser wavelength, the HHG plateau extends to high energy region and the main contributions to the integrated HHG yield can come from harmonics out of the interference-effect-dominating region. As a result, the interference effect plays a smaller role in determining the HHG efficiencies of parallel and perpendicular harmonics, in comparison with the case of short laser wavelength. For large orientation angles, the value of the perpendicular dipole is smaller than the parallel one in a wide energy region, and accordingly, the perpendicular harmonics are weaker than the parallel ones on the whole. As a rule, the parallel efficiency is usually higher than the perpendicular one. As the perpendicular harmonic can contribute importantly to the harmonic emission in some cases, our results suggest that for the complicated molecule, the perpendicular harmonics should be considered in the molecular orbital tomography experiments.
Research of optical short pulse source with tunable repetition rate and ultra-low timing jitter
Jia Shi, Yu Jin-Long, Wang Ju, Wang Zi-Xiong, Chen Bin
2015, 64 (18): 184201. doi: 10.7498/aps.64.184201
Abstract +
The highly stable optical short-pulse generator with high repetition rate is widely applied in many fields of optical communications, such as optical packet switching systems, high-speed analog-to-digital converter systems, wavelength division multiplexing networks, high-speed optical sampling systems and optical time division multiplexing networks. The optical short-pulse generator which is adopted in such systems mentioned above should possess high stability, low timing jitter, the tunability of the repetition-rate, and narrow pulse width. So far, most of the optical short pulses have been generated from the actively mode-locked lasers and the phase-modulated continuous wave lasers. However, both of the two methods require an additional microwave signal source. Consequently, the stability of such an optical short-pulse generator is strictly limited by the phase noise and stability of the additional microwave signal source. Since the concept of an optoelectronic oscillator which includes the generation of low noise optical pulses together with an ultra-low phase noise microwave signal was proposed by Yao in 1996, the optical short-pulse generator based on the optoelectronic oscillator has attracted much attention in recent years. According to this approach, Lasri demonstrated a novel, self-starting optoelectronic oscillator based on an electro-absorption modulator in a fiber-extended cavity for generating an optical pulse stream with high-rate and ultra-low jitter capabilities in 2003. In the scheme, the repetition rate of the generated optical pulses is 10 GHz, and the phase noise is-115 dBc/Hz at 10 kHz. Devgan demonstrated an optoelectronic oscillator by using a gain-switched vertical-cavity surface-emitting laser in a fiber-feedback configuration in 2006. The structure can generate a 2-GHz optical pulse stream with 750-fs timing jitter (over 100 Hz-10 MHz range). In the present paper, a novel optical short-pulse generator with tunable repetition rate and ultra-low timing jitter based on optoelectronic oscillator is demonstrated. The optoelectronic oscillator system can generate the microwave signal with ultra-low phase noise. The continuous wave light directly modulated by this microwave signal is phase modulated twice, and then the optical pulses with remarkable chirping rate are achieved. By optimizing the length of the dispersion compensating fiber, the optical pulses are further compressed. In this experiment, by utilizing a YIG tunable filter, the high-quality and tunable microwave signal within 8-12 GHz is achieved, which demonstrates the tunability of the repetition rate of the optical pulses. When the frequency of the microwave signal (i.e., the repetition rate of the optical pulses) is 9.6 GHz, the measured pulse width and the phase noise are 3.7 ps and-130.1 dBc/Hz at 10 kHz, respectively. Therefore, the timing jitter of the short optical pulse is calculated to be 60.1 fs (over 100 Hz to 1 MHz).
Generation of vortex beams by the four-step phase plates
Shi Jian-Zhen, Yang Shen, Zou Ya-Qi, Ji Xian-Ming, Yin Jian-Ping
2015, 64 (18): 184202. doi: 10.7498/aps.64.184202
Abstract +
The generation and application of the vortex beams are part of the hot topics in the optical field. In this paper, the phase structure of the four-step phase plates, analyzed by Fourier series expansion method, is composed of a series of spiral phase plates. When the phase plate is directly irradiated by linearly polarized light, multi-order diffraction waves with different topological charge numbers are generated. Unlike vortex waves, the intensity distribution of the multi-order diffraction has a deviation from the axial symmetry due to the interference with each other. On this basis, a new scheme is proposed to generate vortex beams by the four-step phase plates. With the help of Mach-Zehnder interferometer, the diffraction waves generated by two pieces of the four-step phase plates overlap each other. By adjusting the phase difference of the Mach-Zehnder interferometer, some orders of diffraction waves generate destructive interference while the others generate constructive interference. Thus the linear polarized light can be converted into vortex beams. The diffraction intensity and angular momentum distributions of the four-step phase plates with different cycle numbers are numerically simulated and compared with the spiral phase plates, we can provethat the vortex beams can be obtained by simple four-step phase plates which are the same as those obtained by spiral phase plates. In addition, the four-step phase plates with a small cycle number can generate vortex beams with a large topological charge number and the fabrication difficulty of the phase plates is reduced.
High sensitive measurement of optical rotation based on photo-elastic modulation
Li Ke-Wu, Wang Zhi-Bin, Chen You-Hua, Yang Chang-Qing, Zhang Rui
2015, 64 (18): 184206. doi: 10.7498/aps.64.184206
Abstract +
In order to realize the continuous and stable, high speed, high precise and high sensitive measurement of optical rotation, and considering the application advantages of photo-elastic polarization modulation technology with high modulation frequency, high modulation purity, high modulation accuracy and good modulation stability, a new scheme about the measurement of optical rotation based on photo-elastic modulation is presented. Probe laser orderly passes through a polarizer, the rotation sample to be measured, a photo-elastic modulator, and a analyzer, and finally reaches the detector, this system uses less optical devices than any others previously reported, so it considerably reduces the measurement error that may be introduced by the optical devices. In the detecting of light path, the polarization axes of the polarizer and analyzer are respectively adjusted with respect to the photo-elastic modulator's fast axis directions 0° and 45°, the optical arrangements make the rotation angle to be measured appear in the alternating current signal, and the photo-elastic modulator's residual birefringence only appears in the odd harmonics. Consequently, the second harmonic signal of photo-elastic modulation is selected as the object to study, which effectively avoids the influence of residual birefringence of the photo-elastic modulator on optical rotation measurement, and efficiently improves the accuracy of optical rotation measurement. What is more, the detector output signal is separated into two parts, the direct current and alternating current signal. The alternating current signal is amplified, then outputs by a lock-in amplifier, which enhances the measurement sensitivity further. A ingenious verification test experiment is done, firstly, the probe laser is modulated into circularly polarized light, and then precisely rotates the polarizer to replace the optical rotation sample. The results show that the new scheme is feasible, this experiment gives the proportion coefficient of the measurement system, the sensitivity of optical rotation measurement increasing up to 3.15× 10-7 rad, and the measurement precision exceeding 0.3%. Therefore, in this scheme achieved is a high sensitive and precise measurement of optical rotation, and it is expected to be applied to the high sensitive and precise rotation measurement. The verification test experiment designed by us can also provide a outstanding calibration reference for high sensitive rotation measurement system.
The -type four-particle entangled state generated by using superconducting artificial atoms with broken symmetry
Leng Chun-Ling, Zhang Ying-Qiao, Ji Xin
2015, 64 (18): 184207. doi: 10.7498/aps.64.184207
Abstract +
We propose a scheme for generating a genuine -type four-particle entangled state of superconducting artificial atoms with broken symmetry by using one-dimensional transmission line resonator as a data bus. With the help of the Circuit quantum electrodynamics system composed of -type three-level artificial atoms and transmission line resonator, our scheme also has long coherence time and storage time. Meanwhile, the -type three-level artificial atom used in the scheme is different from natural atom and has cyclic transitions. Furthermore, our scheme is easy to control and flexible. Through a suitable choice of the Rabi frequencies and detunings of the classical fields, we can use this system to implement the selective coupling between two arbitrary qubits. After suitable interaction time and simple operations, the desired entangled state can be obtained. Since artificial atomic excited states and photonic states are adiabatically eliminated, our scheme is robust against the spontaneous emissions of artificial atoms and the decays of transmission line resonator. We also analyze the performance and the experimental feasibility of the scheme, and show that our scheme is feasible under existing experimental conditions.
Amplifier similariton oscillator using 10 m photonic crystal fiber
Shi Jun-Kai, Chai Lu, Zhao Xiao-Wei, Liu Bo-Wen, Hu Ming-Lie, Li Yan-Feng, Wang Qing-Yue
2015, 64 (18): 184210. doi: 10.7498/aps.64.184210
Abstract +
Ultrashort pulse laser with a repetition rate of below 10 MHz is suitable for a variety of micromachining applications to avoid plasma shielding effects. Besides, the parabolic pulse possesses clean wings, short pulse duration, and large peak power because only the linear chirp is accumulated during the propagation. Based on these two points, a similariton oscillator with a repetition rate of below 10 MHz is a most perfect seed source of an amplification system for micromachining. In this paper, an amplifier similariton oscillator with dispersion map based on a piece of 10 m Yb-doped large-mode-area single-polarization photonic crystal fiber is demonstrated. The semiconductor saturable absorber mirror is employed in the linear cavity as an end mirror to initiate and maintain the mode-locking operation. An adjustable slit is adopted between the end mirror and grating pair in another arm, as a central wavelength adjuster and the spectral filter to ensure the laser operational wavelength in accordance with the working wavelength of semiconductor saturable absorber mirror and the stability of mode-locking operation. The stable single-pulse mode-locking operation can be achieved by adjusting the intracavity dispersion and the operating wavelength. With the net cavity dispersion of-0.89 ps2, a spectrum with steep and smooth edges is obtained, which means that the laser does not operate in the soliton regime but in the dispersion-mapped amplifier similariton regime. A highest output power of 820 mW is obtained with a pulse duration of 6.2 ps and spectral width of 3.84 nm under a pump power of 12.8 W. The repetition rate is 8.6 MHz, corresponding to a pulse energy of 95 nJ. It is the first time that the similariton with a repetition rate of lower than 10 MHz and a highest pulse energy of 95 nJ from a similariton laser has been achieved, to the best of our knowledge. Numerical simulation results confirm that the self-similar evolution is achieved in the gain fiber, and the parabolic-and gauss-shaped pulse can be emitted at the zero-order reflection of the grating and after the slit, respectively.
Raman effect on dark soliton trapping in high birefringence fiber
Yan Qing, Jia Wei-Guo, Yu Yu, Zhang Jun-Ping, Menke Neimule
2015, 64 (18): 184211. doi: 10.7498/aps.64.184211
Abstract +
Not only the interaction between optical pulse and orbital electron but also the interaction between optical pulse and optical phonon needs to be considered when input pulse energy is large. The latter induces the simulated Raman scattering, thus generating the Raman gain. We analyze the effect of Raman gain, especially parallel Raman gain, on dark soliton trapping in high birefringence fiber by analytical method and numerical method. In the first part, we introduce some research results of soliton trapping obtained in recent years. In the second part, the coupled nonlinear Schrödinger equation including Raman gain is utilized for high birefringence fiber. The trapping threshold of dark soliton with considering the Raman gain is deduced by the Lagrangian approach when input pulse is the dark soliton pulse that the amplitude of two polarized components of the dark soliton are the same (see formula (26)). Fig. 1. shows the relation between threshold and parallel Raman gain according to formula (26) when group velocity mismatching coefficient values are 0.15, 0.3, and 0.5 (vertical Raman gains are all 0.1). In the third part, the propagation of the two orthogonal polarization components of dark soliton is simulated by the fractional Fourier transform method. Figures 2-4 show respectively dark soliton trapping with group velocity mismatching coefficient values of 0.15, 0.3 and 0.5. We consider three situations in which Raman gain is not included and the parallel Raman gains are 0.4 and 0.6 when vertical Raman gains are both 0.1 in different group velocity mismatching coefficient values. We obtain the threshold of dark soliton by numerical method under different conditions and analyze the figures. At the same time, we compare the analytical solution with the numerical solution and discuss the effect of Raman gain on dark soliton trapping. The last part focuses on our conclusion. It is found that the threshold which is obtained by analytical method is smaller than that from the numerical solution. The difference between the analytical and numerical dependences decreases with group velocity mismatching coefficient decreases. As a result, formula (26) is in good agreement with numerical data for small group velocity mismatching. The larger the group velocity mismatching, the larger the amplitude threshold of dark soliton is. It also shows that the amplitude threshold of dark soliton can be reduced due to Raman gain and the threshold is reduced faster with the increasing of Raman gain.
Four-wave-mixing generated by femto-second laser pumping based on graphene coated microfiber structure
Feng Qiu-Yan, Yao Bai-Cheng, Zhou Jin-Hao, Xia Han-Ding, Fan Meng-Qiu, Zhang Li, Wu Yu, Rao Yun-Jiang
2015, 64 (18): 184214. doi: 10.7498/aps.64.184214
Abstract +
Nonlinear optics researches of graphene-based four waves mixing (FWM) effect are important for a new generation of photonic devices. Compared with the ordinary graphene materials, the P-doped graphene based hybrid waveguide structure is more conducive to the simulating of the third-order nonlinear effect in low power due to its smaller transmission loss. In this work, we propose a P-doped graphene coated microfiber hybrid waveguide structure for femto-second laser pumping excited FWM. By the simulations, we analyze the HE11 mode distribution and the effective refractive index of the silica microfiber and P-doped graphene coated microfiber hybrid waveguide with different fiber diameters at a wavelength of ~1550 nm. We also implement the fabrication processing and characterize this P-doped graphene coated microfiber hybrid waveguide. In the experiments, we utilize a femto-second laser as the pump laser with a peak power up to kW. As the graphene material and the microfiber contribute to the nonlinearity, the cascade FWM could be obtained. Experimental results demonstrate that when the peak power of the injection pump is fixed at 1.03 kW, by adjusting the detuning in wavelength to the length less than 10.0nm, there are four sets of frequency components that can be observed. In the present paper, we provide the relationship among the detuning in wavelength, the pump power and the the power of stokes peak. These results indicate that under the condition of a few nanometer detuning wavelength, when the pump power is fixed at 14.1 dBm and the detuning wavelength is 6.7 nm, there are second order stokes light and the second order anti-stokes light, which can be observed, here the obtained conversion efficiency is up to-60 dB, which can be improved by optimizing the waveguide structure and increasing the pump power. Meanwhile, this FWM processing is also fast due to the fast pumping laser.#br#The simulation and experimental results demonstrate that this P-doped graphene coated microfiber hybrid structure has the advantages of highly nonlinearity, compact size and withstanding high power ultrafast laser, showing the important research value and potential applications in fields based on ultrafast optics, such as multi-wavelength laser, phase-sensitive amplification, comb filters and all-optical regeneration.
Estimation of steady loss factor with decay rate method
Gu Jin-Tao, Sheng Mei-Ping
2015, 64 (18): 184301. doi: 10.7498/aps.64.184301
Abstract +
Steady loss factor is derived according to its definition, and a conclusion is obtained that steady loss factor is not always among modal loss factors but related to contributions of each modal response to vibration response. To obtain the conclusions about the range of steady loss factor, four cases are discussed according to positions of the two natural frequencies related to the central frequency. 1) Both natural frequencies are lower than the central frequency. 2) Both natural frequencies are higher than the central frequency. 3) One natural frequency, whose modal loss factor is smaller, is higher than the central frequency and the other natural frequency is lower than the central frequency. 4) One natural frequency, whose modal loss factor is larger, is higher than the central frequency and the other natural frequency is lower than the central frequency. Steady loss factor ranges between modal loss factors only if the frequency, whose value of multiplying modal loss factor is largest, is lower than central frequency of frequency band and at the same time, the frequency, whose value of multiplying modal loss factor is the smallest, is higher than the central frequency. Process loss factor which is a time-dependent function is proposed for the description of loss factor of decay process. Meanwhile, the way to calculate process loss factor with modal frequency, loss factor and amplitude is presented. Process loss factor tends to steady loss factor contributed by the mode with smallest loss factor over time. The accuracy is good enough for traditional decay rate method to estimate steady loss factor when there is only one mode or lots of modes in the frequency band. It is difficult for traditional decay rate method to be used to evaluate steady loss factor in the mid-frequency band where frequency density is not enough. A new method is proposed to estimate steady loss factor through separating the smallest modal loss factor components in the frequency band with time decay curve step by step according to the different decay characteristics. Simulation and experimental results indicate that the proposed method can cover the shortage of traditional decay rate method of estimating the steady loss factor in mid-frequency band.
Experimental analysis of auditory mechanism of neural phase-locking based on sample entropy
Zhu Li, Deng Juan, Wu Jian-Hua, Zhou Nan-Run
2015, 64 (18): 184302. doi: 10.7498/aps.64.184302
Abstract +
Phase-locking is a physical phenomenon that refers to a system response which is synchronized with a specific phase of the periodic stimulus. The auditory neural phase-locking plays an important role in revealing the basic neural mechanism of auditory cognition and improving auditory perception. In the existing auditory researches, psychophysical and amplitude spectral methods are mainly adopted. However, those two methods cannot differentiate the envelope-related auditory response from the temporal-fine-structure-related auditory response, and cannot reveal the neural phase-locking mechanism directly either. In this study, a phase locking value (PLV), based on sample entropy, bootstrapping and discrete Fourier transform, is proposed for analyzing the temporal-fine-structure-related frequency following response (FFRT). The proposed PLV is applied to computing neural and physical data. Two electroencephalography experiments are carried out. Results show that the sample entropy of FFRT's PLV is significantly greater than that of FFRE's PLV, and the two PLVs are orthogonal and independent. Thus, the PLVs of FFRE and FFRT reveal the auditory phase-locking mechanisms effectively. In addition, the response to fundamental frequency is mainly attributed to the envelope-related phase locking. And human auditory capability of phase locking to the envelope of the unresolved frequency is superior to the capability of phase-locking to the envelope of the resolved frequency. Moreover, in the case of missing fundamental frequency, the distortion product is the mixture of FFRE in various auditory neural paths. Also, FFRE concentrates at the low harmonic frequencies, while FFRT concentrates at the mid and high order harmonic frequencies. Therefore, the auditory neural phase-locking is related to the frequency resolution of sound. In conclusion, the proposed method overcomes some disadvantages of existing FFR analyses, making it beneficial to exploring auditory neural mechanisms.
Generalized Birkhoff system and a kind of combined gradient system
Mei Feng-Xiang, Wu Hui-Bin
2015, 64 (18): 184501. doi: 10.7498/aps.64.184501
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A combined gradient system which is obtained by adding a gradient system to a skew-gradient system is proposed. The property of the system is studied. Three results on its solution are obtained. Moreover, a criterion on the stability of the system is presented. The generalized Birkhoff system is a more extensive constrained mechanical system than Lagrange system, Hamilton system and Birkhoff system. The conditions under which a generalized Birkhoff system can be considered as a combined gradient system are given. When a generalized Birkhoff system is transformed into a combined gradient system, its integration and stability can be discussed by using the property. Finally, some examples are given to illustrate the application of our results.
Numerical investigation on drag reduction with hydrophobic surface by lattice Boltzmann method
Zhang Ya, Pan Guang, Huang Qiao-Gao
2015, 64 (18): 184702. doi: 10.7498/aps.64.184702
Abstract +
In recent years, hydrophobic surface has attracted much attention for its potential applications in flow drag reduction. This article focuses on the drag reduction mechanism of hydrophobic surface by the multi-relaxation-time scheme and the Shan-Chen multiphase model of lattice Boltzmann method. At first, we validate our method through the multiphase cases of wall adhesion effect and the single-phase cases of flow around a square column, showing that the results from our method are in good consistence with those in previous literature. Then, we simulate and analyze the typical problem of flow around a square column with hydrophobic surface while Reynolds number is 100, in order to investigate the influences of contact angle and gas holdup of the inlet flow on drag coefficient and lift coefficient. The simulation results show that for a given contact angle, hydrophobic surface is capable of reducing drag when gas holdup of the inlet flow is in a certain range; otherwise, drag coefficient will increase. With an appropriate gas holdup of the inlet flow, both drag coefficient and lift coefficient will decrease as the contact angle becomes larger. Finally, we compare gas holdup contours and the corresponding streamline patterns under different drag coefficients. Analyses suggest that the increases of drag coefficient and lift coefficient are related to the gas mass shedding near the square column wall where the eddy forms. Increasing the gas holdup of the inflow is properly conducible to reducing the gas mass shedding and also both drag coefficient and lift coefficient greatly if contact angle is too large. However, if the near-wall gas holdup is saturated, it will aggravate the instability of gas holdup and change the near-wall gas holdup a little, which makes drag coefficient increase slightly. When gas holdup of the inlet flow is appropriate, the near-wall gas holdup becomes steadier with a larger contact angle. Through analysis we note that for hydrophobic surface, the key factor of drag reduction is to keep the near-wall gas layer stable, with which the effect of drag reduction becomes better as the contact angle becomes larger. However, the larger the contact angle, the more sensitive to the change of gas holdup both drag coefficient and lift coefficient are, so it is not recommended to adopt the hydrophobic surface with very large contact angle. With the analysis of the gas holdup near hydrophobic surface with different contact angles, in this article we put forward a new approach to the further exploration of the drag reduction mechanism of hydrophobic surface.
Bending phenomenon of temperature calibration curve in junction temperature measurement by the high transient current
Guo Chun-Sheng, Wang Lin, Zhai Yu-Wei, Li Rui, Feng Shi-Wei, Zhu Hui
2015, 64 (18): 184704. doi: 10.7498/aps.64.184704
Abstract +
To measure the junction temperature of diodes under operating conditions, the temperature calibration curve is studied under large current conditions. To avoid the self heating by the large current conditions, pulsed currents are used in the paper. The temperature calibration curve of TO-247-2L fast recovery diode is investigated in this paper. The 0-1.5 A pulse current, of which the pulse width is 250 μs and the duty cycle is 5%, is chosen to study the temperature calibration curves under 50, 70, 90, 110, 130 ℃ respectively.#br#The results show that under the large current condition, the temperature calibration curve bends. The main reason for the bending phenomenon is that the series resistance changes with temperature increasing, which is affected by the mobilities of electrons and holes in semiconductor material. With the temperature rising, the mobility decreases, which results in the increasing of series resistance. Due to the series resistance increasing The voltage on p-n junction will be reduced. For this reason, a higher voltage is needed to obtain the same current, and the temperature calibration curve will bend.#br#There are two reasons which will lead to the temperature rising. The first reason is self-heating of devices by the power dissipation, and the second reason is that the temperature of device is heated by ambient temperature. Under the same temperature, self-heating behaviors of device by different currents will result in different series resistances. But in the paper, the results show that the series resistances under different currents are the same, which illustrates that self-heating is not the key reason for the change of series resistance. So, the temperature changing of the diode is caused by the ambient temperature rising, which verifies that the bending phenomenon of the temperature calibration curve of TO-247-2L fast recovery diode is caused by the ambient temperature rising.#br#Then, through experimental measurements and theoretical calculations, the accurate nonlinear temperature calibration curve is acquired, which can reduce the measurement errors of high current transient junction temperature.
Broadband anomalous reflector based on cross-polarized version phase gradient metasurface
Fan Ya, Qu Shao-Bo, Wang Jia-Fu, Zhang Jie-Qiu, Feng Ming-De, Zhang An-Xue
2015, 64 (18): 184101. doi: 10.7498/aps.64.184101
Abstract +
Phase gradient meatsurface (PGM) is a new way to control reflective beam and refractive beam. By means of PGM, wave-fronts can be controlled in a more freedom way. The generalized Snell's law was put forward first by Nanfang Yu et al. [Yu N F, Genevet P, Kats M A, Aieta F, Tetienne J P, Capasso F, Gaburro Z 2011 Science 333 334] to describe the anomalous refraction on PGM. Anomalous refraction and out-of-plane reflection were then demonstrated using PGM composed of V-shaped nanoantennas. As deeper research about PGM, many reflective PGMs are also proposed. Typical examples are the reflective PGM using H-shaped resonators by Lei Zhou's group and using split-ring resonators by Shaobo Qu's group, both acting as high-efficiency surface wave couplers. However phase gradient of most PGMs above are achieved in a narrow-band and cannot change the polarizations. Anomalous reflection can only be realized in a certain narrow-band, and anomalous reflective angles cannot be precisely predicted. In this paper, a polarized conversion metasurface based on double-circular metallic resonator is first designed. The conversion successfully achieves ultra-wideband cross-polarization for linearly-polarized waves within a broadband of 12.2 GHz (from 7.9-20.1 GHz) with more than 99% cross-polarized reflectance. On the premise of high efficiency, reflective phase can be regulated by changing geometrical parameter of double-circular metallic structure. Then a broadband one-dimensional dispersive phase gradient metasurface comprised of six unit cells periodically arrayed above substrate is designed and fabricated. The PGM can perfectly achieve anomalous reflection. Measured result about its specular reflectivity is in good agreement with simulated result. Moreover, the measurement results of E-field distribution and anomalous reflective angle nearly accord with simulation results. Anomalous reflective angle is precisely predicted based on the generalized Snell's law. Both simulation and experiment verify that the PGM can make incident waves efficiently coupled as surface waves from 8.9-10 GHz and anomalously reflected in a range from 10 GHz to 18.1 GHz.
Influence of atmospheric refraction on radiative transfer at visible light band
Hu Shuai, Gao Tai-Chang, Li Hao, Liu Lei, Cheng Tian-Ji, Zhang Ting
2015, 64 (18): 184203. doi: 10.7498/aps.64.184203
Abstract +
Refraction is an important factor influencing radiative transfer since it can change both the propagation path and polarization state of electromagnetic wave. In order to discuss the influence of atmospheric refraction on radiative transfer process, a Monte Carlo vector radiative transfer model, which takes atmospheric refraction into account, is introduced. By using this model, photon random movement in uniform atmospheric layer and at the interfaces between adjacent layers is simulated, Stokes vectors and degrees of polarizations of both directly transmitted and diffuse light, and irradiance at the specific layer is also calculated. The model is validated under two conditions: with taking atmospheric refraction into account, and comparing the simulation results with those in the literature; with taking refraction index distributed homogeneously in space, in which case the model is validated against DISORT and RT3. So, the results indicates that our model is accurate and reliable. The influences of atmospheric refraction on the Stokes vectors of diffuse light in different directions are discussed for pure molecular atmosphere, with only Rayleigh scattering considered. Simulations are performed respectively for different solar zenith angles, for different atmospheric profiles, for aerosols with different types and particle shapes, and for clouds with different base heights and optical depths, and correspondingly, the effect of atmospheric refraction on radiative transfer process is discussed as well. Simulation results show that Stokes vector of diffuse light is influenced by atmospheric refraction to a certain extent, especially for light with a zenith angle ranging from 70 to 110, and with the increasing of solar zenith angle, the influence becomes stronger. When atmospheric profile changes, the effect of atmospheric refraction on polarized radiance field is also changed, for which the possible reason is that deference between atmospheric profiles leads to the variation of refraction index profile. When aerosol and cloud are taken into account, the influence of atmospheric refraction is reduced because of the decreasing of the ratio between side-scattering energy and backward scattering energy. Comparing the simulation results for different aerosol particles shows that the influences of atmospheric refractions in mineral and sea salt aerosol are much stronger than that in water soluble aerosol, besides, there is also great discrepancy among results for aerosols with different shapes. This phenomenon may be explained by the differences in scattering ability and spatial distribution of scattering energy among different aerosols. For cloud, there is no significant difference in result among different cloud base heights, while with the increasing of cloud optical depth, the influence of atmospheric refraction on polarized radiance is gradually weakened.
Transportation of Gaussian light beam in two-layer clouds by Monte Carlo simulation
Sun Xian-Ming, Xiao Sai, Wang Hai-Hua, Wan Long, Shen Jin
2015, 64 (18): 184204. doi: 10.7498/aps.64.184204
Abstract +
Based on the radiative transfer theory, the backscattering characteristics of water clouds and ice-water two layers clouds irradiated by infinite narrow collimated light beam are studied by using the Monte Carlo method. The incident wavelength is 0.532 μupm, and the cloud particle shape is assumed to be of sphere or plate. The single scattering characteristics of the clouds are computed based on the Mie theory, and the scattering angle sampling is based on the Mie phase function. The photon step adjustment is considered when the step is large enough to cross the cloud layer. The variations of reflection functions of the water clouds and ice-water two layers clouds with the radial length r and zenith angle are given, and the interior light intensity distribution of clouds are given in two dimensions. From the computed results, we find that the reflection characteristics of the two layer clouds are greatly different from those of the pure water clouds. The reflection intensity of ice clouds covered with water clouds is bigger than that of ice clouds covered with water clouds. This reason is that the sizes of ice clouds are larger than those of the water clouds, so more photons will be scattered into the interior of the clouds.#br#The cloud layer is assumed to be linear and invariant, so the response to an infinitely narrow photon beam will be described by a Green's function of the clouds, and the response to the Gaussian beam can be computed from the convolution of the Green's function according to the profile of the Gaussian photon beam. The multiple scattering characteristics of the Gaussian photon beam are computed from the convolution of the impulse response, i.e., the response to an infinitely narrow photon beam, according to the profile of the Gaussian light photon beam. From the computed results, we find that the reflection function of clouds for Gaussian incidence has a great difference from that for the infinite narrow beam incidence. The reflected light intensity is inversely proportional to the size of the Gaussian beam at the location near r=0. So the laser spot must be considered when detecting the clouds by using of the lidar, and the method presented in this paper can give theoretical support.
Analysis on reconstruction of virtual images of Fourier telescopy
Yu Shu-Hai, Dong Lei, Liu Xin-Yue, Ling Jian-Yong
2015, 64 (18): 184205. doi: 10.7498/aps.64.184205
Abstract +
Fourier telescopy can realize high resolution imaging to remote, small and dim target by using laser as the light source. The signal-to-noise ratio of imaging system is easy to improve by increasing the area of receiver. However, numerical simulation shows that the reconstruction images sometimes has a virtual phenomenon. It reduces the quality of reconstruction images, and even cannot have a resolution to the imaging target. Based on Fourier telescopy imaging principle, using T type transmitting array, the reason of forming virtual images is indicated by mathematical deduction. The spatial frequency error between the actual getting and setting would be produced when the laser beams scan the x or y axis with pitch error. The error would cause the random phase influence when calculating the single Fourier frequency of target by using phase closure on the axis and quadrant scan. Using integrated spatial frequency of transmitter array to reconstruct the image of target would cause a virtual phenomenon. By contrasting reconstruction images it is found that the image quality is reduced seriously, even the virtual phenomenon appears when the transmitting system is axially scanned with pitch error, and it decreases slightly on quadrant scan. In the present paper, we consider the reason of the phase closure of quadrant of T type transmitting array relies on axial frequency. At the same pitch error, different scan methods would cause different reconstruction images. The directions of virtual images are the same within the axial error. The computer simulation confirms the validity of the above analysis by three different modes of transmitter array through using the next field experiment parameters.
Generation and application of two-photon Raman laser for manipulation of internal state of Cs atom
Wang Zhi-Hui, Tian Ya-Li, Li Gang, Zhang Tian-Cai
2015, 64 (18): 184209. doi: 10.7498/aps.64.184209
Abstract +
Two-photon Raman process (TPRP) is an important technique in controlling the atomic internal states. It plays an important role in quantum manipulation and quantum information process. A reliable Raman laser for specific atom is the first step to demonstrate TPRP and quantum manipulation of an atom. In this paper, we theoretically analyze the two-photon Raman process regarding to Cesium “clock states” |6S1/2, F=4, mF=0 and |6S1/2, F=3, mF=0, and we obtain the dependences of the corresponding Rabi frequency on one-and two-photon detunings and one-photon Rabi frequencies in a realistic multi-level Cesium atom system. We find that to obtain an atom state flopping efficiency of 0.99 the Raman laser power fluctuation should be controlled to be smaller than 3.2%. We also report our simple experimental Raman laser system for TPRP of Cesium atom based on a fiber waveguide phase modulator. The phase modulator is driven by a 4.6 GHz microwave source and the two first-order sidebands with a frequency difference of 9.19 GHz are filtered out by a Fabry-Pérot cavity with a finesse of 48. After an amplitude-modulator-based intensity stabilization system, a total power of 73 μupW with a fluctuation of 2.2% within 90 min is obtained. By applying this Raman laser to a single Cesium atom trapped in a micrometer size far-off resonant trap (FORT), we obtain Raman spectra between Cesium “clock states” |6S1/2, F=4, mF=0 and |6S1/2, F=3, mF=0. The discrepancy between the two-photon resonance frequency and the defined clock frequency 9.192631770 GHz is due to the differential Stark shifts by FORT beam and Raman beams as well as the inaccuracy of the microwave source. By varying the Raman pulse length we also show the corresponding Rabi flopping with a rate of 153 kHz, which is consistent with the theoretical calculation. The obtained state transfer efficiency of 0.75 is much smaller than theoretical expectation 0.99, which is mainly limited by the state initialization efficiency. The Raman laser system reported in this paper is simple and reliable to realize and it provides a reliable method to manipulate the Cesium internal state. Moreover it could also be easily extended to other system for quantum manipulation of other species of atom.
Liquid crystal lens with large-range electrically controllable variable focal length
Yin Xiang-Bao, Liu Yong-Jun, Zhang Ling-Li, Lü Yue-Lan, Huo Bo-Fan, Sun Wei-Min
2015, 64 (18): 184212. doi: 10.7498/aps.64.184212
Abstract +
A novel structure model with different disc-ring radii of liquid crystal lens is proposed, in which liquid crystal director could be controlled by changing the electric field intensity dependence. Then the theory of liquid crystal and the geometric optics are analyzed. By using the finite element method, under the same constant voltage, we obtain the distributions of the electric field intensity at different positions of the liquid crystal layer. Then, the simulation results provide a theoretical guideline for the experimental fabrication. Due to the fact that the electric field intensity at the electrode edges is stronger than that at the circular hole, a shorter focal convergence could be realized by controlling the voltage between the upper and the lower substrates. In this paper, the influences of the electrode size and spacer thickness on the focal length of liquid crystal lens are also discussed experimentally and theoretically. Through optimizing various parameters, a prototype liquid crystal lens with a large zoom range and simple structure is obtained. Its focal length can be controlled to be 75-230 mm in a range of 25 VRMS-250 VRMS.
Modification of geometrical attenuation factor of bidirectional reflection distribution function based on random surface microfacet theory
Liu Hong, Zhu Jing-Ping, Wang Kai
2015, 64 (18): 184213. doi: 10.7498/aps.64.184213
Abstract +
The reflection attenuation factor given by Blinn has been widely used as an important factor in the bidirectional reflectance distribution function (BRDF) model based on geometrical optics theory for nearly half a century. However, Blinn's attenuation factor is based on microfacet theory and geometry of equicrural V-grooves caused sharp turning points and obvious error in its function curves. In this paper, a modified geometry attenuation factor based on random surface microfacet theory is presented. We assume that the surface is composed of a large number of microfacets, and the slope of each microfacet is independent of each other and follows Gaussian distribution. The attenuation effect is caused by the masking and shadowing factors in adjacent microfacets. Depending on the slope angles of microfacets, reflection falls in three submodels, i.e., passing model, semi-passing model, and masking/shadowing model when discussing masking/shadowing factor. The modified attenuation factor is given in an integral expression. The modified geometry attenuation factor is simulated and compared with Blinn's geometry attenuation factor. The sharp turning point in Blinn's attenuation factor curve is eliminated and the error of the BRDF curve is reduced. The result shows that the modified geometry attenuation factor reaches better physical rationality and significantly improves the accuracy of BRDF model. Compared with Blinn's geometry attenuation factor, the modification reduces the standard error between BRDF model and existing data from 0.0636 to 0.0084. The cause of the error in Blinn's geometry attenuation factor is analyzed, the equicrural V-grooves assumption of Blinn's geometry attenuation model considers too much reflection attenuation in the condition of small incident angle and large reflect angle. The modification in this paper is based on random surface microfacet theory, in which the angular dependence of adjacent microfacet is eliminated, for this reason the accuracies of geometry attenuation model and BRDF model are improved.
A new free energy model for predicting the qual-biaxial tests of dielectric elastomers
Jiang Shi-Ming
2015, 64 (18): 184601. doi: 10.7498/aps.64.184601
Abstract +
Dielectric elastomeric actuators (DEAs) have been intensely studied in the recent decades. Their attractive features include large deformation(380%), large energy density(3.4 J/g), light weight, fast response( 1 ms), and high efficiency (80%-90%). They can be used in medical devices, space robotices and energy harvesters. The core part of DEAs is a dielectric elastomeric film with two electordes. When pre-stretched forces are exerted on the film in plane direction and voltage is applied across its thickness, the film achieves a large deformation. Usually the effect of electric field is described by Maxwell stress E2, and the effect of mechanical field is described by free energy function models (such as Neo-Hookean model, Arruda-Boyce model and Gent model). There are deviations in varying degree between every models and tests of dielectric elastomer. No model works perfectly. In the present paper, a new free energy function model is given to reduce the deviation. According to the main models above, an undetermined parameter C(1, 2) is introduced. and i (W/i)= C( 1, 2)(i2- 1-2 2-2), pi = C( 1, 2)( pi2- p1-2p2-2)(i/ pi), i = 1, 2, are assumed. The new i ( W/i) and pi are substituted into the equation of equilibrium of dielectric elastomer film pi + E2 = i ( W/i), i = 1, 2. Under equal-biaxial pre-stretched condition, P1 = P2 = P, p1 = p2 = p, C(1, 2) = C(). The parameter C()= (V2/t0)2/( 2- -4-( p- p-4)(/ p)) is obtained. Through analysing the test results of VHB4905 which contains a series of equal-biaxial pre-stretched tests, the data (, C()) are obtained from the test data (, V). C() =a + beI1-3, (I1 = 12 + 22 + 32) can be determined by data points (, C()). By computing the integral of i ( W/i)= a + beI1-3)(i2- 1-2 2-2), i = 1, 2, a new free energy function W = (a/2)(I1-3) + b[eI1-3(I1-3-1) + 1] (the new model) is achieved. The test results of VHB4905 are fitted by Neo-Hookean, Gent model and the new model. Neo-Hookean model fits well only in small deformation. Gent model fits well only in small-middle deformation, and does not work well when stretch 3.5. The new model fits well in small, middle and large deformation. It is better than Neo-Hookean and Gent model. The new model can give big support in the study of dielectric elastomer materials and structure property, and can be used in engineering practice effectively.
Dynamic process and flow-rate regulation mechanism of particle inertial focusing in an asymmetric ally curved microchannel
Tang Wen-Lai, Xiang Nan, Zhang Xin-Jie, Huang Di, Ni Zhong-Hua
2015, 64 (18): 184703. doi: 10.7498/aps.64.184703
Abstract +
In this paper, an asymmetrically curved microchannel device is designed and fabricated to quantitatively characterize the dynamic inertial focusing process of polystyrene particles and blood cells flowing along the channel. The experimental investigations are systematically carried out to probe into the regulation mechanisms of flow rate and particle size. Specifically, based on the particle fluorescent streak images and the corresponding intensity profiles at specific downstream positions, the lateral migration behaviors of particles in the mirochannel can be divided into two stages: the formation of focused streak and the shift of focusing position. It is also found that the channel structures with small radii are dominant during the whole inertial focusing process. A three-stage model is then presented to elucidate the flow-rate regulation mechanism in terms of the competition between inertial lift force and Dean drag force, according to the evolution of particle focusing dynamics with increasing flow rates. By making comparisons of focusing position and focusing ratio between two different-sized particles under various experimental conditions, we find that the larger particles have better focusing performances and stabilities, and the relative focusing position of different-sized particles can be adjusted by changing the driving flow rate. Finally, the applicability of the explored inertial focusing mechanisms for manipulating biological particles with complex features is investigated by analyzing the lateral migration behaviors of blood cells in the asymmetrically curved microchannel. The obtained conclusions are very important for understanding the particle focusing dynamics in micro-scale flows and developing the point-of-care diagnostic instruments.
Modal expansion analysis of nonlinear circumferential guided wave propagation in a circular tube
Gao Guang-Jian, Deng Ming-Xi, Li Ming-Liang
2015, 64 (18): 184303. doi: 10.7498/aps.64.184303
Abstract +
Within the second-order perturbation approximation, the nonlinear effect of primary circumferential guided wave propagation in a circular tube is investigated using modal expansion analysis for waveguide excitation. The nonlinearity of the wave equation governing the wave propagation ensures the second-harmonic generation accompanying primary circumferential guided wave propagation. This nonlinearity may be treated as a second-order perturbation of the linear elastic response. The fields of the second harmonics of primary circumferential guided wave propagation are considered as superpositions of the fields of a series of double frequency circumferential guided wave (DFCGW) modes. Based on the momentum theorem and mathematical formulae of nonlinear stress tensor and its divergence under the cylindrical coordinate system, the mathematical expressions of the corresponding double frequency traction stress tensors and bulk driving forces are deduced for a certain primary circumferential guided wave mode. Subsequently, the equation governing the DFCGW mode expansion coefficient is established. Finally, the mathematical expression of second-harmonic field of the primary circumferential guided wave mode in a tube is derived. The results of the theoretical analyses and numerical calculations indicate that the degree of cumulative growth of the DFCGW mode with circumferential angle is obviously influenced by that of phase velocity matching between the primary and double frequency wave modes. It is found that the amplitude of the DFCGW mode can grow with circumferential angle when its phase velocity matches with that of the primary circumferential guided wave, and that the amplitude of the DFCGW mode will show a beat effect with circumferential angle when its phase velocity is not equal to that of the primary wave mode. The DFCGW mode, whose phase velocity matches with that of the primary wave mode, plays a dominant role in the field of second harmonic generated by the primary wave mode propagation, and the contribution of the other DFCGW modes to the said second-harmonic field is negligible after the primary wave mode has propagated some circumferential angle.
Tuning the electronic property of monolayer MoS2 adsorbed on metal Au substrate: a first-principles study
Zhang Li-Yong, Fang Liang, Peng Xiang-Yang
2015, 64 (18): 187101. doi: 10.7498/aps.64.187101
Abstract +
Using first principles calculations within density functional theory, we investigate the electronic property of a single-layer MoS2 adsorbed on Au. All the quantities are calculated using the Vienna ab initio simulation package. Calculations are performed using the projector augmented wave method with the Perdew-Burke-Ernzerhof functional and a kinetic energy cutoff of 400 eV. The atomic plane and its neighboring image are separated by a 15 Å vacuum layer. The k-meshes for the structure relaxation and post analysis are 9×9×1 and 19×19×1, respectively. The spin-orbit coupling is considered in the calculation. The research includes the binding energy, the band structure, density of states (DOS) and electric charge difference density. Three contact modes between MoS2 (0001) and Au (111) are considered. When the atom S layer and the atom Au layer on the contacting interface have the same structure, the minimum binding energy and distance between MoS2 (0001) and Au(111) are 2.2 eV and 2.5 Å respectively. The minimum binding energy confirms that the absorption is unstable. The band structure demonstrates that the MoS2-Au contact nature is of the Schottky-barrier type, and the barrier height is 0.6 eV which is bigger than MoS2-Sc contact. By comparison with other metal contacts such as Ru(0001), Pd(111) and Ir(111), the dependence of the barrier height on the work function difference exhibits a Fermi-level pinning. But the MoS2 is so thin that the Fermi-level pinning must be very small. Maybe there is a metal induced gap state. DOS points out that the Au substrate has no influence on the covalent bond between Mo and S. The influence of the Au substrate is that it shifts the DOS of monolayer MoS2 left on the axis. The change of DOS results in the increases of electron concentration and electric conductivity. Other calculation points out that Ti substrate can excite more electrons. Electric charge density difference demonstrates that there are a few electric charges that transfer on the contact interface. The conducting path of monolayer MoS2 may emerge at the interface between Au and MoS2. In summary, the Au electrode is not the best electrode in the MoS2 device. The Ti electrode can excite more electrons from MoS2. The work function of Sc electrode is close to the affine of MoS2. The Fermi energy level of graphene can be tuned by external voltage. So the Ti, Sc and graphene will be the better electrodes for MoS2 device. Results of this study may provide a theoretical basis for single-layer MoS2 transistor and guidance for its applications.
First-principles study of electronic structures and phase stabilities of ternary intermetallic compounds in the Mg-Y-Zn alloys
Ma Zhen-Ning, Jiang Min, Wang Lei
2015, 64 (18): 187102. doi: 10.7498/aps.64.187102
Abstract +
In the paper, the first-principles pseudopotential plane-wave method based on density functional theory is used to investigate the crystal structures, enthalpies of formation and electronic structures of X-Mg12YZn phase and W-Mg3Y2Zn3 phase in Mg-Y-Zn alloys. The obtained lattice constants of two phases are in good agreement with the available experimental values, which can reasonably reflect the accuracy of theoretical calculation. The calculated enthalpies of formation indicate that the W-Mg3Y2Zn3 and X-Mg12YZn phases have negative enthalpies of formation, which are-0.2787 eV/atom and-0.0268 eV/atom respectively. Both phases can form stable structures relative to single crystals Mg, Y and Zn, and the enthalpy of formation of W-Mg3Y2Zn3 phase is lower than that of X-Mg12YZn phase. The results for density of states show that the bonding of W-Mg3Y2Zn3 phase occurs mainly among the valence electrons of Mg 2p, Zn 3p and Y 4d orbits, the bonding peaks between-2.53 and 0 eV are derived from the hybridization of Mg 2p, Zn 3p and Y 4d orbits, the peaks between 5.07 and 7.51 eV predominantly originate from the hybridization of Mg 2p and Y 4d orbits. However, the bonding of X-Mg12YZn phase is mainly among the valence electrons of Mg 3s, Mg 2p, Zn 3p and Y 4d orbits. The bonding peaks between-2.30 and 0 eV originate mainly from 2p, 3p, and 4d orbit hybridization of Mg, Zn and Y, the peaks between 0 and 2.08 eV originate from the hybridization of Mg 3s, Mg 2p, Zn 3p and Y 4d orbits. At the same time, there is a pseudo-gap near each Fermi level of W-Mg3Y2Zn3 and X-Mg12YZn phases, which implies the presence of covalent bonding in the two phases. In addition, the charge densities respectively on (011) plane of W-Mg3Y2Zn3 phase and (0001) plane of X-Mg12YZn phase are analyzed, and the results indicate that the Zn-Y band exhibits covalent features in W-Mg3Y2Zn3 phase and X-Mg12YZn phase, the covalent bonding of W-Mg3Y2Zn3 phase is stronger than that of X-Mg12YZn phase. Compared with X-Mg12YZn phase, W-Mg3Y2Zn3 phase has a good phase stability attributed to its more bonding electron numbers in a low-energy region of the Fermi level.
Polarization properties of wurtzite structure Zn1-xMgxO and band offset at Zn0.75Mg0.25O/ZnO interfaces: A GGA+U investigation
Wu Kong-Ping, Qi Jian, Peng Bo, Tang Kun, Ye Jian-Dong, Zhu Shun-Ming, Gu Shu-Lin
2015, 64 (18): 187304. doi: 10.7498/aps.64.187304
Abstract +
Two-dimensional (2D) electron gas with high-mobility is found in wurtzite ZnO/Zn(Mg)O heterostructure, which probably arises from the polarization discontinuity at the ZnO/Zn(Mg)O interface, and the 2D electron gas in the heterostructure is usually also regarded as resulting from polarization-induced charge. In order to explore both the formation mechanism and the origin of the 2D electron gas in ZnMgO/ZnO heterostructure, it is necessary to study the polarization properties of Zn1-xMgxO alloy and energy band alignment of ZnO/Zn1-xMgxO super-lattice. In this paper, we study the polarization properties of Zn1-xMgxO alloy with different Mg compositions by using first-principles calculations with GGA+U method, and the polarization properties are calculated according to Berry-phase method. Owing to the excellent match between the in-plane lattice constants of ZnO and Zn1-xMgxO, the lattice constants of the ZnO and Zn1-xMgxO interface are similar, ZnO/Zn1-xMgxO super-lattice could be constructed easily. The planar-averaged electrostatic potential for the Mg0.25Zn0.75O/ZnO super-lattice and the macroscopically averaged potential along Z(0001) direction are calculated. The large size of (5+3) Mg0.25Zn0.75O/ZnO super-lattice ensures the convergence of potential to its bulk value in the region of the ZnO layer and Mg0.25Zn0.75O layer far from ZnO/Zn1-xMgxO interface. Besides, the valence band offset at the Mg0.25Zn0.75O/ZnO interface is calculated to be 0.26~eV based on the macroscopically averaged potential mentioned above, and the ratio of conduction band offset (EC) to valence band offset (EV) is in a reasonable range, and this is in substantial agreement with the values reported in recent experimental results. Because strain induces additional piezoelectric polarization in MgxZn1-xO, which is introduced by Mg dopant, the lack of inversion symmetry and the bulk ZnO induce its spontaneous polarization in the [0001] direction. The polarization discontinuity at the Mg0.25Zn0.75O/ZnO interface leads to the charge accumulation in the form of interface monopoles, giving rise to built-in electric fields in the super-lattice. In addition, energy alignment determination of the Mg0.25Zn0.75O/ZnO super-lattice is performed, which shows a type-I band alignment with EV=0.26 eV and EC=0.33 eV. The determination of the band alignment indicates that the Mg0.25Zn0.75O/ZnO super-lattice is competent to the confining of both electron and hole. These findings will be useful for designing and optimizing the 2D electron gas at Mg0.25Zn0.75O/ZnO interface, which can be regarded as an important reference for studying the 2D electron gas at MgxZn1-xO/ZnO super-lattices for electronics and optoelectronics applications.


Changes of micro zone luminescent properties and stress of GaN-based light emitting diode film grown on patterned silicon substrate, induced by the removal of the substrate and AlN buffer layer
Zhang Chao-Yu, Xiong Chuan-Bing, Tang Ying-Wen, Huang Bin-Bin, Huang Ji-Feng, Wang Guang-Xu, Liu Jun-Lin, Jiang Feng-Yi
2015, 64 (18): 187801. doi: 10.7498/aps.64.187801
Abstract +
At present, there are mainly two kinds of methods to prevent crack and reduce tensile stress of the silicon substrate GaN based light emitting diode (LED) epitaxial films: one is to use the patterned silicon substrate and the other is to grow a thick AlGaN buffer layer. The two kinds of methods have their own advantages and disadvantages. Although the patterned silicon substrate GaN based LED has industrialized and is gradually accepted by the market, there remain many scientific and technical problems, to be resolved, and a lot of research gaps worth studying deeply. Among these problems, to clearly investigate the different micro zone photoluminescence and the stress states in a single-patterned GaN based LED film grown on patterned silicon substrate. The studies of the stress interaction between the buffer layer and the quanturn well layer and the effect on the luminescent properties have important guiding significance for improving the quality and performance of the devices. Different micro zone photoluminescence (PL) properties in single-patterned GaN-based LED films grown on patterned silicon substrates, nondestructive free-standing LED thin film after removing away the silicon substrate, and the free-standing LED films after removing away the AlN buffer layer are studied. The variations of the bending degree of the free-standing LED thin films before and after removing away AlN buffer layer are inverstigated by using fluorescence microscopy and scanning electron microscopy. The results show as follows. 1) After removing away the silicon substrate, the free-standing LED film bends to the substrate direction in a cylindrical bending state. After removing away the AlN buffer layer, the LED film bends into flat. 2) For LED thin films on silicon substrates or off silicon substrates, their PL spectra have significant differences in different micro zones for the same pattern. When the AlN buffer layer is removed from the substrate its PL spectrum tends to be consistent in the different micro zones of the same pattern. When the patterned silicon substrate GaN-based LED thin film is removed from the silicon substrate, the PL spectrum is redshifted in each micro zone. After AlN buffer layer is removed from the substrate, the PL spectra present different degrees of blueshift in each micro zone. 3) The LED films before and after removing away the AlN buffer layer show some differences in droop effect.
Complete three-dimensional reduced surface field super junction lateral double-diffused metal-oxide-semiconductor field-effect transistor with semi-insulating poly silicon
Cao Zhen, Duan Bao-Xing, Yuan Xiao-Ning, Yang Yin-Tang
2015, 64 (18): 187303. doi: 10.7498/aps.64.187303
Abstract +
Lateral double-diffused metal-oxide-semiconductor field-effect transistor (LDMOS) is a key device for the power integrated circuit (PIC) and high voltage integrated circuit (HVIC) technologies. In order to break through the limit relation of 2.5 power between breakdown voltage (BV) and specific on-resistance (Ron,sp) for the traditional LDMOS, and improve the driving capability for the PIC by reducing the power consumption, the new SJ-LDMOS with the semi-insulating poly silicon (SIPOS SJ-LDMOS) is proposed in this paper for the first time, to the best of the authors' knowledge. In order to take full advantage of super junction concept, the SIPOS layer is used for SJ-LDMOS to achieve the effect of the complete three-dimensional reduced surface field (3D-RESURF) for the SJ-LDMOS. The substrate assisted depletion is effectively eliminated by the buffer layer under the super junction. The overall performances of the SIPOS SJ-LDMOS are improved by the uniform and high resistance of the SIPOS layer. The surface electric field is modulated to be uniform by the electric field modulation effect due to the SIPOS layer covering the field oxide. The higher BV would be achieved for the more uniform surface electric field because of the increased average lateral electric field. The BV for the unit length of the drift region is improved to 19.4 V/μupm. The SIPOS SJ-LDMOS along the 3D are subjected to the electric field modulation by the SIPOS layer, which achieves the complete 3D-RESURF effect, thus the drift region with the high concentration can be depleted completely to obtain the high BV. Moreover, in the on-state the majority carrier accumulation can be formed in the drift region of the SIPOS SJ-LDMOS due to the SIPOS layer, so that the specific on-resistance decreases further. In virtue of the ISE simulation, by optimizing the SIPOS layer of the proposed SIPOS SJ-LDMOS, the results show that the specific on-resistance of the SIPOS SJ-LDMOS is 20.87 mΩ·cm2 with a breakdown voltage of 388 V, which is less than 31.14 mΩ·cm2 for the N-buffer SJ-LDMOS with a breakdown voltage of 287 V, and far less than 71.82 mΩ·cm2 for the conventional SJ-LDMOS with a breakdown voltage of only 180 V with the same drift length.
Electron spin resonance study on charge ordering and spin ordering in nanocrystalline Bi0.2Ca0.8MnO3
Wang Qiang
2015, 64 (18): 187501. doi: 10.7498/aps.64.187501
Abstract +
Bi-based manganites Bi0.2Ca0.8MnO3 samples with different paiticle sizes were prepared by the sol-gel technique. The effect of paiticle size on the charge ordering (CO) and spin correlations of Bi0.2Ca0.8MnO3 was investigated by electron spin resonance (ESR). The variation in ESR intensity with temperature shows that the long-range CO transition is suppressed by the size reduction, and completely disappears as the paiticle size is reduced to 40 nm. In the paramagnetic (PM) region, the ESR intensity is fitted by the Arrhnius formula. The result shows that the activation energy is significantly enhanced with decreasing of paiticle size, especially in the 40 nm sample, indicating the enhancement of ferromagnetic (FM) correlations. However, the temperature dependence of ESR line width displays a typical CO characteristics for all samples. Thus, it is suggested that there is the short-range CO state in the 40 nm sample, while the long-range CO transition is completely suppressed. It is found that the onset temperatures of CO states are almost the same in all samples, indicating that the strength of CO correlations is not influenced by size reduction in this compound. A positive Curie-Weiss (CW) temperature is obtained from the line width in high-temperature PM regime, which confirms the existence of FM correlations in this system. Moreover, the value of CW temperature shows a significant decrease with particle size reduction, which indicates that the FM interactions can be weakened by size reduction in this system. Based on the research of ESR intensity and line width, it is concluded that the suppressed CO cannot be attributed to the enhancement of double-exchange FM interactions in Bi0.2Ca0.8MnO3 nanoparticles. To explain these behaviors, a core-shell model based on surface effect is proposed. In nanosized CO manganites, the disordered surface spins destroy the collinear antiferromagneitc (AFM) configuration, and favor FM surface layer coupled with the inner AFM core. With the reduction of paiticle size, the weakening of long-range AFM CO is more significant than that of short-range FM ordering due to the increase of surface spin disorders. With the reduction of paiticle size, the FM ordering will gradually dominate in the competition between FM ordering and AFM ordering, which results in the disappearance of CO transition peak in the ESR intensity curve.
Shock plasticity design of brittle material
Jiang Tai-Long, Yu Yin, Huan Qiang, Li Yong-Qiang, He Hong-Liang
2015, 64 (18): 188301. doi: 10.7498/aps.64.188301
Abstract +
The mechanical properties of a material are closely related to its internal micro-structure. Enhancing shock plasticity by designing appropriate micro-structure will help to slow down or delay shock failure of brittle material. In this paper, we put forward a method of designing and improving shock plasticity of brittle material by implanting specific micro-voids. A lattice-spring model is adopted, which can represent mechanical properties of brittle materials quantitatively. Simulations reveal how the arrangement modes of micro-voids can affect the shock response of brittle material. By implanting randomly arranged voids, porous brittle material has significantly higher shock plasticity than dense brittle material and the design of the regular arrangement mode of voids will help to enhance the shock plasticity further. The dominant mechanism corresponding to the void collapse in the shocked brittle material is shear slip caused by shear stress concentration, which features the occurrence of shear cracks around the voids. Features of mesoscopic deformation in the sample with 5% porosity indicate that the shock plasticity of porous brittle material comes from the volume shrinkage deformation caused by void collapse and the slippage and rotation deformation caused by extension of shear cracks. The inter-permeation of voids and volume shrinkage deformation play a leading role in the sample with regularly arranged voids. While the shear cracks extends over long distance, slippage and rotation deformation take place dominantly in the sample with randomly arranged voids. The two samples with different arrangement modes of voids both have three stages of response in the Hugoniot stress-strain curves in this paper, i. e., linear elasticity stage, collapse deformation stage, and slippage and rotation deformation stage. The sample with higher porosity has a higher shock plasticity than the sample with lower porosity. When the samples have the same porosity, the collapse deformation stage makes greater contribution to the overall shock plasticity if voids are regularly arranged, while the slippage and rotation deformation stage make greater contribution to the overall shock plasticity if the voids are randomly arranged. The principle of enhancing shock plasticity of brittle material by arranging voids regularly in this paper provides physical knowledge for the designing and preparing new types of brittle materials, thereby helping to prevent the function failure induced by shock in brittle material.
Recent progress in controllability of complex network
Hou Lü-Lin, Lao Song-Yang, Xiao Yan-Dong, Bai Liang
2015, 64 (18): 188901. doi: 10.7498/aps.64.188901
Abstract +
The model, structure and dynamics of complex systems and networks are studied to control complex systems, which reflects the ability to understanding complex systems. Recently, the research on controllability of complex networks by using control theory and complexity science has attracted much attention. It has been investigated extensively by many scientists from various fields, and many meaningful achievements have been obtained in the past few years. In this paper, the process of controllability of complex networks is discussed, the framework of structural controllability based on maximum matching is introduced in detail, and the relevant research status since 2011 is summarized. Controllabilities of complex networks are introduced in the following aspects: different types of controllabilities, relationship between controllability and network statistical characteristics, classification and measures based on controllability, robustness of controllability, and optimization methods of controllability. Finally, the questions urgent to solve in controllability are discussed, so as to give a help to the the study in this respect.#br#There are five sections in this paper, which involve with different aspects of controllability. In the introduction section, the research work of controllability since 2011 is briefly mentioned, and the difference between controllability and previous pinning controllability is clarified. In the second section, the concept of controllability and different types of controllabilities are discussed in detail, including structural controllability, exact controllability, controllability with edge dynamics and controllability with nodal dynamics. In the third section, the relationship between controllability and network structure is investigated, especially the effects of common statistical characteristics and low-degree nodes on controllability. In the fourth section, the measures based on controllability are introduced, which includes control profiles, control range, control centrality, control capacity and control modality. In the fifth section, the research work about control robustness is discussed from robustness measures to optimization methods. In the fifth section, the optimization methods of controllability are introduced, which are classified into two different strategies: topology and edge direction.
Design and verification of microwave low frequency band-pass and high frequency band-stop composite structure
Huang Da-Qing, Kang Fei-Yu, Zhou Zhuo-Hui, Liu Xiang, Cheng Hong-Fei
2015, 64 (18): 188401. doi: 10.7498/aps.64.188401
Abstract +
It is important and urgent to develop microwave low frequency band-pass and high frequency band-stop composite structures according to the needs of marine environment stealth weapons and equipment constructions. In this paper, a hollow hexagonal periodic structure is originally designed and the microwave band-pass and band-stop characteristics are investigated through the CST software simulation. As an optimization result, the numerical periodic structure parameters of hexagon ring are as follows: hexagon ring side length is 2.7 mm, line width 0.5 mm and gap width 0.15 mm, which shows a transmission of 83% at 0-2 GHz, and meanwhile a shielding efficiency of more than 10 dB at 8-18 GHz, thereby basically justifying our design target. On this basis, a new type of double-layers' composite frequency selective surface (FSS) structure which is composed of facial layer, hollow hexagon ring array 1, middle spacer layer, hollow hexagon ring array 2 and another facial layer stacked layer by layer, is creatively designed, which displays excellent microwave low frequency band-pass and high frequency band-stop performances compared with a single layer hollow hexagonal periodic structure, and by simulation and optimization, structural parameters of the upper FSS structure are as follows: hexagon ring side length is 3.0 mm, line width 0.5 mm, gap width 0.4 mm, and the lower FSS structure parameters are as follows: hexagon ring side length is 3.2 mm, line width 0.5 mm, gap width 1.0 mm; simulation results show itself that dual different layers' FSS design presents itself excellent low frequency band-pass and high frequency band-stop transformation characteristics, and the fast switch capacity is the basic foundation for both excellent low frequency band-pass and outstanding high frequency band-stop characteristics. The effects of wave incidence angle (TE) on electrical performance of dual-layers FSS are investigated and the results indicate that the designed dual-layers' FSS possesses a wide angle insensitivity in a range of 0-45°, which is especially beneficial to engineering applications. Finally the composite structures with dual-layers' FSSs are manufactured and verified, and high transmission up to 95.6% at 0-2 GHz frequency band and more than 10 dB shielding efficiency at 7.05-18 GHz are obtained, which strongly testifies our design idea and has important significance for developing the high performance band-pass and band-stop composite structure and new electromagnetic functional composite materials.


Mechanism of neural information processing of sound localization by interaural level difference
Zhang Hong, Ding Jiong, Tong Qin-Ye, Cheng Qian-Liu
2015, 64 (18): 188701. doi: 10.7498/aps.64.188701
Abstract +
More attention should be paid to the neural system, which is a quantitative system. There are few reports about the quantitative research on neural system. It will hinder the quantitative studies on animal binaural sound localization. The existing physiological experiments have found that there is a monotonic increase/decrease relationship between the input sound levels and the output spike frequencies of auditory neurons, so the variations of input sound level can be simplified into the change of output spike frequency of auditory neurons. In this paper, based on the theory of circle map and symbolic dynamics, a quantitative model of auditory neural circuitry is presented. In this neural circuit model, the neurons of ipsilateral input circuit propagate the action potentials as excitatory inputs, the neurons of heterolateral input one propagate the action potentials as inhibitory inputs. We also use a chemical neuronal model, which shows that the neurotransmitters released from pre-synapse vesicles have characteristics of quantitative release. The strength of the coupling between two neurons is represented by a coupling coefficient. The relationship between the input/output spike sequences of neural circuitry is simulated by using an Hodgkin-Huxley equation. In the range of simulating parameters, there is a monotonic increase/decrease phenomenon between input and output spike frequency. For the neuron with a single input and output structure, it is symbolized according to the method of symbolic dynamics; for the neurons with multi-input and single output structure, the output spike time will be used to detect the input spike frequency variations which are caused by the changes of interaural level difference (ILD), and the binaural level differences from those spike sequences are analyzed to locate the source of the sounds. With the increase of output spikes, the length of symbolic sequence increases, the symbolic sequence is sensitive to the variation of input signal. The simulation results show that the quantitative model proposed in this paper is able to detect the ILD signals by neural spike sequences.
Properties of excited state of polaron in quantum disk in finite depth parabolic potential well
Zhao Cui-Lan, Wang Li-Li, Zhao Li-Li
2015, 64 (18): 186301. doi: 10.7498/aps.64.186301
Abstract +
Studies of single quantum state measurement and the relevant physics are very important for the fields of quantum information and quantum coupution. In recent years, quantum dots as information carrier have become a hotpoint of research. The study on quantum dot properties has atracted a lot of attetion and made a series of progress.#br#In this paper, we formulate a theoretical method that can be used to investigate polaron properties in low-dimensional structures in finite depth potential well. We assume that an electron in a quantum disk which is in other medium is in parabolic potential field, but the effect of the medium on the electron in quantum disk is equivalent to a potential barrier with height V1 and width d. By expanding the finite height potential barrier as plane waves and Lee-Low-Pines unitary transformation for Hamiltonian, as well as variation for expectation value of Hamiltonian where trial wave functions are obtained by solving the energy eigen-value equation, the ground state energy, the first excited state energy, and excitation energy of polaron are drived.#br#Numerical calculation by using polaron unit, numerical results indicate that the first excited state energy and excitation energy of polaron increase with increasing the width or height of the potential barrier, because the probability of electron penetrating potential barrier will decrease as the width or height of potential barrier increases, so that electronic energy, the first excited state energy and excitation energy of polaron all increase. Numerical results also show that energies mentioned earlier decrease with increasing radius of quantum disk, which illustrates that the quantum disk has obvious quantum size effect.#br#It is also found from numerical results that the first excited state energy of polaron decreases with increasing effective confine length, it falls quickly when effective confine length is less than 0.3 and is a little change when effective confine length is more than 0.3. The longer the effective confine length, the more weakly the electron is bounded and the smaller the potential energy is, so that the first excited state energy of polaron decreases. Oppositely, the excitation energy of polaron increases with increasing effective confine length, because the first excited state energy decreases more slowly than the ground state energy.
SPECIAL ISSUE—Recent developments and applications of computational physics
A fully quantum description of the free-energy in high pressure hydrogen
Chen Ji, Feng Ye-Xin, Li Xin-Zheng, Wang En-Ge
2015, 64 (18): 183101. doi: 10.7498/aps.64.183101
Abstract +
Hydrogen is the lightest and most abundant element in the universe. Ever since Wigner and Huntington's prediction that pressure induced metallization might happen in solid hydrogen, understanding the hydrogen phase diagram has become one of the greatest challenges in condensed matter and high pressure physics. The light mass of hydrogen means that the nuclear quantum effects could be important in describing this phase diagram under high pressures. Numerical evaluations of their contributions to the structural, vibrational, and energetic properties, however, are difficult and up to now most of the theoretical simulations still remain classical. This is particularly true for the energetic properties. When the free-energies of different phases are compared in determining the ground state structure of the system at a given pressure and temperature, most of the theoretical simulations remain classical. When nuclear quantum effects must be taken into account, one often resorts to the harmonic approximation. In the very rare case, the anharmonic contributions from the nuclear statistical effects are considered by using a combination of the thermodynamic integration and the at initio molecular dynamics methods, which helps to include the classical nuclear anharmonic effects. Quantum nuclear anharmonic effects, however, are completely untouched. Here, using a self-developed combination of the thermodynamic integration and the at initio path-integral molecular dynamics methods, we calculated the free-energies of the high pressure hydrogen at 100 K from 200 GPa to 300 GPa. The harmonic lattice was taken as the reference and the Cmca phase of the solid hydrogen was chosen. When the bead number of the path-integral (P) equals one, our approach reaches the so-called classical limit. Upon increasing P until the results are converged, our approach reaches the limit when both classical and quantum nuclear anharmonic effects are included. Therefore, by comparing the free-energy of the harmonic lattice and the thermodynamic integration results at P equals one, we isolate the classical nuclear anharmonic effects. By comparing the thermodynamic integration results at P equals one and with those when they are converged with respect to P, we isolate the quantum nuclear anharmonic effects in a very clean manner. Our calculations show that the classical nuclear anharmonic contributions to the free-energy are negligible at this low temperature. Those contributions from the quantum nuclear anharmonic effects, however, are as large as ~15 meV per atom. This value also increases with pressure. This study presents an algorithm to quantitatively calculate the quantum contribution of the nuclear motion to free-energy beyond the often used harmonic approximation. The large numbers we got obtained also indicate that such quantum nuclear anharmonic effects are important in describing the phase diagram of hydrogen, at/above the pressures studied.
Dirac-like cones at k=0
Huang Xue-Qin, Chan Che-Ting
2015, 64 (18): 184208. doi: 10.7498/aps.64.184208
Abstract +
Dirac cones and Dirac points are found at the K (K') points in the Brillouin zones of electronic and classical waves systems with hexagonal or triangular lattices. Accompanying the conical dispersions, there are many intriguing phenomena including quantum Hall effect, Zitterbewegung and Klein tunneling. Such Dirac cones at the Brillouin zone boundary are the consequences of the lattice symmetry and time reversal symmetry. Conical dispersions are difficult to form in the zone center because of time reversal symmetry, which generally requires the band dispersions to be quadratic at k=0. However, the conical dispersions with a triply degenerate state at k=0 can be realized in two dimensional (2D) photonic crystal (PC) using accidental degeneracy. The triply degenerate state consists of two linear bands that generate Dirac cones and an additional flat band intersecting at the Dirac point. If the triply degenerate state is derived from the monopolar and dipolar excitations, effective medium theory can relate this 2D PC to a double zero-refractive-index material with effective permittivity and permeability equal to zero simultaneously. There is hence a subtle relationship between two seemingly unrelated concepts: Dirac-like cone and zero-refractive index. The all-dielectric double zero-refractive-index material has advantage over metallic zero-index metamaterials which are usually poorly impedance matched to the background and are lossy in high frequencies. The Dirac-like cone zero-index materials have impedances that can tune to match the background material and the loss is small as the system has an all-dielectric construction, enabling the possibility of realizing zero refractive index in optical frequencies. The realization of Dirac-like cones at k=0 can be extended from the electromagnetic wave system to acoustic and elastic wave systems and effective medium theory can also be applied to relate these systems to zero-index materials. The concept of Dirac/Dirac-like cone is intrinsically 2D. However, using accidental degeneracy and special symmetries, the concept of Dirac-like point can be extended from two to three dimensions in electromagnetic and acoustic waves. Effective medium theory is also applicable to these systems, and these systems can be related to isotropic media with effectively zero refractive indices. One interesting implication of Dirac-like cones in 2D PC is the existence of robust interface states. The existence of interface states is not a trivial problem and there is usually no assurance that localized state can be found at the boundary of photonic or phononic crystal. In order to create an interface state, one usually needs to decorate the interface with strong perturbations. Recently, it is found that interface state can always be found at the boundary separating two semi-infinite PCs which have their system parameters slightly perturbed from the Dirac-like cone formation condition. The assured existence of interface states in such a system can be explained by the sign of the surface impedance of the PCs on either side of the boundary which can be derived using a layer-by-layer multiple scattering theory. In a deeper level, the existence of the interface state can be accounted for by the geometric properties of the bulk band. It turns out that the geometric phases of the bulk band determine the surface impedance within the frequency range of the band gap. The geometric property of the momentum space can hence be used to explain the existence of interface states in real space through a bulk-interface correspondence.
Computational prediction of lattice defects in multinary compound semiconductors as photovoltaic materials
Yuan Zhen-Kun, Xu Peng, Chen Shi-You
2015, 64 (18): 186102. doi: 10.7498/aps.64.186102
Abstract +
In the past 60 years development of photovoltaic semiconductors, the number of component elements has increased steadily, i.e., from silicon in the 1950s, to GaAs and CdTe in the 1960s, to CuInSe2 in the 1970s, to Cu(In, Ga) Se2 in the 1980s, to Cu2ZnSnS4 in the 1990s, and to recent Cu2ZnSn(S, Se)4 and CH3NH3PbI3. Whereas the material properties become more flexible as a result of the increased number of elements, and multinary compound semiconductors feature a dramatic increase of possible point defects in the lattice, which can significantly influence the optical and electrical properties and ultimately the photovoltaic performance. It is challenging to characterize the various point defects and defect pairs experimentally. During the last 20 years, first-principles calculations based on density functional theory (DFT) have offered an alternative method of overcoming the difficulties in experimental study, and widely used in predicting the defect properties of semiconductors. Compared with the available experimental methods, the first-principles calculations are fast, direct and exact since all possible defects can be investigated one by one. This advantage is especially crucial in the study of multinary compound semiconductors which have a large number of possible defects. Through calculating the formation energies, concentration and transition (ionization) energy levels of various possible defects, we can study their influences on the device performance and then identify the dominant defects that are critical for the further optimization of the performance. In this paper, we introduce the first-principles calculation model and procedure for studying the point defects in materials. We focus on the hybrid scheme which combines the advantages of both special k-points and -point-only approaches. The shortcomings of the presentcalculation model are discussed, with the possible solutions proposed. And then, we review the recent progress in the study of the point defects in two types of multinary photovoltaic semiconductors, Cu2ZnSn(S,Se)4 and H3NH3PbI3. The result of the increased number of component elements involves various competing secondary phases, limiting the formation of single-phase multinary compound semiconductors. Unlike ternary CuInSe2, the dominant defect that determines the p-type conductivity in Cu2ZnSnS4 is Cu-on-Zn antisite (CuZn) defect rather than the copper vacancy (VCu). However, the ionization level of CuZn is deeper than that of VCu. The self-compensated defect pairs such as [2CuZn+SnZn] are easy to form in Cu2ZnSnS4, which causes band gap fluctuations and limits the Voc of Cu2ZnSnS4 cells. Additionally the formation energies of deep level defects, SnZn and VS, are not sufficiently high in Cu2ZnSnS4, leading to poor lifetime of minority carriers and hence low Voc. In order to enhance the formation of VCu and suppress the formation of CuZn as well as deep level defects, a Cu-poor/Zn-rich growth condition is required. Compared with Cu2ZnSnS4, the concentration of deep level defects is predicted to be low in Cu2ZnSnSe4, therefore, the devices fabricated based on the Se-rich Cu2ZnSn(S,Se)4 alloys exhibit better performances. Unlike Cu2ZnSnS4 cells, the CH3NH3PbI3 cells exhibit rather high Voc and long minority-carrier life time. The unusually benign defect physics of CH3NH3PbI3 is responsible for the remarkable performance of CH3NH3PbI3 cells. First, CH3NH3PbI3 shows that flexible conductivity is dependent on growth condition. This behavior is distinguished from common p-type photovoltaic semiconductor, in which the n-type doping is generally difficult. Second, in CH3NH3PbI3, defects with low formation energies create only shallow levels. Through controlling the carrier concentration (Fermi level) and growth condition, the formation of deep-level defect can be suppressed in CH3NH3PbI3. We conclude that the predicted results from the first-principles calculations are very useful for guiding the experimental study.
Recent progress in Rashba spin orbit coupling on metal surface
Gong Shi-Jing, Duan Chun-Gang
2015, 64 (18): 187103. doi: 10.7498/aps.64.187103
Abstract +
Spin-orbit coupling (SOC) is a bridge between the spin and orbital of an electron. Through SOC, spin of the electron can possibly be controlled throuth external electric fields. It is found that many novel physical phenomena in solids are related with SOC, for example, the magnetic anisotropy of magnetic materials, the spin Hall effect, and the topological insulator, etc. In the surface of solid or at the interface of heterostructure, Rashba SOC is induced by the structure inversion asymmetry. It was observed first in semiconductor heterostructure, which has an inversion asymmetric potential at the interface. Because Rashba SOC at the interface can be easily controlled through gate voltage, it is of great significance in the field of electric control of magnetism. Metal surface subsequent to semiconductor becomes another main stream with large Rashba SOC. In this paper, we review the recent progress in Rashba SOC in metal surfaces, including both the magnetic and nonmagnetic metal surfaces. We demonstrate the findings in Au(111), Bi(111), Gd(0001), etc., and discuss the possible factors that could influence Rashba SOC, including the surface potential gradient, atom number, the symmetry of the surface wavefunction, and the hybridization between the different orbitals in the surface states, etc. We also discuss the manipulation of Rashba SOC through electric field or surface decoration. In addition, on magnetic surface, there coexist Rashba SOC and magnetic exchange interaction, which provides the possibility of controlling magnetic properties by electric field through Rashba SOC. The angle-resolved photoemission spectroscopy and the first-principles calculations based on density functional theory are the two main methods to investigate the Rashba SOC. We review the results obtained by these two approaches and provide a thorough understanding of the Rashba SOC in metal surface.
The valley degree of freedom of an electron
Sun Jia-Tao, Meng Sheng
2015, 64 (18): 187301. doi: 10.7498/aps.64.187301
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Under the periodic potential of solid, the movement of an electron obeys the Bloch theorem. In addition to the charge and real spin degree of freedom, Bloch electrons in solids are endowed with valley degree of freedom representing the local energy extrema of the Bloch energy bands. Here we will review the intriguing electronic properties of valley degree of freedom of solid materials ranging from conventional bulk semiconductors to two-dimensional atomic crystals such as graphene, silicene, and transition metal dichalcogenides. The attention is paid to how to break the valley degeneracy via different ways including strain, electric field, optic field, etc. Conventional semiconductors usually have multiple valley degeneracy, which have to be lifted by quantum confinement or magnetic field. This can alleviate the valley degeneracy problem, but lead to simultaneously more complex many-body problems due to the remnant valley interaction in the bulk semiconductor. Two-dimensional materials provide a viable way to cope with the valley degeneracy problem. The inequivalent valley points in it are in analogy with real spin as long as the inversion symmetry is broken. In the presence of electric field, the nonvanishing Berry curvature drives the anomalous transverse velocity, leading to valley Hall effect. The valley degree of freedom can be coupled with other degree of freedom, such as real spin, layer, etc, resulting in rich physics uncovered to date. The effective utilization of valley degree of freedom as information carrier can make novel optoelectronic devices, and cultivate next generation electronics–valleytronics.
Material modeling for large scale and complex nanostructures: A semi-empirical Hamiltonian method
Yu Ming, Wu Shi-Yu
2015, 64 (18): 187302. doi: 10.7498/aps.64.187302
Abstract +
The advent of the era of nano-structures has also brought about critical issues regarding the determination of stable structures and the associated properties of such systems. From the theoretical perspective, it requires to consider systems of sizes of up to tens of thousands atoms to obtain a realistic picture of thermodynamically stable nano-structure. This is certainly beyond the scope of DFT-based methods. On the other hand, conventional semi-empirical Hamiltonians, which are capable of treating systems of those sizes, do not possess the rigor and accuracy that can lead to a reliable determination of stable structures in nano-systems. During the last dozen years, extensive effort has been devoted to developing methods that can handle systems of nano-sizes on the one hand, while possess first principles-level accuracy on the other. In this review, we present just such a recently developed and well-tested semi-empirical Hamiltonian, referred in the literature as the SCED-LCAO Hamiltonian. Here SCED is the acronym for self-consistent/environment-dependent while LCAO stands for linear combination of atomic orbitals. Compared to existing conventional two-center semiempirical Hamiltonians, the SCED-LCAO Hamiltonian distinguishes itself by remedying the deficiencies of conventional two-center semi-empirical Hamiltonians on two important fronts: the lack of means to determine charge redistribution and the lack of involvement of multi-center interactions. Its framework provides a scheme to self-consistently determine the charge redistribution and includes multi-center interactions. In this way, bond-breaking and bond-forming processes associated with complex structural reconstructions can be described appropriately. With respect to first principles methods, the SCED-LCAO Hamiltonian replaces the time-consuming energy integrations of the self-consistent loop in first principles methods by simple parameterized functions, allowing a speed-up of the self-consistent determination of charge redistribution by two orders of magnitudes. Thus the method based on the SCED-LCAO is no more cumbersome than the conventional semi-empirical methods on the one hand and can achieve the first principle-level accuracy on the other. The parameters and parametric functions for SCED-LCAO Hamiltonian are carefully optimized to model electron-electron correlations and multi-center interactions in an efficient fitting process including a global optimization scheme. To ensure the transferability of the Hamiltonian, the data base chosen in the fitting process contains large amount of physical properties, including (i) the binding energies, the bond lengths, and the symmetries of various clusters covering not only the ground state but also the excited phases, (ii) the binding energies as a function of atomic volume for various crystal phases including also the high pressure phases, and (iii) the electronic band structures of the crystalline systems. In particular, the data bases for excited phases of clusters and high pressure phases in bulk systems are more important when performing molecular dynamics simulations where correct transferable phases are required, such as the excited phases. The validity and the robustness of the SCED-LCAO Hamiltonian have been tested for more complicated Si-, C-, and B-based systems. The success of the SCED-LCAO Hamiltonian will be elucidated through the following applications: (i) the phase transformations of carbon bucky-diamond clusters upon annealing, (ii) the initial stage of growth of single-wall carbon nanotubes (SWCNTs), (iii) the discovery of bulky-diamond SiC clusters, (iv) the morphology and energetics of SiC nanowires (NWs), and (v) the self-assembly of stable SiC based caged nano-structures. A recent upgrade of the SCED-LCAO Hamiltonian, by taking into account the effect on the atomic orbitals due to the atomic aggregation, will also be discussed in this review. This upgrade Hamiltonian has successfully characterized the electron-deficiency in trivalent boron element captured complex chemical bonding in various boron allotropes, which is a big challenge for semi-empirical Hamiltonians.
Spin-polarization of organic molecules at the ferromagnetic surface
Yi Ding, Wu Zhen, Yang Liu, Dai Ying, Xie Shi-Jie
2015, 64 (18): 187305. doi: 10.7498/aps.64.187305
Abstract +
Study on organic/ferromagnetic interface is helpful for understanding the effects of magnetoresistance in organic spin-valve, because one of the reasons of leading to this phenomenon is due to the spin injection at the interface. However, the interactions at the organic/ferromagnetic interface are complicated and full of possibilities, and the effects are still under debate till now. One possible cause is that the adsorption of organic molecules on the ferromagnetic surface is random, which leads to various adsorbing configurations. Therefore, in this paper we select some typical adsorbing configurations of benzene/Co system to reveal the effect of spin-polarization of organic molecules at the ferromagnetic surface by using first-principles calculations. It is obtained that the spin degenerated electronic states of benzene molecule will be broken due to the coupling between the 3d electrons of Co atoms and the 2p electrons of benzene molecule. The density of states at the Fermi level becomes spin related and a spin polarization appears in the benzene molecule. For both of the configurations T1T2 and T1H12, from the projected density of states we can find that the majority-spin electrons of the benzene molecule is oriented in opposition to the direction of the ferromagnetic electrode at the Fermi level, which means that the organic molecules filter and reverse the original spin direction of the injected electrons from the ferromagnetic electrode. As mentioned above, the adsorbing configurations are different, so we consider three kinds of configurations with different adsorbing distances for further studying the spin polarization at the interface. On the basis of the configuration T1T2, distances of 2.0 Å, 2.2 Å and 2.4 Å are studied, where 2.0 Å is the equilibrium position we obtained with full relaxation. It should be noted that we do not relax the geometric structure of the system in this part of study. It is found that the spin polarization is sensitively dependent on the distance between benzene and Co surface. The spin-polarization near the Fermi level even changes its direction from positive to negative with the increase of the distance in such a small range. Our studies reflect the complexity of organic molecule/ferromagnetic electrode interfaces, and enrich the understanding of this field.
Progess of discrete Boltzmann modeling and simulation of combustion system
Xu Ai-Guo, Zhang Guang-Cai, Ying Yang-Jun
2015, 64 (18): 184701. doi: 10.7498/aps.64.184701
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Detonation is a kind of self-propagating supersonic combustion where the chemical reaction is rapid and violent under an extreme condition. The leading part of a detonation front is pre-shocked by a strong shock wave propagating into the explosive and triggering chemical reaction. The combustion system can be regarded as a kind of chemical reactive flow system. Therefore, the fluid modeling plays an important role in the studies on combustion and detonation phenomena. The discrete Boltzmann method (DBM) is a kind of new fluid modeling having quickly developed in recent thirty years. In this paper we review the progress of discrete Boltzmann modeling and simulation of combustion phenomena. Roughly speaking, the discrete Boltzmann models can be further classified into two categories. In the first category the DBM is regarded as a kind of new scheme to numerically solve partial differential equations, such as the Navier-Stokes equations, etc. In the second category the DBM works as a kind of novel mesoscopic and coarse-grained kinetic model for complex fluids. The second kind of DBM aims to probe the trans- and supercritical fluid behaviors or to study simultaneously the hydrodynamic non-equilibrium (HNE) and thermodynamic non-equilibrium (TNE) behaviors. It has brought significant new physical insights into the systems and promoted the development of new methods in the fields. For example, new observations on fine structures of shock and detonation waves have been obtained; The intensity of TNE has been used as a physical criterion to discriminate the two stages, spinodal decomposition and domain growth, in phase separation; Based on the feature of TNE, some new front-tracking schemes have been designed. Since the goals are different, the criteria used to formulate the two kinds of models are significantly different, even though there may be considerable overlaps between them. Correspondingly, works in discrete Boltzmann modeling and simulation of combustion systems can also be classified into two categories in terms of the two kinds of models. Up to now, most of existing works belong to the first category where the DBM is used as a kind of alternative numerical scheme. The first DBM for detonation [Yan, et al. 2013 Front. Phys. 8 94] appeared in 2013. It is also the first work aiming to investigate both the HNE and TNE in the combustion system via DBM. In this review we focus mainly on the development of the second kind of DBM for combustion, especially for detonation. A DBM for combustion in polar-coordinates [Lin, et al. 2014 Commun. Theor. Phys. 62 737] was designed in 2014. It aims to investigate the nonequilibrium behaviors in implosion and explosion processes. Recently, the multiple-relaxation-time version of DBM for combustion [Xu, et al. 2015 Phys. Rev. E 91 043306] was developed. As an initial application, various non-equilibrium behaviors around the detonation wave in one-dimensional detonation process were preliminarily probed. The following TNE behaviors, exchanges of internal kinetic energy between different displacement degrees of freedom and between displacement and internal degrees of freedom of molecules, have been observed. It was found that the system viscosity (or heat conductivity) decreases the local TNE, but increases the global TNE around the detonation wave. Even locally, the system viscosity (or heat conductivity) results in two competing trends, i.e. to increase and decrease the TNE effects. The physical reason is that the viscosity (or heat conductivity) takes part in both the thermodynamic and hydrodynamic responses to the corresponding driving forces. The ideas to formulate DBM with the smallest number of discrete velocities and DBM with flexible discrete velocity model are presented. As a kind of new modeling of combustion system, mathematically, the second kind of DBM is composed of the discrete Boltzmann equation(s) and a phenomenological reactive function; physically, it is equivalent to a hydrodynamic model supplemented by a coarse-grained model of the TNE behaviors. Being able to capture various non-equilibrium effects and being easy to parallelize are two features of the second kind of DBM. Some more realistic DBMs for combustion are in progress. Combustion process has an intrinsic multi-scale nature. Typical time scales cover a wide range from 10-13 to 10-3 second, and typical spatial scales cover a range from 10-10 to 1 meter. The hydrodynamic modeling and microscopic molecular dynamics have seen great achievements in combustion simulations. But for problems relevant to the mesoscopic scales, where the hydrodynamic modeling is not enough to capture the nonequilibrium behaviors and the molecular dynamics simulation is not affordable, the modeling and simulation are still keeping challenging. Roughly speaking, there are two research directions in accessing the mesoscopic behaviors. One direction is to start from the macroscopic scale to smaller ones, the other direction is to start from the microscopic scale to larger ones. The idea of second kind of DBM belongs to that of the first direction. It will contribute more to the studies on the nonequilibrium behaviors in combustion phenomena.
Quantum thermal transport and spin thermoelectrics in low-dimensional nano systems: application of nonequilibrium Green's function method
Chen Xiao-Bin, Duan Wen-Hui
2015, 64 (18): 186302. doi: 10.7498/aps.64.186302
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Emerging novel properties of nanomaterials have been attracting attention. Besides quantum electronic transport properties, the breakdown of classical Fourier’s law and other significant quantum thermal behaviors such as quantized thermal conductance, phonon subbands, size effects, the bottleneck effect, and even interaction between heat and spin degrees of freedom have also been revealed over the past two decades. These phenomena can be well captured by the nonequilibrium Green’s function (NEGF) method, which is pretty simple under ballistic or quasi-ballistic regimes. In this review, we mainly focus on two aspects: quantum phonon transport and thermal-spin transport in low-dimensional nanostructures. First, we present a brief history of researches on thermal transport in nanostructures, summarize basic characteristics of quantum thermal transport, and then describe the basic algorithm and framework of the phonon NEGF method. Compared with other methods, the NEGF method facilitates numerical calculations and can systematically incorporate quantum many-body effects. We further demonstrate the power of phonon NEGF method by recent research progress: from the phonon NEGF method, distinct behaviors of phonon transport compared with those of electrons, intrinsic anisotropy of phonon transport, radial strain within elastic regime as quantum perturbation, two kinds of interfacial transport behaviors, defect-induced localization of local phonon density of states, unobservable phonon localization, etc, have been discovered in some particular low-dimensional nanomaterials or nanostructures. Second, the new concept of “spin caloritronics”, which is devoted to the study of thermally induced spin-related transport in magnetic systems and offers a brand-new way to realize thermal-spin or thermoelectric energy conversion, is also introduced. After concisely discussing the spin Seebeck effect, spin-dependent Seebeck effect, and magneto-Seebeck effect, we present the linear response theory with spin degree of freedom and show that by combining with linear response theory, NEGF method is also applicable for studying spin caloritronics, especially spin thermoelectrics. Finally, recent research on quantum dot models or numerical calculation of real materials give hints to the searching for high-ZT materials. With the ever-increasing demand for energy and increasing power density in highly integrated circuits, quantum thermal transport properties are not only of fundamental interest, but also crucial for future developing electronic devices. Relevant researches also pave the way to spin thermoelectrics, which has vast potential in thermoelectric spintronic devices and energy harvesting.
Novel properties of 5d transition metal oxides
Du Yong-Ping, Liu Hui-Mei, Wan Xian-Gang
2015, 64 (18): 187201. doi: 10.7498/aps.64.187201
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The spin-orbit coupling (SOC) in the 5d transition metal element is expected to be strong due to the large atomic number and ability to modify the electronic structure drastically. On the other hand, the Coulomb interaction in 5d transition is non-negligible. Hence, the interplay of electron correlations and strong spin-orbit interactions make the 5d transition metal oxides (TMOs) specially interesting for possible novel properties. In this paper, we briefly summarize our theoretical studies on the 5d TMO. In section 2, we systematically discuss pyrochlore iridates. We find that magnetic moments at Ir sites form a non-colinear pattern with moment on a tetrahedron pointing to all-in or all-out from the center. We propose that pyrochlore iridates be Weyl Semimetal (WSM), thus providing a condensed-matter realization of Weyl fermion that obeys a two-component Dirac equation. We find that Weyl points are robust against perturbation and further reveal that WSM exhibits remarkable topological properties manifested by surface states in the form of Fermi arcs, which is impossible to realize in purely two-dimensional band structures. In section 3, based on density functional calculation, we predict that spinel osmates (AOs2O4,A=m Ca,Sr) show a large magnetoelectric coupling characteristic of axion electrodynamics. They show ferromagnetic order in a reasonable range of the on-site Coulomb correlation U and exotic electronic properties, in particular, a large magnetoelectric coupling characteristic of axion electrodynamics. Depending on U, other electronic phases including a 3D WSM and Mott insulator are also shown to occur. In section 4, we comprehensively discuss the electronic and magnetic properties of Slater insulator NaOsO3, and successfully predict the magnetic ground state configuration of this compound. Its ground state is of a G-type antiferromagnet, and it is the combined effect of U and magnetic configuration that results in the insulating behavior of NaOsO3 We also discuss the novel properties of LiOsO3, and suggest that the highly anisotropic screening and the local dipole-dipole interactions are the two most important keys to forming LiOsO3-type metallic ferroelectricity in section 5. Using density-functional calculations, we systematically study the origin of the metallic ferroelectricity in LiOsO3. We confirm that the ferroelectric transition in this compound is order-disorder-like. By doing electron screening analysis, we unambiguously demonstrate that the long-range ferroelectric order in LiOsO3 results from the incomplete screening of the dipole-dipole interaction along the nearest-neighboring Li-Li chain direction.
Higgs mode near superfluid-to-Mott-insulatortransition studied by the quantum Monte Carlo method
Chen Kun, Deng You-Jin
2015, 64 (18): 180201. doi: 10.7498/aps.64.180201
Abstract +
In additional to the phonon (massless Goldstone mode) in Galilean invariant superfluid, there is another type of mode known as the Higgs amplitude mode in superfluid with emergent Lorentz invariance. In two dimensions, due to the strong decay into phonons, whether this Higgs mode is a detectable excitation with sharp linear response has been controversial for decades. Recent progress gives a positive answer to this question. Here, we review a series of numerical studies of the linear response of a two-dimensional Lorentz invariant superfluid near the superfluid-Mott insulator quantum critical point (SF-MI QCP). Particularly, we introduce a numerical procedure to unbiasedly calculate the linear response properties of strongly correlated systems. The numerical procedure contains two crucial steps, i.e., one is to use a highly efficient quantum Monte Carlo method, the worm algorithm in the imaginary-time path-integral representation, to calculate the imaginary time correlation functions for the system in equilibrium; and then, the other is, based on the imaginary time correlation functions, to use the numerical analytical continuation method for obtaining the real-time (real-frequency) linear response function. Applying this numerical procedure to the two-dimensional Bose Hubbard model near SF-MI QCP, it is found that despite strong damping, the Higgs boson survives as a prominent resonance peak in the kinetic energy response function. Further investigations also suggest a similar but less prominent resonance peak near SF-MI QCP on the MI side, and even on the normal liquid side. Since SF-MI quantum criticality can be realized by ultracold aotms in optical lattice, the Higgs resonance peak can be directly observed in experiment. In addition, we point out that the same Higgs resonance peak exists in all quantum critical systems with the same universality, namely (2 + 1)-dimensional relativistic U(1) criticality, as SF-MI QCP.
Atomic and electronic structures of silicene and germanene on GaAs(111)
Zhang Xian, Guo Zhi-Xin, Cao Jue-Xian, Xiao Si-Guo, Ding Jian-Wen
2015, 64 (18): 186101. doi: 10.7498/aps.64.186101
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By using first-principles method in the density-functional theory, we clarify the atomic and electronic structures of silicene and germanene on 1×1 GaAs(111). We find stable structures for silicene and germanene on both the As-terminated and Ga-terminated GaAs surfaces. The structures of silicene and germanene are similar to those of the free-standing ones, which present a honeycomb-hexagonal geometry. The cohesive energies of silicene and germanene on both As and Ga sides of GaAs surfaces are comparable to those of their bulk structures and/or those on Ag(111) substrates which have been widely observed in experiment, showing the possibility of synthesizing them on both sides of GaAs surfaces in experiment. The corresponding binding energies are in a range of 0.56-1.37 eV per Si (Ge) atom, 10 times larger than the usual van der Waals interaction, showing the covalent interaction between silicene (germanene) and GaAs surfaces. The band structure calculations show that such a covalent interaction induces the absence of Dirac electrons for silicene and germanene on GaAs surfaces. We then explore the method of recovering the Dirac electrons by using hydrogen (H) intercalation. It is found that the intercalated H atoms are chemically bonded to GaAs surface, and the silicene (germanene) shifts upward distance from GaAs surface increasing from 2.50-2.58 Å to 3.49-3.86 Å, where a covalent van-der-Waals interaction transition happens between silicene (germanene) and GaAs surface. Moreover, the distances between silicene (germanene) and H atoms are 30% and 8% larger than the atomic-radius sum of Si (Ge) and H on As-terminated and Ga-terminated GaAs surfaces, respectively. This shows that the interaction between silicene (germanene) and H on the As-terminated GaAs surface is obviously weaker than the typical covalent interaction, while on the Ga-terminated GaAs surface, it is comparable to the typical covalent interaction. This difference is induced by the difference in electronegativity between As and Ga atoms. We further find that the H intercalation recovers the Dirac electrons well on the As-terminated GaAs(111) due to the weaker Si (Ge)-H interaction, while it does not on the Ga-terminated GaAs(111) due to the stronger Si (Ge)-H interaction. The results are confirmed by performing calculations for silicene (germanene) on larger GaAs(111) surfaces, i.e., the 3×3 GaAs surface. Our study provides the theoretical basis for the preparation and application of silicene and germanene on semiconductor surfaces.
Introduction to first-principles simulation package ABACUS based on systematically improvable atomic orbitals
Liu Xiao-Hui, Chen Mo-Han, Li Peng-Fei, Shen Yu, Ren Xin-Guo, Guo Guang-Can, He Li-Xin
2015, 64 (18): 187104. doi: 10.7498/aps.64.187104
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With the rapid development of supercomputers and the advances of numerical algorithms, nowadays it is possible to study the electronic, structural and dynamical properties of complicated physical systems containing thousands of atoms using density functional theory (DFT). The numerical atomic orbitals are ideal basis sets for large-scale DFT calculations in terms of their small base size and localized characteristic, and can be mostly easily combined with linear scaling methods. Here we introduce a first-principles simulation package “Atomic-orbital Based Ab-initio Computation at UStc (ABACUS)”, developed at the Key Laboratory of Quantum Information, University of Science and Technology of China. This package provides a useful tool to study the electronic, structural and molecular dynamic properties of systems containing up to 1000 atoms. In this paper, we introduce briefly the main algorithms used in the package, including construction of the atomic orbital bases, construction of the Kohn-Sham Hamiltonian in the atomic basis sets, and some details of solving Kohn-Sham equations, including charge mixing, charge extrapolation, smearing etc. We then give some examples calculated using ABACUS: 1) the energy orders of B20 clusters; 2) the structure of bulk Ti with vacancies; 3) the density of states of a model protein; 4) the structure of a piece of DNA containing 12 base pairs, 788 atoms. All results show that the results obtained by ABACUS are in good agreement with either experimental results or results calculated using plane wave basis.
Phase separation induced by density-spin interaction in one-dimensional extended t-J model
Mao Bin-Bin, Cheng Chen, Chen Fu-Zhou, Luo Hong-Gang
2015, 64 (18): 187105. doi: 10.7498/aps.64.187105
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t-J model is one of important theoretical models in the study of high temperature superconductivity. Recent cold molecule experiments indicate that t-J model can be simulated by ultracold polar molecules. In the simulated t-J model, besides long-range dipolar interaction, density-spin interaction has also been introduced. In the present we study the effect of density-spin interaction in the one-dimensional extended t-J model by using the density matrix renormalization group method. We choose three sets of representative parameters, which correspond to three different phases in the ground state phase diagram of t-J model, to calculate the charge and spin density distribution in real space and the structure factor of density-density and spin-spin correlation functions. The results indicate that the nature of the system will not change if the intensity of the density-spin interaction is small, however if the intensity is large enough, the system enters the phase separation, in which the character is quiet different form that of the phase separation in the traditional t-J model.
Measurement of ion temperature by ion-acoustic waves Landau damping in oxide cathode plasma
Hu Guang-Hai, Jin Xiao-Li, Zhang Qiao-Feng, Xie Jin-Lin, Liu Wan-Dong
2015, 64 (18): 189401. doi: 10.7498/aps.64.189401
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Ion temperature is one of the fundamental plasma parameters, which is important for studying the plasma behavior and instabilities. The measurement of ion temperature is very difficult especially in a low temperature plasma. The traditional passive and active (laser induced fluorescence) spectral diagnostics are complex and expensive because of the low value of the ion temperature, while the resolution of the retarding energy analyzer is not fine enough to measure the small T_i. Here we utilize the method of ion acoustic wave Landau damping to measure the ion temperature in the linear magnetized plasma device, where the 2 meter long plasma column with 12 cm in diameter is produced by an indirectly heated oxide cathode plasma source. The device provides a wide range of plasma parameters for many fundamental issues of plasma research. The typical plasma density is 2×1017 m-3 and neutral argon pressure is 0.02 Pa. Discharge pulse length is 5.8 ms with a plateau period of 4.8 ms. Ion acoustic waves (IAWs) are excited via biased plane stainless mesh grid with a high transparency of 80%. The grid with 10 cm in diameter is located in the center of the device (1.5 m away from the plasma source), while its normal axis is parallel to the magnetic field lines. Ion acoustic waves are excited during the discharge pulse via the sine signals applied to the grid. The biasing peak-peak voltage is 12 V with frequencies of 800 kHz and 1 MHz. IAW is also excited with biasing voltage 24 V and frequency 800 kHz, while the experimental results exclude the existence of the ion burst mode. A movable Langmuir probe controlled by a step motor is used to measure the spatial evolution of the IAW along the magnetic field. Thus the damping length and the phase velocity of the IAW propagating in the magnetic field are measured under different conditions. The measured phase velocity is around 3200 m/s in plasma coordinate. The electron temperature is measured to be 2.9 eV resulting from the V-I curve of single probe. Based on the measured damping length, the ion temperature is measured to be 0.3 eV, which is very consistent with the results measured by spectral diagnostics on other similar linear machines.