Vol. 65, No. 11 (2016)
2016-06-05
GENERAL
2016, 65 (11): 110501.
doi: 10.7498/aps.65.110501
Abstract +
The tracking control of chaotic system has been one of the research focus areas of nonlinear control in recent years, in which the vital problem is to enable chaotic system to stabilize to an equilibrium point or to track a deterministic trajectory quickly. The conventional chaos control methods make the control power unnecessarily large and generate the phenomenon of chattering easily, resulting in the instabilities of the systems.The problems above can be transformed into the solutions of differential algebraic equations effectively. Considering that the group preserving scheme not only approximates the original system, but also preserve as much as possible the geometric structure and invariants of the original system, this paper takes advantage of the group preserving method to study the control method in chaotic system from two different perspectives.A new group preserving scheme based on the fast descending control method is presented, which enables chaotic system to stabilize to an equilibrium point quickly. Firstly, we introduce a novel approach to replace the optimal control problem of nonlinear system by directly specifying a time-decaying Lagrangian function, which helps us to transform the optimal control problem into a system of differential algebraic equations. Then we derive a modified group preserving scheme for the system.Similarly, we propose a new group preserving scheme based on the sliding mode control method for chaotic system to track a deterministic trajectory quickly. Owing to numerical discretization errors, signal noises and structural uncertainties in dynamical systems, the conventional sliding mode control method cannot guarantee to maintain the trajectories on the sliding surface, unless the numerical integration method is designed to do so. On the other hand, the conventional sliding mode control method easily induces high frequency chattering of the control force. Therefore, we modify the conventional sliding mode control method and use the modified group preserving scheme to find the control force.The above two methods are the combination of traditional control method and the Lie-group method. An invariant manifold is properly designed, and the original system is transformed into the differential algebraic system, in which the modified group preserving scheme can be used to find the control force. The resulting controlled system is stable.Finally, the proposed methods are applied to the classic Lorenz system and Duffing system correspondingly. Numerical experimental results show that the new approaches are very accurate and stable. Since the two controlled methods are fast in convergence and chattering-free, each of them has a good application prospect in the tracking control of chaotic systems.
2016, 65 (11): 110502.
doi: 10.7498/aps.65.110502
Abstract +
Considering the periodically driving linear potential, we study the interactions between two solitons in Bose-Einstein condensate (BEC). By using Darboux transformation, the double bright soliton solution of Gross-Pitaevskii equation is obtained. Then we numerically calculate the properties of interaction between the two bright solitons in BEC, and obtain a critical value of the S-wave scattering length (SL). It is shown that, when the SL is more than the critical value, the attractive interaction and the atom transfer between two bright solitons can be observed. While the SL is less than the critical value, two bright solitons can keep stable and localized. Furthermore, the stable periodic oscillations of two solitons can be observed by slowly changing the potential. These results will be conducive to the BEC soliton experiments.
2016, 65 (11): 110503.
doi: 10.7498/aps.65.110503
Abstract +
Photoionized plasmas widely exist nearby strong radiative sources in the universe. With the development of the high energy density facilities, photoionized plasmas related to astrophysical objects are generated in laboratories accordingly. RCF (radiative collisional code based on the flexible atomic code) is a theoretical model applied to steady-state photoionized plasmas. Its rate equation includes five groups of mutually inverse atomic processes, which are spontaneous decay and photoexcitation, electron impact excitation and deexcitation, photoionization and radiative recombination, electron impact ionization and three body recombination, autoionization and dielectronic capture. All of the atomic data are calculated by FAC (the flexible atomic code), and with four input parameters, RCF can calculate the charge distribution and emission spectrum of the plasma. RCF has well simulated the charge state distribution of a photoionizing Fe experiment on Z-facility and the measured spectrum of photoionizing Si experiment on GEKKO-XII laser facility. According to the simulation results, the importance of photoexcitation and electron impact excitation processes in the two photoionization experiments is discussed. In the photoionizing Fe experiment condition, high energy photons not only ionize the ions by photoionization directly, but also excite the ions to autoionizing levels, ionizing the ions indirectly. What is more, far from ionizing the ions, electrons even suppress the ionization of the plasma by exciting the ions to levels with small ionization cross sections. In the photoionizing Si experiment condition, because of high photoexcitation rate, strong resonance line of He-like ion and some Li-like ion lines, which have similar spontaneous decay rates as the resonance line, are emitted. Although the intercombination line of He-like ion has lower spontaneous decay rate than the resonance lines, strong recombination makes them have comparable strengthes. Electron impact excitation can influence the line ratio of He-like ion lines by affecting the distribution of 1s2l (l=s,p) levels.
2016, 65 (11): 110601.
doi: 10.7498/aps.65.110601
Abstract +
The calculable capacitor is a classical and fundamental experimental apparatus in precision electromagnetic measurements. It is the alternating current (AC) impedance primary standard, and an important tool in measuring the fine structure constant. The calculable capacitor provides a way to directly link the capacitance unit to the mechanical unit of length. In the calculable capacitor, the displacement measurement of the guard electrode is an essential part, because the average value of the cross capacitances is directly proportional to the linear displacement of the moving guard electrode. In order to measure the displacement with a high accuracy of 10-9 or lower, a Fabry-Perot interferometer, whose cavity length is traceable to a stabilized laser by the phase sensitive detection technique, is employed. Considering that the Fabry-Perot interferometer is irradiated by the Gaussian laser beam, the effect of the phase shift of the Gaussian field, relative to the plane wave, should be carefully considered in the displacement measurement. The amplitude of the Gaussian laser beam disperses out of the region where it can be assumed to be plane-wave propagation, so its wavefronts bend and their spacing is different from that of the plane wave. As a result, the corresponding distance of an interference fringe from the coherent Gaussian laser beams is not strictly equal to /2, and it means that the displacement correction based on the phase shift of the Gaussian laser beam in the Fabry-Perot interferometer is inevitable. Therefore, the measured result should add or subtract the correction value to obtain the actual displacement of the interferometer. In order to determine the Gouy phase correction, an interferometer model based on the calculable capacitor is studied analytically and numerically. Using the free space propagation and lens transformation of the Gaussian beam field, the complex amplitude of the partial beam transmitted through the interferometer is obtained, and its phase versus the longitude propagation distance is analyzed. The amplitude and phase of the total transmitted beam, which is the coherent superposition of all the partial beams, are presented. Since the Fabry-Perot interferometer in the calculable capacitor is actively locked to a stabilized laser at two different cavity lengths, the phase of the transmitted beam at each cavity length is calculated individually. The phase difference between the two transmitted beams versus the longitude propagation distance is also analyzed numerically. The simulation result demonstrates that the minimum value of the displacement correction can be obtained by actively detecting the laser light at a distance of 560 mm from output mirror, when the Fabry-Perot interferometer moves from the cavity length of 111.3 mm to 316.3 mm, and it means that a displacement correction value of 0.7 nm, with a relative value of |L|/|L| = 3.410-9, should be added to the measured displacement of the guard electrode.
2016, 65 (11): 110701.
doi: 10.7498/aps.65.110701
Abstract +
The terahertz (THz) radiation becomes an attractive light source utilized in molecular dynamic spectroscopy, remote sensing, medicine, communication and fundamental research. The controlling of the THz spectrum is necessary for the applications. In this paper, a method is proposed for controlling the terahertz spectra generated from the laser induced plasma by two-color pluses based on the contribution of plasma oscillation. The plasma current oscillation can shift the THz spectrum when the length of medium is less than plasma skin depth. Experimentally we use a short length of molecules by means of the molecular beam method. We investigate the changing spectrum of broadband ultrashort terahertz THz generated from a jet of nitrogen (N2) molecules pumped with the two-color laser pulses following the varying plasma density and plasma length. With the increase of plasma density and the decrease of the plasma length, we observe the increase of THz central frequency (0.8-1.4 THz) and the broadening of the THz spectral width (0.78-1.53 THz). The analysis and the calculation show that the THz spectrum changes due to the frequency and the width of the plasma resonance. This scheme of controlling the THz spectrum by changing the plasma density and length is easier to implement and do not need to use complex shaped optical pulses. The discovery provides a new way of controlling the low-frequency broadband THz spectrum.
2016, 65 (11): 110504.
doi: 10.7498/aps.65.110504
Abstract +
Quantum criticality emerges when the collective fluctuations of matter undergo a continuous phase transition at zero temperature and has been a research focus in conventional condensed-matter physics over the past several decades. In the quantum critical regime, the exotic and universal properties are expected. These properties are independent of the microscopic details of the system, but depend only on a few general properties of the system, such as its dimensionality and the symmetry of the order parameter. The research of quantum criticality can not only help us to understand quantum phase transitions, but also provide a novel route to new material design and discovery.Ultracold bosonic gases have provided a clean system for studying the quantum critical phenomena. The critical behavior of a weakly interacting three-dimensional (3D) Bose gas should be identical to that of 4He at the superfluid transition, which belongs to the 3D XY universality class. From the normal fluid to the superfluid, the system undergoes a phase transition from completely disorder to long-range order, while in the vicinity of the phase transition point, the system parameters will show some singularity characteristics. In this paper, we observe the critical behavior of 87Rb Bose gas in a quadrupole-Ioffe configuration (QUIC) trap near the phase transition temperature Tc. A novel singularity behavior of the full width at half maximum of momentum distribution (FWHMMD) of atomic gas is discovered in the experiment. Prior to our experiment, we prepare a sample with 7.8105 87Rb atoms in the 5S1/2 |F=2, mF=2 state. Then the sample is held in a QUIC trap for a presetting period of time to control the temperature of atom sample precisely. During the holding time, the sample is heated up due to background gas collisions or fluctuations of the trap potential. In our experiment, the heating rate is deduced to be 0.3480.078 nK/ms from the absorption image. For a bosonic gas in a harmonic trap, critical gas can only cover a finite-size region due to a spatially varying density. We define the finite-size region as a critical region determined by the Ginzburg criterion. Then the FWHMMDs of atomic gas in the critical region are measured for different temperatures near the critical point. To this aim, we first extract the momentum distribution of atomic gas from the absorption image of the atomic clouds released from the QIUC trap after free expansion. Thus momentum distribution of atomic gas in the critical region can be extracted from the absorption image by subtracting the momentum distribution of thermal gas outside the critical region. According to the statistical results of the FWHMMD at different temperatures, we find that the FWHMMD suddenly reduces, thus revealing a very notable singularity behavior when the temperature is very close to the phase transition temperature Tc.
2016, 65 (11): 110602.
doi: 10.7498/aps.65.110602
Abstract +
The semiconductor industry is rapidly developing in the global market, and chip design companies usually purchase the third-party EDA tools in order to shorten the design cycle of IC and reduce manufacturing cost. Therefore, in the IC chip production procedure there exist a lot of insecurity factors, and the hardware security of IC chips becomes the most important issue of the national security defense. Physical hardware trojan will modify the value of register, leak sensitive data and cause device degradation failure. Furthermore, the physical hardware trojans only modify the physical properties of the circuit chip rather than injects the malicious functional circuit. They are hidden more deeply than logical hardware trojans. Therefore, it is far-reaching significance issues for the hardware trojan detection methods and national security to study logic circuit transmission characteristics and the chip degradation failure physical mechanism which are caused by injection physical hardware trojans. In this paper, a metal-oxide-semiconductor field-effect transistor (MOSFET) device with injection dopant hardware trojan is realized by using ATHENA process simulation system to achieve the ion implantation process. The ATLAS simulation devices are tested using hot carrier injection degradation (hot carrier degradation is denoted by HCD) stress model for the degradation failure process which is caused by injecting the hot carrier injection hardware trojan (HCHT) into the MOSFET device. Another normal MOSFET combines with dopant hardware trojan MOSFET or hot carrier injection hardware trojan MOSFET to comprise the same inverter logic circuit by using the ATLAS two-dimensional (2D) device simulation system with SmartSpice instructions mode. The effect on logic circuit output characteristics caused by physical hardware trojan is studied by using Spice simulation to output the DC and AC transient time characteristics. It is also studied how the W/L value of a hardware trojan transistor influences the output characteristics of the logic circuit. We design an experiment to study transient characteristics of the same inverter logic module which consists of different W/L values of a transistor at different temperatures. The experiment is realized by Spice circuit simulation. In this paper, the effects of the variations of the HCD stress intensity and temperature on output characteristic are analyzed for hot carrier injection hardware trojan. The results indicate that the negative effect of hardware trojan on logic circuit DC current output characteristic is more obvious than AC transient time characteristic. Thus, we propose an effective method and a convenient procedure to detect the injection physical hardware trojan in packaged chips. Furthermore, the test process is a feasible operation method of detecting physical hardware trojan.
2016, 65 (11): 110301.
doi: 10.7498/aps.65.110301
Abstract +
In this paper, we first make a brief review of the general method of solving master equation of density operator, which includes the C-number method method and the super-operator method. The C-number can transform quantum master equation into Fokker-Plank equation or the differential equation of density matrix elements, and this method has a wide applicable range but the Fokker-Plank equation and differential equation are difficult to solve. Besides, the solution is not always applicable for any initial condition. The super-operator method can solve master equation efficiently compared with C-number method, however the solving process of super-operator method mostly depends on the characteristics of Lie algebra. For instance, if the corresponding Lindblad operator can be divided into the generators of Su(2) or Su(1,1) Lie group, the super-operator is no longer applicable. Thus although super-operator is more efficiently than C-number method, it has a narrow applicable range. Furthermore, other researchers have made much effort to develop super-operator method, for instance, S.J. Wang proposed the left and right action operator, the left operator is the same as the general operator, while the right action operator from the right side acts on other general operator, thus the explicit formation of super-operator can be given by this method. Fan proposed the thermal entangled state representation which can convert operator between real mode and fictitious mode. All these developments depend on Lie algebra, thus they all have a narrow applicable range just like super-operator method.
We introduce a new Ket-Bra entangled state (KBES) method in this paper, which can transform master equation into Schrodinger-like equation with the corresponding Ket-Bra entangled state. Then one can use the method of Schrodinger equation such as time evolution method, perturbation method, etc. to solve the master equation. Compared with C-number method and super-operator method, the KBES method has several merits. 1) A wide applicable range, KBES method is applicable for any master equation of finite-level system in theory. 2) Compatibility with computer programming, the most crucial procedure is to calculate the exponent of Lindblad operator eFt which needs the diagonalization of F, and all this can be finished by computer. 3) Most mature methods of Schrodinger equation can be used to solve master equation because of the KBES method can transform master equation into Schrodinger-like equation. Then we study the model which two-level qubit is coupled with reservoir under time-varying external field, the corresponding master equation is deduced and solved by KBES method. Furthermore, we analyze the decoherence evolution of density operator and we consider the entanglement evolutions of two uncoupled qubit cases. We find that the external field seriously influences the decoherence process. The off-diagonal elements 10(t) become damply oscillated when the external field exists, and the frequency of oscillate keeps growing along with . Besides, the dynamic evolution of concurrence is also influenced by the external field, which leads to the occurrence of both entanglement sudden death and entanglement sudden birth, while the last ESB phenomenon only happens under the external field. Thus, we thought that one can suppress the decoherence and disentanglement process by exerting suitable time-varying external field on the open system, of course, the suitable external field can also be obtained by our KBES method in theory.
ATOMIC AND MOLECULAR PHYSICS
2016, 65 (11): 113101.
doi: 10.7498/aps.65.113101
Abstract +
The high-temperature piezoelectric crystal Ga3PO7is a versatile functional material widely used in many electromechanical devices. As the Curie temperature of this crystal is as high as 1346 ℃, it can break through the current temperature limitations(1200 ℃) and then be used in extremely high-temperature condition. However, it is very difficult to explore its properties in such a high-temperature environment. Moreover, the relevant theoretical research has not been reported to date. Aiming at this problem, the density function theory combined with quasi harmonic approximation theory is used to investigate the structural, thermal and surface acoustic wave (SAW) properties of Ga3PO7. Firstly, the Gibbs energies of Ga3PO7 crystal with different stains are calculated, and the equilibrium structures of Ga3PO7 crystal at different temperatures (from 0 ℃ to 1200 ℃) are found according to minimal energy principle. Secondly, based on the result above, we optimize Ga3PO7 crystal at different temperatures, and then, the thermal and elastic properties of Ga3PO7 crystal within 0-1200 ℃ are calculated using CASTEP package based on the density functional theory in the generalized gradient approximation. The results show that its lattice constants increase almost linearly as temperature increases while its density decreases. Owing to anisotropy, its lattice constant along the c axis increases much more greatly than along the a axis. The coefficients of thermal expansion along the a and c axis are evaluated to be 1.6710-6 K-1 and 3.5810-6 K-1, respectively, and the volumetric heat capacity is evaluated to be 2.067 J/gK. These values all agree well with the experimental values. Finally, the elastic constants, bulk modulus and SAW properties of Ga3PO7 crystal at different temperatures (from 0 ℃ to 1200 ℃) are calculated. The results show that the bulk modulus can reach 175 GPa, and it changes very little as temperature increases. The fluctuation of elastic constants has slight influences on SAW velocity and the electric-mechanical coupling factor. When the propagation angle is 151, it possesses the stablest SAW properties and the largest electric-mechanical coupling factor which can reach 0.7%. The comprehensive analyses of the thermal, mechanical and SAW properties show that Y-cut Ga3PO7 possesses a greater potential application in high temperature environment.
2016, 65 (11): 113201.
doi: 10.7498/aps.65.113201
Abstract +
Ion acceleration is of interest for applications in fast ignition, compact particle sources, medical science, and others. The formation of plasma is of fundamental importance for understanding ion acceleration driven by intense laser. In order to further understand the solid dense material ionization dynamics under ultra-strong field, we use two-dimensional particle-in-cell code to study the ionization process of ultra-thin carbon film, driven by ultra-short intense laser pulse, particularly to see the plasma generation and distribution during the interaction. When an ultra-intense short pulse laser irradiates a solid dense nm-thick film target, the collisional ionization can be ignored for such a thin film target. If the target thickness is larger than laser pulse skin depth, the formation of plasma is contributed from laser field direct ionization and the ionization of electrostatic field inside the target, both of which are discussed and compared by the simulation results in this work. The ionization directly stimulated by laser field happens only near the laser-target interaction surface. After the generation of plasma on the target surface, electrons are accelerated into the target because of laser ponderomotive force. A huge electrostatic field is formed inside the target as a result of hot electron transport in it, and ionizes the target far from the interaction surface. It is found that a bigger fraction of ionization is contributed from electrostatic field ionization inside the target. The effect of laser pulse intensity on ionization is studied in detail, in which the laser pulse intensity is changed from 11018 W/cm2 to 11020 W/cm2. Comparing the results obtained under different intensities, we can see that higher intensity results in higher ionization speed, and much higher-order ions can be generated. At an intensity of 11020 W/cm2, although the intensity much higher than the threshold can generate C+6, only a small part of ions can be ionized into C+6. The reason is that the C+6 ions can be generated directly only by laser field, and the total number of C+6 ions is determined by laser pulse skin depth and spot size. We also consider the effect of laser pulse duration from 30 fs to 120 fs at an intensity of 11020 W/cm2. It is found that higher ionization speed can be obtained, while much less higher-order ions can be generated under shorter laser pulse duration. This description of the generation of solid density plasma driven by intense laser interacting with nm-thick target helps us to further understand the material characteristic under ultra-strong field. This work also benefits the numerical model of plasma in application, namely laser driven ultra-thin film ion acceleration.
2016, 65 (11): 113301.
doi: 10.7498/aps.65.113301
Abstract +
The ozone layer which absorbs harmful solar UV radiation is an essential umbrella for human beings. However, a large number of exhausts of chlorine compounds including freon released by people in the atmosphere pose a great threat to the ozone layer. Freon dissociates into the product of chlorine radicals induced by UV sunlight radiation, which are found to be the main culprit for the destruction of atmospheric ozone. In this paper, time-of-flight mass spectrometry and velocity map imaging technique are coupled for investigating the multiphoton dissociation dynamics of Freon 1110 (C2Cl4, Tetrachloroethylene) induced by ultrafast short laser pulse on a femtosecond time scale at 800 nm. Fragments mass spectra of C2Cl4 are measured by time-of-flight mass spectrometry. Together with the parent ion C2Cl4+, two dominant fragment ions C2Cl3+ and C2Cl2+ are discovered in the multi-photon ionization and dissociation process in the experiment. By analyzing the above mass spectra, two corresponding photodissociation mechanisms are discussed and listed as follows: 1) C2Cl4+C2Cl3+ +Cl with single CCl bond breaking and direct production of Cl radical; 2) C2Cl4+C2Cl2+ +2Cl with double CCl bonds breaking and production of two Cl radicals. Also, ion images of these two observed fragment ions C2Cl3+ and C2Cl2+ are measured by velocity map imaging apparatus. The kinetic energy distributions of these two fragment ions are determined from the measured velocity map images. The kinetic energy distributions of both C2Cl3+ and C2Cl2+ can be well fitted by two Gaussion distributions. It indicates that both fragments C2Cl3+ and C2Cl2+ are from two production channels. The peak energies for each channel are fitted. More detailed photodissociation dynamics is obtained by analyzing the angular distribution of the generated fragment ions. The anisotropy parameter values are measured to be 0.46 (low energy channel) and 0.52 (high energy channel) for the fragment C2Cl3+, and 0.41 (low energy channel) and 0.66 (high energy channel) for the fragment C2Cl2+, respectively. The ratios between parallel transition and perpendicular transition are determined for all the observed channels for producing fragments C2Cl3+ and C2Cl2+. In addition, density functional theory calculations at a high-precision level are also performed on photodissociation dynamics for further analysis and discussion. The optimized geometries of ground state and ionic state of C2Cl4 are obtained and compared with density functional theory calculation at the level of B3LYP/6-311G++(d,p). The different structures of the ground and ionic states are given and discussed. The calculated information about ionic states of C2Cl4, including energy level and oscillator strength for the ionic excited states, is also given for analyzing the photodissociation dynamics of the C2Cl4 ions.
2016, 65 (11): 113401.
doi: 10.7498/aps.65.113401
Abstract +
Bremsstrahlung emission produced by electron impact on thick or thin targets is one of the fundamental radiation processes, and the interest in its study continues to grow because of its importance for understanding the interaction of electrons with matter and also for many practical applications. Nowadays, there has been some disagreement concerning whether or not the polarization bremsstrahlung, which is emitted by the atomic electrons in a target polarized by the incident charged particles, contributes to the total bremsstrahlung when the incident electrons bombard a solid target. Some reports suggested that the polarization bremsstrahlung does not significantly contribute to the total bremsstralung in experiments involving solid targets. However, some recent experimental data indicated that a significant amount of polarization bremsstrahlung contributes to the total bremsstrahlung when electrons from -decays of radioactive nuclei bombard solid targets. In other papers, the comparison between the bremsstrahlung spectra produced by electron impact on different thick solid targets from low-Z to high-Z elements and the simulation spectra of Monte Carlo code PENELOPE showed that there are certain discrepancies between the experimental and simulation results, and on the whole the factors required for the experimental results and simulation spectra to match with each other seem to increase slightly with the target atomic number increasing and for high-Z elements experimental results are about 10% higher than simulation results. PENELOPE is a general-purpose Monte Carlo code that simulates coupled electron-photon transportation, in which simulation for bremsstrahlung is only based on ordinary bremsstrahlung and any contribution from polarization bremsstrahlung is not included Therefore, whether the discrepancies between the experimental and simulation spectra are caused by the polarization bremsstrahlung or by other reasons remains to be further studied. In this paper, we improve the Faraday cup to measure the incident electron charges more accurately Meanwhile, a highpurity Al film of 7.05 m thickness is placed in front of the ultra-thin window of the X-ray silicon drifted detector (SDD) to prevent the backscattered electrons that escape from the side hole of the Faraday cup entering into the SDD detector. The Al film thickness is measured by the method of Rutherford backscattering. In addition, we adopt a data processing method which is different from previous one, to take into account the interaction between backscattered electrons and the window of the SDD detector. New measurements of bremsstrahlung spectra generated by 10-25 keV electron impact, respectively, on thick targets of tungsten and gold are reported in this paper. The experimental data are compared with the simulation results of X-ray spectra obtained from the PENELOPE code, and they are in very good agreement except for the lower energy region ( 3 keV) where the experimental spectra are slightly lower than the simulation spectra. The reason for the small discrepancy for the lower energy region ( 3 keV) is also discussed. The results presented in this paper indicate that the X-ray spectra, which are produced by electron impact on solid targets, do not include obvious contribution of polarization bremsstrahlung, and the PENELOPE code can reliably describe the bremsstrahlung produced by electron impact on solid thick targets.
2016, 65 (11): 114101.
doi: 10.7498/aps.65.114101
Abstract +
In order to generate a submicron localized hollow laser beam and realize the more efficient laser cooling and trapping of a single atom, a simple and promising scheme with using the system of a single mode fiber a circle binary phase plate and a microlens is proposed in this paper. From Rayleigh-Sommerfeld diffraction theory, the intensity distribution of the generated localized hollow laser beam near the focal plane and its propagating properties in free space are calculated. Also, the dependences of the dark-spot size of the localized hollow beam on the mode radius of single mode fiber and the focal length of the mocrolens are studied. The calculated results show that the intensity distribution of the localized hollow beam presents approximately symmstrical distribution near the focal plane. In the center of the focal plane, the light intensity is 0 and increases gradually around it. So a closed spherical light field (i.e., localized hollow laser beam) with a radius of 0.4 m is generated. The calculated results also show that the dark-spot size of the localized hollow laser beam decreases with the increasing of the microlens focal length and the decreasing of the single mode fiber mode radius. So proper parameters of this optical system can be chosen to generate localized hollow laser beams with different sizes for various applications. When the localized hollow laser beam is blue detuned, atoms will be trapped in the minimum light filed. If a repumping laser beam is applied, the trapped atoms will be also cooled by the intensity-gradient Sisyphus cooling. In this paper, we build a device for trapping and cooling a single atom by using the generated blue detuned submicron localized hollow laser beam. We study the dynamical process of intensity-gradient cooling of a single 87Rb atom trapped in the localized hollow beam by Monte-Carlo method. Our study shows that a single 87Rb atom with a temperature of 120 K (the corresponding momentum is 30ħk) from a magneto-optical trap (MOT) can be directly cooled to a final tempreture of ～ 5.8 K (the corresponding momentum is ～ 6.6ħk). So an ultracold single atom is generated and trapped in our submicro localized hollow beam. This device for obtaining ultralcold single atom can be widely uesd in the regions of the optical physics, the atom and molecule optics, such as the detecting of the fundamental physical parameters, realizing the quantum computer, studying the cold collision of singe atoms, and realizing the single atom laser.
ELECTROMAGNETISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
2016, 65 (11): 114203.
doi: 10.7498/aps.65.114203
Abstract +
Wavefront measurement is widely used in the field of optical manufacturing, military, astronomy, medical treatment, etc., and it reflects the performance of the optical system through evaluating aberrations. Relevant studies have been carried out by many researchers. Among them, point-diffraction interferometer and spatial phase-shifting interferometer are two significant instruments for the wavefront measurement. Point-diffraction interferometer is a simple self-referencing configuration with high precision, and spatial phase-shifting interferometer can be used in the vibration environment or for measuring the dynamic object. Owing to these advantages, they have been widely used in the field of wavefront measurement. In this paper, to realize the combination of these two techniques, we propose a new method of fabricating a polarization point-diffraction plate. Through laser drilling technology, we fabricate a pinhole at a micron level on a wire grid polarizer with a period and depth of 140 nm and 100 nm respectively, and fabricate a polarization point-diffraction plate. We analyze the principle of laser drilling, the orthogonally polarized reference beam and test beam generation mechanism of the polarization point-diffraction plate. The principle of spatial phase-shifting interferometer is deduced by adopting Stokes vector and mueller matrix. Combining with the spatial phase-shifting system with beam splitter, a spatial phase-shifting polarization point-diffraction interferometer is built. In the experimental apparatus, the diameter of the pinhole on the polarization point-diffraction plate is 10.2 m, the beam splitter is a chessboard phase grating whose period, duty cycle, and etched depth are 34 m, 0.5, and 577 nm respectively, and the phase-shifting component is a 22 wave-plate array which is glued together with a 1/4 wave-plate, a 1/2 wave-plate, a 3/4 wave-plate and a full wave-plate; the four fast-axes of the wave-plates are all along the horizontal direction. The spatial phase-shifting polarization point-diffraction interferometer is used to measure the transmitted wavefront through a collimating lens with a focal length of 550 mm and F#10 which are used on a collimator. The measured peak-to-valley value, root-mean-square value and Zernike fitting coefficients are in good agreement with those obtained by SID4 wavefront sensor made by Phasics corporation in France, which verifies the reliability of the measuring results obtained by spatial phase-shifting polarization point-diffraction interferometer. The spatial phase-shifting polarization point-diffraction interferometer introduces spatial phase-shifting technology into traditional point-diffraction interferometer, thereby achieving real-time wavefront measurement with high resolution and precision, and also improving the immunity to vibration, air turbulence, etc.
2016, 65 (11): 114206.
doi: 10.7498/aps.65.114206
Abstract +
Owing to space optical system working for a long time outside the field of view, where there is strong background radiation, and the fact that the point source transmittance (PST) is an important evaluation indicator for stray light suppression of this optical system, the development of stray light equipment for testing PST has received more and more attention. Though the development of PST testing system has been extensively studied, none of them elaborate on the calibration of the PST testing system. Besides, most of the PST testing systems are at a laboratory research stage, and the calibration of neither testing stability nor accuracy is recognized. Therefore, on the basis of the PST testing system established, one calibration lens is designed to calibrate the PST testing system. By comparing the measured PST values of the calibration lens with the analyzed values, the PST testing system can be evaluated. The calibration lens model is built to analyze PST values at different off-axis angles by using the ray tracing software Tracepro. We consider the accuracy of modeling, and on the basis of simplifying the structure design, we measure bidirectional reflectance distribution function (BRDF) values of the painted surface of the calibration lens, and then estimate values of lens surface from Harvey-Shack model and PSD theory by taking these property data into the model of simulation. Ultimately PST analyzed values of calibration lens can be obtained. Finally, by comparing the measured values of calibration lens, which are tested by using PST testing system, with the analyzed values, the calibration of the PST testing system is completed. In the PST testing process of calibration lens, by analyzing the data at different off-axis angles, the accuracy of repeated measurements and threshold of PST testing system can be obtained. At the same time, testing errors caused by the stability of light source, detector linearity, air scattering and structure of double cylindrical chamber are analyzed through the testing data. The data show that when double cylindrical chamber clean class is ISO7, the PST threshold of this equipment is 10-8, and the accuracy of repeated measurements is 7.9%. Taking into account the detection capability, the PST threshold of this equipment is 10-10 when environmental condition is better than ISO6.
2016, 65 (11): 114201.
doi: 10.7498/aps.65.114201
Abstract +
The Fourier telescopy is a kind of active illumination imaging with high resolution by using multi-interfering fringes generated by the multi-beams from the large transmitter arrays. According to the imaging principle, the beams from one laser source are split and each beam is applied with a different tiny frequency shift so that the interfering fringes may moving across the target. The configuration of the beams changes so that they would generate fringes in different spatial frequencies and different directions. Recently, most of researches focused on the factors such as the baseline scale and data sampling efficiency that may affect the imaging quality. However, there are other two factors, i.e., the configuration of the transmitter and its redundancy, which need studying. In Fourier telescopy, if the direction and spatial frequency of the fringe patterns that are generated by the change of different baseline configurations match each other, the target surface information would be a crucial factor that affects the image quality.In the first part of this article, the practicability of zero redundancy of baseline is analyzed. The results show that the baseline cannot have zero redundancy due to the iteration algorithm. Then the minimum redundancy is analyzed and the minimum redundancy line is proposed. By using the Strehl ratio as the merit of the imaging quality, the concept of redundancy-strehl ratio-target texture distribution (RST) and calculation method are proposed. This method integrates the transmitter redundancy, target detail information and image quality together. The distribution of RST value on the frequency plane is compared with the minimum redundancy line. If the RST point is located on the horizontal side compared with the line, the target detail information on this baseline is mainly in the horizontal direction. On the other hand, if the RST point is located on the longitude side, the target information is mainly in the longitude direction. Therefore this new proposed method reveals the relationship between target spatial information and the baseline configuration. In this article T-shaped transmitter array is adopted, and the Fourier components are mainly distributed on the rectangle plane. According to this relationship and calculated RST value, the working transmitter may continuously rectify its scale and shifting patterns so that the spatial frequencies and directions of fringes may match the target Fourier components in time. In this article, three simulated images and two real images are tested by the proposed method, and the results show that the RST values and the distributions well reveale the relationship between the detailed information and the baseline configurations.Now the Fourier telescopy follows the procedure from laboratory setup to the real system research. Considering the convenience and cost of project realization, this method is helpful for analyzing the real system of the transmitter configuration and enhancing working efficiency.
2016, 65 (11): 114202.
doi: 10.7498/aps.65.114202
Abstract +
Localized surface plasmon resonance of cylindrical magneto optical particles provides an important mechanism for the formation of chiral edge states in two-dimensional magneto-optical photonic crystals. These states are an electromagnetic analogy of the so-called chiral edge state's (CESs) in a quantum Hall system where the power transmission is unidirectional due to particular topological properties of the bands. Just like their electronic counterpart, the number of optical CESs in the band gap opened by an applied magnetic field is determined by the sum of the Chern numbers of the lower bands. For a two-dimensional photonic crystal composed of ferrite rods magnetized along their axis, the coupling of the localized surface plasmon resonance states of each rod results in a narrow flat band-gap, which contains one-way edge modes arising from the circulation of the energy flow around each rod excited by the resonance with broken time-reversal symmetry. So far the interpretation of the resonance-related chiral edge states are based on the long-wavelength approximation of the localized surface plasmon resonance of an individual magneto-optical particle. Though the results agree with the experimental results qualitatively, an obvious quantitative deviation is still obvious. In this work we apply the scattering theory to analyze the resonance condition and the features of both the far-field and the near-field at resonance for cylindrical magneto-optical particles. Our calculation shows that the splitting of scattering peaks of different orders will occur due to the magneto-optical effect. Such a split is observed between an (+n)-peak and an (-n) peak, as a sign of the broken time-reversal symmetry, and also between peaks of lower-order and higher-order. Another important feature is the simultaneous occurring of the far-field resonance and the near-field resonance, where the latter is characterized by a peak of energy-flow circulation around the particle. Based on this model the effects of particle size on the resonance peaks are discussed. It is shown that the resonance peaks are moved and broadened with the particle size increasing. The results explain the obvious deviation of the position of the resonance band-gap from the predicted frequency according to the previous long-wavelength approximation. Furthermore, the calculation of a particle of moderately-large size (nearly one-tenth of the incident wavelength) demonstrates the appearance of higher-order modes up to n=4 circling around the particle surface. This implies that these higher-order modes may also make non-trivial contribution to the formation of the flat band-gap observed in a photonic crystal of ferrite-rods and affect the behaviours of the chiral-edge state existing in such a gap. Particularly, it may be helpful in realizing the multimodes of chiral edge states in magneto-optical photonic crystals.
2016, 65 (11): 114204.
doi: 10.7498/aps.65.114204
Abstract +
The entangled state of continuous variables of microwave frequency is an important resource in the field of quantum. In order to apply it to quantum communication protocol and quantum radar, the entanglement between two spatially separated subsystems, namely dual-path entangled quantum microwave is needed. However, for the circuit that generates the entangled quantum microwave, there is no suitable method to indicate whether the quality of the entangled microwave signal is good or not. Aiming at this problem, we put forward a method of evaluating the quality of dual-path entangled quantum microwave signals generated based on von Neumann entropy. The origin of the entangled quantum microwave is that vacuum state signals are transformed into squeezed state signals in driven pump, so in this paper we use a two-mode squeezed vacuum state to describe the formation of dual-path entangled quantum microwave signal, thus providing the function relation between the photon number and the squeezed parameter. In a communication system, the signal-noise ratio is usually used to express the reliability of system. Entropy is a measure of disorder degree in information. If both of them can be made the analogy, the entropy is used to evaluate the proportion of entangled state signals, the quality of original signals will be evaluated and the relationship among the entropy and squeezed parameter and the photon number will be analyzed. The simulation results show that the photon number in the entangled quantum microwave signal is determined by the squeezed parameter, and there is an index change with the square rule between them. Entropy decreases with the increase of squeezed parameter: its minimum value is 0, and its maximum value can be found from 0.9 to 1. The slope of curve is steep near the maximum, which reflects that the influence of squeezed parameter on the degree of entanglement is obvious, and that the range of optimal value choices in squeezed parameter is very narrow. The optimal value of squeezed parameter is dependent on photon number; it increases with the increase of the photon number. Entropy tends to decrease smoothly with the increase of squeezed parameter and it approximately has a negative exponent relation. The photon number in an actual signal is limited, so the limit value of entropy is estimated to be about 65%. The research shows that the quality of the entangled microwave signal can be improved by choosing appropriate squeezed parameter in different circuits that generate dual-path entangled quantum microwave signals for meeting the actual needs. Therefore, the research can provide the method of choosing the parameters of dual-path entangled quantum microwave circuit and improve the availability of system.
2016, 65 (11): 114205.
doi: 10.7498/aps.65.114205
Abstract +
High-order harmonic generation (HHG) is one of the hottest topics in strong field atomic and molecular physics. In this paper, frequency domain theory which is based on formal scattering theory is extended to study the HHG of O2 molecules under a linearly polarized single mode laser field. The dependence of HHG on the angle 0 between the laser polarization direction and nuclear axis is investigated. In our calculation, we only consider the contribution of highest occupied molecular orbital (HOMO) and use the single electron approximation. The HOMO is obtained from quantum chemical software Molpro. The intensity of the laser is 5.181014 W/cm2 and the wavelength is 800 nm. On the one hand, in the case that the nuclear axis lies in the plane perpendicular to the laser propagation direction, we find that the yields of all order harmonics increase with 0 increasing until the yields reach the maximum values when 0 is equal to about 45. Then the yields decrease with 0 increasing and have the minimum values when 0 is equal to about 90. The analysis shows that the yield of HHG is dominated by the density of electrons in HOMO along the laser polarizing direction in momentum space. On the other hand, in the case that the nuclear axis lies in the plane parallel to laser propagation direction, the dependence of HHG on 0 is the same as that when the nuclear axis is in the plane perpendicular to laser propagation direction. The reasons for the same results for the two cases lie in the following fact. The HOMO of O2 molecule has g symmetry which is not rotationally symmetric around nuclear axis. So HHG yield relies on the g extension orientation. Since the g extension orientation cannot be fixed, the HHG of O2 should be averaged over the contributions to HHG at all possible g extension orientations. This average is equivalent to that the electron density is rotationally symmetric around the nuclear axis and hence leads to the fact that the HHG yield of O2 depends on 0 rather than the plane that the nuclear axis lies in.
2016, 65 (11): 114207.
doi: 10.7498/aps.65.114207
Abstract +
Permittivity depends on the electric field intensity in a nonlinear material, and it changes with the incident intensity of the electromagnetic wave. This phenomenon leads to the occurrence of optical bistability. The optical bistable threshold value depends on the localized degree of electromagnetic field in the nonlinear material, and the stronger the localized field, the lower the threshold value is. However, the loss of material is one of the important factors influencing the strength of the local field. It is commonly believed that the loss is not conducible to reducing the threshold value because increased loss can weaken the localized degree of fields. For the lossy single-negative metamaterial, the transmission is nonmonotonic as the loss varies. That is to say, the transmission first decreases and then increases in the lossy single-negative metamaterial. Therefore, the nonlinear transmission in the lossy single-negative metamaterial may lead to novel physical phenomena. Permeability-negative material and permittivity-negative material are two kinds of different single-negative metamaterials. In this paper, the optical bistable phenomena in the heterostructure of permeability-negative material and permittivity-negative material are studied by using the transfer matrix method. Here, the permittivity-negative material is nonlinear material. The results show that the optical bistable threshold value first increases and then falls down as the loss increases. The variance of the localized electromagnetic field at the interface between the permeability-negative layer and the permittivity-negative layer at the discussed frequency is discussed in the present paper to understand the nonmonotonic phenomenon. Further studies indicate that the nonmonotonic localized electromagnetic field is also presented at the interface between the permeability-negative layer and permittivity-negative layer. That is to say, the enhancement of the localized field can be obtained when the loss is increased, which results in the nonmonotonic optical bistable threshold value in the heterosturcture composed of the single-negative metamaterials. In the final analysis, the abnormal phenomenon is induced by the loss in the single-negative metamaterial, which is the special property of single-negative metamaterial.
2016, 65 (11): 114701.
doi: 10.7498/aps.65.114701
Abstract +
Owing to the fact that the increasing amount of attention has been focused on numerical weather forecast and climate change research, it is desired that the observation error of upper air temperature with using sounding temperature sensors can be reduced down to 0.1 K. However, the temperature measurement errors of bead thermistor sounding temperature sensors, induced by solar radiation, are on the order of 1 K or more, which is a few orders of magnitude larger than the errors produced by the measurement circuits and digital signal processing systems in radiosondes. Hence, the solar radiation error poses an important bottleneck for improving the measurement accuracy. To tackle this problem, a numerical analysis method is proposed in this research. By employing a computational fluid dynamics (CFD) method, the influences of various solar radiation intensity, sensor angles, and air pressures from sea level to 20 km altitude on temperature measurement accuracy are studied. In this CFD model, the boundary conditions of external convection and solar radiation of the bead thermistor are taken into consideration. The modeling results indicate that solar radiation intensity and altitude are important factors that affect the amplitude of the radiation error. With the elevation increasing from sea level, the solar heating error appears to have an exponential correlation with the altitude, which exhibits a growing slop rate. When the sensor angle is 90o, the radiation error of a bead thermistor sensor probe is minimal. The simulation results are fitted by a Levenberg-Marquardt method and a global optimization method. A correction equation of the radiation error is obtained, where the altitude of the sensor and solar radiation intensity act as two major variables in the equation. In order to verify the equation obtained in this study, an experimental platform for solar radiation error, which includes a low-pressure temperature chamber, a rotation apparatus, an LED-based radiation source, and a wireless communication system, is designed and constructed. It can be found that the solar radiation errors of the bead thermistor based on fluid dynamics numerical calculation are generally consistent with experimental data. The average offset and root mean square error between the correction equation and experimental results are 0.017 K and 0.023 K, respectively, which can demonstrate the accuracies of the computational fluid dynamics method, the Levenberg-Marquardt method and the global optimization method proposed in this research. The methods and techniques introduced in this paper may open the way for correcting the solar radiation errors of the bead thermistor sounding temperature sensors.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
2016, 65 (11): 116101.
doi: 10.7498/aps.65.116101
Abstract +
Grain boundaries (GBs) are known to have an important influence on material properties, so understanding how GBs in graphene change its physical properties is important both scientifically and technologically. In this paper, we perform a series of molecular dynamics simulations to investigate the energies, mechanical properties and fracture process of 29 graphene GBs (symmetric and nonsymmetric) under tensile strains. With different arrangements of the pentagonal and heptagonal rings, the misorientation angle () ranges from 3.5 to 27.8. The GBs defects in graphene can produce a pre-strain that will lead to an increase of the energy of GBs. We study the atomic energy distribution around GBs and define a new parameter: single defect energy (Esingle) to calculate the average energy per GBs defect. It is found that Esingle shows a clear linear relation between and defect density (), because pre-strain filed can be cancelled out locally with the increase of defect density. And this pre-stain can reduce the strength of the C-C bond contained in GBs defects. Hence, with very few exceptions, mechanical failure always starts from the defective region. Furthermore, the energy of GBs can be used to reflect the strength of GBs indirectly. The simulated results show that the tensile strength of GBs is linearly related to the highest atomic energy (Emax), and it also depends on Esingle monotonically. Owing to the pre-strain, load distribution along GBs is uneven. Because some bonds are stretched while others are compressed, that is, the real number of bearing carbon bonds is less than the nominal number. Therefore, at the beginning of tension, the Young's modulus of polycrystalline graphene is significantly lower than that of the monocrystal one. But with the increase of strain, it becomes comparable to that of the monocrystal graphene at sufficiently large strain. The results of fracture process indicate that formation and propagation of crack are both dependent on strength GBs. For low GB strength, the fracture mechanism is transgranular fracture in the form of direct fracture of C-C bonds. When stress reaches a critical value, the weakest C-C bonds in GBs will breakdown and form a fracture site. Because of the uneven bearing condition, the C-C bonds in front of the crack possess considerable residual strength and could prevent crack from propagating. As a result, many other fracture sites in the GBs defect can be produced with the increase of strain, and finally, these sites emerge gradually along GBs and form a sawtooth crack. In contrast, the fracture process of high strength GBs is always accompanied with the variations of Stone-Wales (S-W) transformation and direct fracture of C-C bonds. Once the fracture site forms, the crack will propagate rapidly predominantly along armchair or zigzag direction and finally could cross GBs, this process can be called intergranular fracture. Our present work provides fundamental guidance for understanding how defects affect the mechanical behaviour, which is important for further research and application of graphene devices.
2016, 65 (11): 116201.
doi: 10.7498/aps.65.116201
Abstract +
Sound speed is of great importance for high velocity impact phenomena because it is a fundamental parameter to deduce the shear moduli, strengths and phase transitions of materials at high pressure. It has attracted much attention because of significant challenges to experiment and simulation. In practice, with the development of laser interferometer measurement system, one can obtain velocity-time histories of windowed-surfaces or free surfaces with high resolution in shock or ramp compression and unload experiments. This development provides a possible way to infer the sound speed from these velocity profiles. The key problem is to build valid analysis technique to extract the sound speed. Commonly used Lagrangian analysis methods include backward integration method, incremental impedance matching method, transfer function method and backward characteristic analysis method. However, all of these methods hardly infer the right results from the nonsymmetric impact and release experiment with only one depth of material due to the complex impedance mismatch among a flyer, sample and window. Some decreasing impedance mismatch techniques have been developed for the experiments including reverse impact or using a high strength flyer, but these techniques will limit the pressure range or need a newly designed gun with large caliber. In fact, the traditional backward characteristic analysis method only considers the sample/window interaction while bending of the incoming characteristics due to impedance difference between the flyer and sample is always ignored, which causes a distortion to the loading condition of samples. Thus in this work, we add forward characteristics to describe rarefaction wave reflection at the flyer/sample interface. Then a reasonable loading-releasing in-situ velocity profile of the interface can be derived from this improvement. We use the improved/tradition characteristics and incremental impedance matching method to analyze a synthetic nonsymmetric impact experiment in which the flyer, sample and window are of Al, Cu and LiF, respectively. Synthetic analyses suggest that the modified characteristic method can give more accurate results including sound speed-particle velocity and release path at high pressure. Compared with other methods, the new characteristic method just needs to know the release path of flyer and window that can be calibrated by well-developed technique, moreover, this method also does not need to know the form of equation of state and constitutive model of the sample. Calculation of this method is not complex and the iterative approach usually achieves convergence in less than 10 steps. All of these features will facilitate using this method to infer sound speed from the velocity profile of nonsymmetric impact experiments.
2016, 65 (11): 116501.
doi: 10.7498/aps.65.116501
Abstract +
To guide the experiment research, the thermal stability of composite silicon nanowire encapsulated in carbon nanotubes is investigated by computer simulation. The cubic-diamond-structured silicon nanowires with the same diameter and [111] orientationt are filled in some armchaired single-walled carbon nanotubes. The heat process of compound structure of silicon nanowire encapsulated in carbon nanotubes is simulated by classical molecular dynamic method. Through the visualization and energy analysis method, the thermal stability of composite structure is studied. The changes in the thermal stability of silicon nanowires and carbon nanotubes are explained by the relationship between carbon nanotube space constraint and van der Waals force. It is found that the diameter of the carbon nanotubes is closely related to the thermal stability of silicon nanowires inside. When the nanotube diameter is small, thermal stability of silicon nanowires increases; when the nanotube diameter increases up to a certain size, the thermal stability of silicon nanowires will suddenly drop significantly: until the distance between silicon nanowires and the wall of carbon nanotube is greater than 1 nm, the thermal stability of silicon nanowires will be restored. On the other hand, silicon nanowires filled into the carbon nanotubes have an effect of reducing the thermal stability of carbon nanotubes.
2016, 65 (11): 116801.
doi: 10.7498/aps.65.116801
Abstract +
Analogous to graphite, hexagonal boron nitride (h-BN) has a layered structure composed of boron and nitrogen atoms that are alternatively bond to each other in a honeycomb array. As the layers are held together by weak van der Waals forces, h-BN thin films can be grown on surfaces of various metal crystals in a layer-by-layer manner, which is again similar to graphene sheets and thus attracts a lot of research interests. In this work, scanning tunneling microscope and spectroscope (STM and STS) were applied to the study of an h-BN thin film with a thickness of about 10 nm grown on Cu foil by means of chemical vapor deposition. X-ray diffraction from the Cu foil shows only one strong peak of Cu(200) in the angle range of 40-60, indicating that the Cu foil is mainly Cu(100). After sufficient annealing in an UHV chamber, the h-BN film sample is transferred to a cooling stage (77 K) for STM/STS measurement. Its high quality is confirmed by a large-scale STM scan that shows an atomically flat topography. A series of dI/dV data taken within varied energy windows all exhibit similar U shapes but with different bottom widths that monotonously decrease with the sweeping energy window. The dI/dV curve taken in the energy window of [-1 V, +1 V] even shows no energy gap in spite that h-BN film is insulating with a quite large energy gap of around 6 eV, as observed in a large-energy-window dI/dV curve (from -5 V to +5 V). These results indicate that the STM images reflect the spatial distribution of tunneling barriers between Cu(100) substrate and STM tip, rather than the local density of states of the h-BN surface. At high sample biases (from 4 V to 1 V), STM images exhibit an electronic modulation pattern with short range order. The modulation pattern displays a substructure in low-bias STM images (less than 100 mV), which finally turns to the (11) lattice of h-BN surface when the sample bias is extremely lowered to 3 mV. It is found that the electronic modulation pattern cannot be fully reproduced by superimposing hexagonal BN lattice on tetragonal Cu(100) lattice, no matter what their relative in-plane crystal orientation is. This implies that the electronic modulation pattern in the STM images is not a Mori pattern due to lattice mismatch. We speculate that it may originate from spatial distribution of tunneling barrier induced by adsorption of H, B and/or N atoms on the Cu(100) surface in the CVD growth process.
EDITOR'S SUGGESTION
2016, 65 (11): 116802.
doi: 10.7498/aps.65.116802
Abstract +
Both high-efficient thermoelectric materials and thermal insulating coatings requiring low thermal conductivities, layered materials and superlattices prove to be an efficient multiscale material design for such requirements. The interfaces are artificially introduced to scatter thermal phonons, thus hindering thermal transport. Very recently, it has been found that interface modulation can further reduce the thermal conductivity. All of the recent advances originate from highly demanding numerical computations. An efficient estimate of the thermal properties is important for fast and/or high-throughput calculations. In this article, the phonon transport on layered material is studied theoretically for general purposes, based on the fact that long-wavelength phonons contribute dominantly in general. According to the Debye hypothesis, the classical wave equation can describe phonon transport very well. This fact has been very recently used to model phonon transport carbon nanotubes, which justifies the applicability of continuum mechanics for nanomaterials. Furthermore, Kronig and Penny have solved the electron transport on periodic lattices. In a very similar way, for the periodic layered materials and superlattices, with Floquet and linear attenuation theory, the wave equations with and without damping are solved analytically. The wave equation decouples to Helmholtz equations in each direction with periodic excitation functions. In this paper, we propose to model the phonon transport by using Matthew-Hill equation, with which we can obtain the phonon spectrum (i.e. phonon dispersion relation). The proposed theory is justified by two-dimensional (2D) graphene/hexagon boron nitride superlattice and three-dimensional (3D) silicon/germanium superlattices. Like the carbon nanotube cases, using this continuum-mechanics method, we can reproduce the previous numerical results very quickly compared with using published molecular dynamics and density functional theory The effects of interface modulation and phonon localization are shown over full phase space, which further enables the calculating of both high and low bounds of thermal conductivity for all possible superlattices and layered materials. In order to model real interfaces, with considering possible mixing and transition due to other mechanisms, we use the smooth transition function, which is further modeled via sinusoidal series. Very interestingly, interface grading is shown to erase band gaps and delocalize modes. This fact has been seldom reported and can be helpful for designing real materials. Likewise, we take phonon damping (equivalent to inter-phonon scattering) into account by adding damping into the wave equation. It is observed that phonon damping smears the originally sharp boundaries of phonon phase space. In this way, evanescent phonons and transporting phonons can be treated simultaneously on the same footing. The proposed method can be used for modeling the efficient and general thermal materials
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
2016, 65 (11): 118101.
doi: 10.7498/aps.65.118101
Abstract +
In recent years, a variety of new-concept solar cells have attracted the attention of many scholars. The CdS-CdTe ferroelectric-semiconductor coupled (FSC) solar cell is a novel concept of photovoltaic device that is designed with ferroelectric nano particles of S-rich CdS1-xTex, which are embedded in the light-absorbing semiconductors of Te-rich CdSyTe1-y. In our previous work, we have developed a two-step process to fabricate a nano-dipole photovoltaic device, including a thin film deposition in vacuum and high-temperature phase segregation at elevated temperature in sequence. The X-ray diffraction (XRD) and high-resolution scanning transmission electric microscopy (STEM) results confirm the formation of S-rich CdS1-xTex particles with a wurtzite structure embedded in a Te-rich CdSyTe1-y film with a zinc blend structure. The localized ferroelectric hysteretic behavior of these particles is confirmed through piezoelectric force microscopy (PFM). Meanwhile, a set of CdS-CdTe FSC devices with a symmetrical structure of ITO/FSC/ITO is fabricated. We observe not only a reasonable photovoltage output on the order of hundreds of mV but also the hysteretic behavior of photovoltage through external electric field post-fabrication.
To search for direct evidence of the working mechanism of the FSC solar cell, we further study the film surface micro current distribution of the FSC thin film solar cell. In this work, we adopt the CAFM method to acquire electron distribution of the FSC thin film surface and STEM, the electron diffraction for element distribution, and crystal structure of FSC thin film. Also, Schottky solar cell of FTO/pure CdTe/metal structure which is fabricated by the same process as the FSC solar cell is used as reference sample in the CAFM analysis.
In this work, we fabricate the CdS-CdTe FSC film solar cell through a radio-frequency magnetron sputtering method, whose structure is a glass/FTO/CdSTe/back contact (Cu/Au) configuration. In order to enhance the polarization of nano dipole particles in the device, an electric field bias across the FSC film is applied in the high-temperature phase segregation process. Micro-current distribution in CdS-CdTe FSC solar cell is investigated by CAFM. Grain boundaries of the FSC film are found to be non-conductive with high current corridors adjacent to them. And some small particles with diameter about 100 nm are embedded in grain boundaries (GBs) of CdTe grains. By applying positive and opposite voltage separately between measurement tip and TCO of sample, we find that the non-conductive GBs have a strong piezoelectric response, which are most likely S-rich CdS1-xTex in wurtzite structure. By contrast with pure CdTe film, the possibility that the non-conductive particles are CdCl2 residuals is excluded. We also find by STEM that many particles with sizes about 100-200 nm are embedded in FSC thin film, mostly at the GBs. The XRD results confirm that the small particles are S-rich CdS1-xTex particles with a wurtzite structure and the big grains are Te-rich CdSyTe1-y with a zinc blend structure. We could conclude reasonably that the small particles observed in CAFM image probably are S-rich CdS1-xTex:The apparent correlation between the carrier transport channel and nano-dipole material is also established. An interesting discovery from such devices is that such cells exhibit performance improvement over time in months after storage with encapsulation in ambient environment. A linear relationship between Voc and the external field strength is observed and the best conversion efficiency is improved from 11.3% to 13.2% further after 6-month storage. We believe that all these microscopic and macroscopic evidences are consistent with the FSC photovoltaic mechanism.
2016, 65 (11): 118105.
doi: 10.7498/aps.65.118105
Abstract +
Materials with large perpendicular magnetic anisotropies (PMAs) have drawn great attention because of their potential applications in advanced spintronic devices such as spin-transfer-torque magnetic random access memory (STT-MRAM) and ultrahigh-density perpendicular magnetic recording. To date, a large variety of PMA materials have been investigated, such as L10-ordered FePt, CoPt granular films, Co/(Pt,Pd,Ni) multilayers, ultra-thin CoFeB alloys and perpendicularly magnetized Co2FeAl films. Among the various kinds of materials with PMA, MnGa film with L10-structure has received the most attention because it has large PMA (Ku～107 erg/cm3), ultralow Gilbert damping constant (0.008) and theoretically predicted high spin polarization (more than 70%). All these properties make L10-ordered MnGa a good candidate for spintronic devices such as STT-MRAM and spin-torque oscillators. Meanwhile, from the viewpoint of STT related spintronic device, it is necessary to fabricate ultrathin perpendicularly magnetized L10-MnxGa films to lower the critical current for magnetization reversal. However, up to now, in the main researches the ultrathin L10-MnxGa films have been grown on MgO substrates, which makes it difficult to integrate the MnGa-based magnetic tunnel junctions into the semiconductor manufacturing process.In this work, ultrathin L10-Mn1.67Ga films with different thickness values (1-5 nm) are grown on traditional GaAa (001) substrates by a molecule-beam epitaxy system. During the deposition, in situ streaky surface reconstruction patterns are observed from reflection high-energy electron diffraction, which implies high crystalline quality of the L10-Mn1.67Ga film. Only MnGa superlattice (001) and MnGa fundamental (002) peaks can be observed in the X-ray diffraction patterns in a range from 20 to 70, which means that the L10-Mn1.67Ga film is a good single-crystalline with c-axis along the normal direction. The magnetic properties of these films are measured by superconductor quantum interference device magnetometer in a field range of 5 T. The perpendicular M-H curves are almost square, while the in-plane curves are nearly hysteresis-free, each with a remnant magnetization (Mr) of around zero, which clearly evidences the PMA of the ultrathin L10-Mn1.67Ga film. Moreover, as the thickness of L10-Mn1.67Ga film decreases from 5 nm to 1 nm, the ratio of Mr/Ms also decreases from 1 to 0.72, which indicates that the PMA loses as thickness decreases. We also estimate the perpendicular anisotropy constant (Ku) from the relation Ku=Keff+2 Ms2, and the maximum Ku of 14.7 Merg/cm3 is obtained for the 5 nm MnGa film. Although the Ku decreases with thickness decreasing, a Ku value of 8.58 Merg/cm3 is observed in a 2 nm thick film. The obtained results are important for developing the L10-MnGa-based spin-transfer torque Gbit class magnetic random access memory.
2016, 65 (11): 118501.
doi: 10.7498/aps.65.118501
Abstract +
Graphene is predicted to hold a promising use for developing future miniaturized electronic devices. However, the magnetic transport properties based on the armchair-edged graphene nanoribbons (AGNRs) is less studied in currently existing work. So in this work the special chemical modified nanoribbons based on the edge of the AGNR bridged by the transition metal Mn atom and passivated subsequently by two F atoms or two H atoms (AGNR-Mn-F2 or AGNR-Mn-H2) are proposed theoretically. Our calculations from first-principle method based on the spin-polarized density functional theory combined with the non-equilibrium Green's function technique show that the heterojunction F2-AGNR-Mn-H2 consisting of such two types of nanoribbons possesses the excellent magnetic device features, namely, the spin polarization is able to reach almost 100% in a very large bias region, and under P magnetic configuration (the external magnetic fields applied perpendicularly to two electrodes are set to point to the same direction), the single spin filtering effects can be realized, while under the AP configuration (the external magnetic fields applied perpendicularly to two electrodes are set to point to the opposite directions), the dual spin filtering effects can be realized. It is also found that such a heterojunction features dual diode-like effect, and its rectification ratio is up to be 108. Additionally, changing the direction of switching magnetic field, namely, changing the magnetic configurations from one kind of case to another, would lead to an obvious spin valve effect, and the giant magnetoresistace approaches to 108%. These findings suggest that the excellent spin polarization, dual diode-like effect, and giant magnetoresistace effect can be realized simultaneously for this heterojunction, therefore, it holds good promise in developing spintronic devices.
2016, 65 (11): 118801.
doi: 10.7498/aps.65.118801
Abstract +
Highly-catalytic, cost-effective, well process-compatible, and highly-stable hydrogen-evolving catalysts are increasingly becoming key catalysts in realizing monolithic electrochemical solar water-splitting devices. However, the typical noble metallic catalysts seriously restrict the industrialization of electrochemical solar water-splitting devices on account of their poor storages and high costs. Low-cost, high-catalytic and non-metallic catalysts pave the promising way for the industrialization process. Molybdenum sulfide has emerged as a type of potential catalyst with high-activity and stability for the hydrogen-evolving reaction (HER) in the acidic condition, nowadays gradually becoming a research hotspot in solar-water-splitting. The process preparation of high-efficient molybdenum sulfide catalyst is consequently extremely important for enhancing the solar-to-hydrogen efficiency. In this paper, we synthesize highly-catalytic, low-cost, and highly-compatible non-metallic amorphous molybdenum trisulfide catalyst based on a simple wet chemical approach at room temperature for hydrogen-evolving reaction, followed by extensive studies of the effects of the mass loading of catalyst on the catalytic capacity and the solar-to-hydrogen performance of solar-water-splitting devices in series. When the mass loading is 0.5 mgcm-2, the MoS3 catalyst exhibits the promising HER activity. the surface of catalyst appears to be rough, porous, nano-sized architecture and the thickness is around 2.0 m, which simultaneously enlarges the electrochemically active area and reduces charge transfer impedance, accelerating the electron transport to electrochemically active site and improving the interfacial charge transfer. Besides, the HER catalytic activity is illustrated in a wired solar-water-splitting device. The current density can achieve the maximum values of 7.51 and 3.28 mA/cm2 corresponding to 0 and 0.8 V vs. RHE, and the onset potential is 1.83 V, comparable to the open circuit voltage (1.90 V) of two amorphous silcon cells in series. Therefore, we conclude that for amorphous molybdenum trisulfide catalyst there exists an optimized mass loading, with which an optimized catalytic capacity (260 mV vs. RHE at 10 mA/cm2 and tafel slope of 68 mV/dec) can be achieved. Further, by using the catalyst as a cathode for the solar-water-splitting devices in series, the catalyst can efficiently reduce the overpotential and improve the current output for the device, thereby potentially achieving a higher solar-to-hydrogen efficiency.
2016, 65 (11): 118802.
doi: 10.7498/aps.65.118802
Abstract +
In recent years, dye-sensitized solar cells (DSSCs) have attracted much attention because of their easy fabrication, good flexibility low cost and relatively high efficiency. As a crucial component, the function of counter electrode (CE) is to collect the electrons from external circuits and transfer them to electrolyte by catalyzing the reduction of I3- into I-. Platinum (Pt) is a conventional material of CE in DSSCs due to its high conductivity and outstanding catalytic activity towards the reduction of triiodide (I3-). However, the high cost and low abundance of Pt restrict the commercial application of DSSCs. Moreover, Pt could be dissolved slowly in the I-/I3- redox electrolyte, which will deteriorate the long term stability of DSSCs. Therefore, it is necessary to explore novel material with high conductivity, catalytic activity and stability to replace Pt. In this paper, with Fe(NO3)39H2O and graphene oxide (GO) serving as raw materials and deionized water as the solvent, we synthesize iron diselenide (FeSe2) nanorods (with diameters in a range of about 100-200 nm)/reduced graphene oxide (rGO) composite through a facile hydrothermal method and use the composite as CE material of DSSCs for the first time. The structure and morphology of FeSe2/rGO are characterized by using X-ray diffraction (XRD), Raman spectrum, field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). The XRD pattern shows that the FeSe2 is typically orthorhombic phase. The SEM images show that the FeSe2 has a structure of nanonods and can be attached to the surface of rGO closely The surface of FeSe2/rGO composite is rough and exhibits a porous structure. The TEM image shows that the FeSe2 has a high degree of crystallinity and orientation. To evaluate the catalytic activity and conductivity of FeSe2/rGO, we perform cyclic voltammetry (CV) measurements, electrochemical impedance spectroscopy and obtain Tafel polarization curves for FeSe2/rGO electrode and also for Pt, FeSe2 and rGO electrodes for comparison. The results indicate that the CE based on FeSe2/rGO composites has the lowest peak-to-peak voltage separation (E_{pp}) charge transfer resistance (Rct) and series resistance (Rs) in the four different CEs, suggesting that the FeSe2/rGO CE has an excellent electrocatalytic performance for the reduction I3-. The current density-voltage (J-V) curves of DSSCs with different CEs under the illumination of 1 sun (100 mW cm-2) show that the cell with FeSe2/rGO CE has an open-circuit voltage (Voc) of 0.727 V, a short-circuit current (Jsc) of 18.94 mA cm-2, a fill factor (FF) of 0.65 and an excellent power conversion efficiency (PCE) of 8.90%, which is a notable improvement compared with the PCE of the cell with an FeSe2 CE (7.91%) and an rGO CE (5.24%). It can be attributed to the synergetic effects between the FeSe2 nanorods and rGO which eventually improve the PCE of DSSC We also conducte the experiments on the electrochemical stability of FeSe2/rGO CE by sequential CV measurements the result indicates that the FeSe2/rGO composite has a better stability than Pt in I-/I3- electrolyte In summary, we synthesize a novel FeSe2/rGO conductive catalyst. This hybrid material possesses the features of FeSe2 and rGO, exhibiting both highly catalytic activity and high conductivity Therefore, the low-cost and high-performance FeSe2/rGO composite can be a promising CE material to replace Pt in the large-scale industrial production of DSSCs.
2016, 65 (11): 118901.
doi: 10.7498/aps.65.118901
Abstract +
The security of information transmission is of paramount importance in all sectors of society, whether civilian or defence related. In ancient times the encryption of secret messages was mainly realized by physical or chemical means, but this was later supplemented by mathematical techniques. In parallel, the breaking of enemy codes has also been a subject of intense study. To date, the only known absolutely secure means of encryption is through quantum cryptography, However, this still has to be implemented by equipment that is vulnerable to various physical attacks, so it is important to study these methods of attack, both for legitimate users and for the surveillance of criminal activities. Today, nearly all transactions have to be realized through the computer and much effort has been devoted to cracking the software. However, little attention has been paid to the hardware, and it has only recently been realized that computer chips themselves can leak sensitive information, from which a code may even be deciphered.
By studying the photonic emission and the data dependency of a cryptographic chip during operation, the correspondence between the Hamming weight of the operand and the number of photons emitted may be established, based on which a simple and effective method is proposed to crack the Advanced Encryption Standard (AES) cipher chip. An experimental platform has been set up for measuring and analyzing the leaked photonic emission using time-correlated single-photon counting. An AT89C52 microcontroller implementing the operation of the AES cipher algorithm is used as a cipher chip. The emitted photons are collected when the first AddRoundKey and SubBytes of the AES encryption arithmetic are executed, and their respective numbers are found to have a linear relationship with the operand Hamming weight. The sources of noise affecting the photon emission trace have been analyzed, so that the measurement error and uncertainty can be reduced effectively. With the help of our Hamming weight simulation model, by selecting one or several groups of plain text and comparing the corresponding relationship between the Hamming weight of the intermediate values and the number of photons emitted by the cipher chip, the key of the AES encryption algorithm has been successfully recovered and cracked. This confirms the effectiveness of this method of attack, which can therefore pose a severe threat to the security of the AES cipher chip. For the next step in the future, our method will be optimized to narrow the search range, and also combined with other photonic emission analysis attacks (such as simple photonic emission analysis and differential photonic emission analysis) to improve the efficiency. A comparison and evaluation of the various methods will be made. At the same time, our current experimental configuration will be improved to obtain a better collection efficiency and signal-to-noise ratio.
2016, 65 (11): 118102.
doi: 10.7498/aps.65.118102
Abstract +
Motivated by exploring new high temperature ceramics which have excellent mechanical properties, we systematically search for all the stable compounds and their crystal structures in the binary Hf-N system by combining the evolutionary algorithm with first principle calculation. In addition to the well-known rock-salt HfN, we find five other novel compounds, i.e., Hf6N(R-3), Hf3N(P6322), Hf3N2(R-3m), Hf5N6(C2/m), and Hf3N4(C2/m). Then, their phonon frequencies are calculated so that the dynamical stabilities are known. Their high temperature thermodynamic stabilities are further confirmed and the Gibbs free energies are calculated in thequasi-harmonic approximation. All of these structures are thermodynamic stable when the temperature is lower than 1500 K. However, as temperature increases, the structuresHf5N6(C2/m) and Hf3N4(C2/m) become meta-stable. Meanwhile, some meta-stable structures, including Hf2N (P42/mnm), Hf4N3 (C2/m), Hf6N5(C2/m), Hf4N5(I4/m), Hf3N4 (I-43d), and Hf3N4 (Pnma), each of which has higher symmetry and lower formation enthalpy, are all listed. At the same time, our results of Hf3N4 testify that C2/m structure is stabler than Pnma and I-43d structures when the temperature is lower than 2000 K, which is different from the conclusion given by Bazhanov [Bazhanov D I, Knizhnik A A, Safonov A A, Bagatur'yants A A, Stoker M W, Korkin A A 2005 J. Appl. Phys. 97 044108]. The results also show that the difference in Gibbs free energy between C2/m and Pnma Hf3N4 structure decreases with temperature increasing. Thus, we speculate that the C2/m Hf3N4 transforms into Pnma Hf3N4 when the temperature is above 2000 K. The mechanical properties, including the elastic constant, bulk modulus, shear modulus, Young's modulus and hardness, are systematically investigated. The hardness first increases, reaching a maximum at Hf5N6 (21 GPa), and then decreases with increasing nitrogen content. Besides, Hf3N2 and Hf4N5 both exhibit relatively high hardness value of 19 GPa, while the hardness of HfN is 15 GPa. Finally, the electron densities of states and crystal orbital Hamilton populations are calculated so that the mechanic origins can be analyzed from the electronic structures of these phases. The crystal orbital Hamilton populations show that the strength of Hf-N covalent bonding increases with increasing nitrogen content, however, it has an exceptional peak for Hf3N2, which can be used to explain the relatively high hardness of this structure. Beside covalent bonding strength, structural vacancy can also affect their mechanical properties. It is concluded that the strong covalent bonding and low structural vacancy both can explain the good mechanical performance of Hf5N6.
2016, 65 (11): 118103.
doi: 10.7498/aps.65.118103
Abstract +
Diamond is well known for its excellent properties, such as its hardness, high thermal conductivity, high electron and hole mobility, high breakdown field strength and large band gap (5.4 eV), which has been extensively used in many fields. However, its application in semiconductor area needs to be further understood, because it is irreplaceable by conventional semiconductor materials, especially in the extreme working conditions. In order to obtain diamond semiconductor with excellent electrical performances, diamond crystals co-doped with boron (B) and hydrogen (H) are synthesized in an FeNi-C system by temperature gradient growth (TGG) at pressure 6.0 GPa and temperature 1600 K. Fourier infrared spectra (FTIR) measurements displayed that H is the formation of sp3 CH2-antisymmetric and sp3 -CH2-symmetric vibrations in the obtained diamond. Furthermore, the corresponding absorption peaks of H element are located at 2920 cm-1 and 2850 cm-1, respectively. Hall effects measurements demonstrated that the co-doped diamond exhibited that p- type material semiconductor performance, and the conductivity of the co-doped diamond is significantly enhanced comparing tocompared with the conductivity of the B-doping diamond. The results indicated that the Hall mobility mobilities is nearly equivalent between B-doped and co-doped diamond crystals are nearly equivalent, while the concentrations of the carriers and conductivity of the co-doped diamonds are higher than those of the B-doped diamond crystals. It is also noticed that the nitrogen concentration of the co-doped diamond decreases obviously, when the H and B are introduced into the diamond structure. Additionally, the change of the conductivity is investigated by first-principles calculation. In the B-doping diamond, two impurity levels are located in the forbidden band with small gaps. These impurity states above the Fermi level couldcan trap the photo-excited electrons, while those below Fermi level can trap the photo-excited vacancies, improving the transfer of the photo-excited carriers to the reactive sites. With the H co-doped diamond, the two impurity states moved to the valance band maximum and merged into each other, which extends the valance band and improves the charge transfer efficiency. From the perspective of energy band, for the co-doped of B and N atoms co-doped diamond, the impurity states are contributed by N/B-2p states while the overlop and splitting of N/B-2p in the band gap appeared. For the H co-doped diamond, the splitting of the N/B-2p states vanishes and shifts to the lower energy level, which was due to the fact that the excess charge transferred from N to H. The calculation results above are in qualitatively agreement with experimental results. We hope that this investigation would be meaningful for the application of diamond in semiconductor field.
2016, 65 (11): 118104.
doi: 10.7498/aps.65.118104
Abstract +
In this paper, we integrate nano technology into traditional microelectronic processing, and develop an on-chip UV sensor based on lateral growth ZnO nanowire arrays. Traditional procedures are used to fabricate the interdigital electrodes, and ZnO nanowires are self-organized and grown between electrodes laterally by hydrothermal method. Additional inclined nanowires are removed during the post-processing procedures, such as ultrasound cleansing and electrode reinforcement. Two kinds of electrode structures are applied, i.e., Cr and Au. For the Cr electrode device structure, because Cr will restrain nanowires from growing vertically on its top, the laterally grown nanowire is long enough to reach the other side of the electrode. The corresponding photoelectric response mechanism is photoconduction controlled by surface oxide ion adsorption. Although the photocurrent is large, the gain is low, and the response speed is slow. Under the UV radiations of 20 mW/cm2 and of 365 nm in wavelength, the dark current is 2.210-4 A with 1 V bias voltage, the gain is up to 64, the photocurrent cannot reach saturation after 25 s, and the recovery time is 51.9 s. A secondary electrode can be fabricated after growing the nanowire arrays to reinforce the connection between the electrode and the ends of the nanowires. However, the direct contact between metal and semiconductor will form a Schottky contact. The photoelectric response mechanism is then changed to photovoltaic effect, which can greatly improve the gain and response speed. Under UV radiations of 20 mW/cm2 and of 365 nm in wavelength, the dark current is 4.310-8 A with 1 V bias voltage, the gain is up to 1300, the respond time is 3.8 s, and the recovery time is 5.7 s. For the Au electrode device structure, because Au is catalysis for ZnO nanowire growth, nanowires grown in lateral direction will compete with those grown in vertical direction, and hence the laterally grown nanowires are not long enough to reach the other side of the electrode. Nanowires grown from two sides of the electrodes will meet each other and form a bridging junction, however, this will turn the photoconduction mechanism from surface ion controlled into a bridging junction controlled, which yields the best device performance. Before removing the inclined nanowires by ultrasound cleansing, under UV radiations of 20 mW/cm2 and of 365 nm in wavelength, the dark current is 8.310-3 A with 1 V bias voltage, the gain is up to 1350, the respond time is 3.3 s, and the recovery time is 3.4 s. After removing the inclined nanowires, under UV radiations of 20 mW/cm2 and of 365 nm in wavelength, the dark current is 10-9 A with 1 V bias voltage, the gain is up to 8105, the respond time is 1.1 s, and the recovery time is 1.3 s.
2016, 65 (11): 118701.
doi: 10.7498/aps.65.118701
Abstract +
The functional corticomuscular coupling (FCMC) is defined as the interaction, coherence and time synchronism between cerebral cortex and muscle tissue, which could be revealed by the synchronization analyses of electroencephalogram (EEG) and electromyogram (EMG) firing in a target muscle. The FCMC analysis is an effective method to describe the information transfer and interaction in neuromuscular pathways. Forthermore, the multiscaled coherence analyses of EEG and EMG signals recorded simultaneously could describe the multiple spatial and temporal functional connection characteristics of FCMC, which could be helpful for understanding the multiple spatial and temporal coupling mechanism of neuromuscular system. In this paper, based on the adaptively decomposing signal into frequency band characteristis of variational mode decomposition (VMD) and the quantitatively detecting the directed exchange of information between two systems of transfer entropy (TE), a new methodvariational mode decomposition-transfer entropy (VMD-TE) is proposed. The VMD-TE method could quantitatively analyze the nonlinear functional connection characteristic on multiple time-frequency scales between EEG over brain scalp and surface EMG signals from flexor digitorum surerficialis, which are recorded simultaneously during grip task with steady-state force output.In this paper, application of VMD-TE method consists of two steps. Firstly, the EEG and EMG signals are adaptively decomposed into multi intrinsic mode functions based on variational mode decomposition method, respectively, to describe the information on different time-frequency scales. Then the transfer entropies between the different timefrequency scales of EEG and EMG are calculated to describe the nonlinear corticomuscular coupling characteristic in different pathways (EEGEMG and EMGEEG), to show the functional coupling strength (namely VMD-TE values). finally, the maximum VMD-TE values between the different time-frequency scales of EEG and EMG signals among the eight subjects are selected, to describe the discrepancies of FCMC interaction strength between all time-frequency scales. The results show that functional corticomuscular coupling is significant in both descending (EEGEMG) and ascending (EMGEEG) directions in the beta-band (15-35 Hz) in the static force output stage. Meanwhile, the interaction strength between EEG signal and the gamma band (50-72 Hz) of EMG signal in descending direction is higher than in ascending direction. Our study confirms that the beta oscillations of EEG travel bidirectionally between sensorimotor
GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS
2016, 65 (11): 119701.
doi: 10.7498/aps.65.119701
Abstract +
In order to improve the time delay estimation accuracy of the observed profile in the X-ray pulsar based navigation, the spectral characteristics of the observed profile of X-ray pulsar and the drawback of the classical Taylor fast Fourier transform (FFT) time delay estimation method are analyzed. It is found that when estimating the time delay, we can abandon the higher frequency components that are always affected by noise seriously, but only utilize the information about the low frequency part. Based on this idea, by modifying the weigh function of the classical Taylor FFT time delay estimation method, a new time delay estimation algorithm based on the optimal frequency band is proposed, in which the optimal frequency band is determined by establishing the relationship between the selected frequency band and the time delay estimation accuracy under different signal-to-noise ratios (SNRs). Then by using the real data obtained with the proportional counter array, the low-energy (2-60 keV) detection instrument boarded on the Rossi X-ray Timing Explorer satellite, the optimal frequency as a function the SNR of observed profile is given for the PSR B0531+21 (namely the Crab pulsar) through the Monte-Carlo technique. Since the parameters of different pulsars are known, in practical navigation, the optimal frequency in an observation time for a certain pulsar can be estimated in advance by using the simulation data or the obtained real data of the pulsar, which can remarkably alleviate the onboard computational burden. Finally, a series of numerical simulations and experiments using real data of Crab pulsar are designed to evaluate the performance of the proposed time delay estimation algorithm. The main results can be summarized as follows: the proposed estimator outperforms the normally used fast approximate maximum-likelihood (FAML), cross correlation (CC), nonlinear least square (NLS) and weighted nonlinear least-square (WNLS) estimators when the observation time is short or the source flux is small; when the observation time is long or the source flux is large, its estimation accuracy is almost the same as those of CC and NLS estimators and lower than those of the FAML and WNLS estimators, but its computational complexity is smaller than those of NLS, FAML and WNLS estimators. The above results indicate the high estimation accuracy and high computational efficiency of the proposed time delay estimation method, which can be used in the case that the observation time is restricted to be short or the source flux of the usable pulsar is small in X-ray pulsar based navigation.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
EDITOR'S SUGGESTION
2016, 65 (11): 117101.
doi: 10.7498/aps.65.117101
Abstract +
The mobility edges which separate the localized energy eigenstates from the extended ones exist normally only in three dimensional systems. For one-dimensional systems with random on-site potentials, one never encounters mobility edges, where all the eigenstates are localized. However, there are two kinds of 1D systems such as correlated disordered models, and the systems of exponentially decaying hopping kinetics, features of mobility edges at some specific values become possible. We study in this paper the properties of the mobility edges in a one-dimensional p-wave superfluid on an incommensurate lattice with exponentially decaying hopping kinetics. Without the p-wave superluid, the system displays a single mobility edge, which separates the extended regime from the localized one at a certain energy. Without the exponentially decaying hopping term, the system displays a phase transition from a topological superconductor to an Anderson localization at a certain disorder strength, where no mobility edge exists. We are interested in the influence of the p-wave superfluid on the mobility edge. By solving the Bogoliubov-de Gennes equation, the eigenvalues and the eigenfunctions are obtained. In order to identify the extending or the localized properties of the eigenvectors, we define an inverse participation ratio IPR. For an extended state, IPRn～1/L which goes to zero at a large L, and for a localized one, IPRn being constant. Therefore, the IPR can be taken as a criterion to distinguish the extended state from the localized one, while the mobility edge is defined as the boundary between two different states. We find that, with a p-wave superfluid, the system changes from a single mobility edge to a multiple one, and the number of mobility edges increases with the increased superfluid pairing order parameter. To further obtain the energy or the location of the mobility edge, we investigate the scaling behavior of wave functions by using a multifractal analysis, which is calculated through the scaling index . The minimum value of the index, with the values min= 1, 0min1, and min= 0, mean the extended, critical, and localized states, respectively. For the two consecutive states, the minima of the scaling index min when extrapolating to the large size limit between 0 and 1 signal the mobility edge. By exploring the corresponding Bogoliubov quasi-particle wave functions for the system under open boundary conditions together with the multifractal analysis for the system under periodic boundary conditions, we identify two mobility edges for the system of the p-wave superfluid pairing. Furthermore, we will investigate how the existence of the mobility edges influences the p-wave superfluid, and identify the phase diagram at the given parameters. We will in the future try to understand the relationship between the topological superfluid and the mobility edges.
EDITOR'S SUGGESTION
2016, 65 (11): 117701.
doi: 10.7498/aps.65.117701
Abstract +
Multiferroics simultaneously exhibit several order parameters such as ferroelectricity and antiferromagnetism, representing an appealing class of multifunctional material. As the only multiferroics above room temperature, BiFeO3 (BFO) becomes an attractive choice for a wide variety of applications in the areas of sensors and spintronic devices. The coexistence of several order parameters brings about novel physical phenomena, for example, the magnetoelectric coupling effect. It allows the reversal of ferroelectric polarization by a magnetic field or the control of magnetic order parameter by an electric field. Heterostructure interface plays an important role in enhancing the ferroelectric and magnetic properties of multiferroic materials. Furthermore, the magnetoelectric coupling at the interface between the antiferromagnetism BFO and a ferromagnetic film has the close relation with achieving a functional multiferroic-ferromagnetic heterostructure.
In order to determine the relationship between the multiferroic property and the interface experimentally, we prepare the Bi0.8Ba0.2FeO3(BBFO)/La2/3Sr1/3MnO3(LSMO) heterostructure on an SrTiO3(STO) substrate by pulsed laser deposition, and the structure characteristics and ferroelectric and magnetic properties are investigated. X-ray diffraction analysis shows that BBFO and LSMO films are epitaxially grown as single-phase. The further study by high-resolution transmission electron microscopy determines that the BBFO film has a tetragonal structure. The ferroelectric and magnetic measurements show that the magnetic and the ferroelectric properties are simultaneously improved, and the maximum values of the remnant polarization (2Pr) and the saturation magnetization of the heterostructure at room temperature are about 3.25 C/cm2 and 112 emu/cm3, respectively. The reasons for enhancing the ferroelectric and ferromagnetic properties of heterostructure are demonstrated by X-ray photoelectron spectrum that shows being unrelated to the valence states of Fe element. On the contrary, interface effect plays a major role. In addition, the magnetic resistivities and dielectric properties of BBFO/LSMO heterostructure are investigated at temperatures in a range of 50 K to 300 K, finding that magnetoresistance (MR) and magnetodielectric (MD) are respectively about -42.2% and 21.9% at 70 K with a magnetic field of 0.8 T, and the transition of magnetic phase takes place near 180 K. Furthermore, the temperature dependences of magnetodielectric and magnetoloss (ML) present opposite tendencies, suggesting that magnetodielectric is caused by Maxwell-Wagner effect and the magnetoresistance. Experimental results reveal that heterogeneous interface effect shows the exceptional advantages in enhancing multiferroic property and magnetoelectric coupling effect of complex heterostructure material. It is an effective way to speed up the application of multiferroic materials.
2016, 65 (11): 117801.
doi: 10.7498/aps.65.117801
Abstract +
InAs/GaAs quantum dots (QDs) have been extensively applied to high-performance optoelectronic devices due to their unique physical properties. In order to exploit the potential advantages of these QD-devices, it is necessary to control the QDs in density, uniformity and nucleation sites. In this work, a novel research of in-situ pulsed laser modifying InAs wetting layer is carried out to explore a new controllable method of growing InAs/GaAs(001) QDs based on a specially designed molecular beam epitaxy (MBE) system equipped with laser viewports. Firstly, a 300 nm GaAs buffer layer is grown on GaAs (001) substrate at 580 ℃ and the temperature decreases to 480 ℃ to deposit InAs. As soon as the amount of InAs deposition reaches 0.9 ML, a single laser pulse ( =355 nm, pulse duration ～ 10 ns) with an energy intensity of ～ 40.5 mJ/cm2 is in-situ introduced to irradiate the surface. Then, the sample is taken out and then its surface modification is immediately evaluated by atomic force microscope measurement. Atomic layer removal nano-holes elongated in the direction, and a surface density of ～2.0109 cm-2 are observed on the wetting layer. We attribute the morphology change to being due to laser-induced atom desorption. Because indium atoms should be easily desorbed away at substrate temperature of 480 ℃ during the laser irradiation, some vacancy defects are created. Then atoms adjacent to those defects would become weakly bounded, resulting in preferential desorption around the defect sites in sequence. Therefore, atomic layer removal is intensified by such a kind of chain effect and finally nano-holes are developed on the surface. In order to make clear how these nano holes of special kind influence the InAs/GaAs (001) QD growth, we perform another study by continuously depositing the InAs after the irradiation at the same thickness of 0.9 ML. It is found that when 1.7 ML InAs is deposited, QDs start to nucleate into some nano-holes and then are further deposited with an InAs coverage of 1.9 MLs, all the nano holes would be completely nucleated by QDs with a good uniformity, and there are no QDs in the remaining area. Such an effect of QD preferential nucleation in nano-holes could be explained by the following two causes. Firstly, adsorbed indium atoms tend to immigrate into nano-holes for lower surface energy induced by the concave surface curvature. The enhanced accumulation of Indium is in favor of the preferential nucleation of QDs in nano-holes. On the other hand, QD growth in areas outside the nano holes is depressed for indium desorption in pulsed laser irradiation process. In conclusion, our studies of in-situ laser-induced surface modification reported here provide a potential solution of controllable InAs/GaAs (001) QD growth.
