## Vol. 65, No. 10 (2016)

##### 2016-05-20

###### GENERAL

2016, 65 (10): 100201.
doi: 10.7498/aps.65.100201

Abstract +

A novel cylindrical composition-gradient electrode is considered to be one of most potential structures in lithium-ion battery. To investigate the mechanism of a cylindrical composition-gradient electrode under potentiostatic operation, we take Li1.2(Mn0.62Ni0.38)0.8O2 for example. The effects of the three main factors, i.e., diffusion coefficient, Youngs modulus, partial molar volume of solute, on the stress field in the cylindrical electrode are discussed. Each of the three material parameters is assumed to be a linear function of the distance from the center to surface. The small deformation theory and thermodynamic theory are employed to establish the mathematical model of composition-gradient cylindrical electrode. The mechanics equations and diffusion equation of cylindrical electrode are derived for an inhomogeneous material in plane strain condition. By comparing with single-phase electrode, it is found that Youngs modulus increasing from the center to the surface greatly reduces the max tensile radial stress and tensile hoop stress and changes the location of max radial stress since the radial displacement of the center is restricted. The time for the lithium-ions to reach the center is longer and the tensile stress near the center decreases at dimensionless time =0.0574 when diffusion coefficient decreases along the radial direction. Owing to the smaller diffusion coefficient at the surface, there is a reduction in the number of lithium-ions through the unit area in unit time when their corresponding concentration gradients are the same. The variation of partial molar volume means that the volume expansion caused by the intercalation of lithium-ions decreases along the radial direction. Therefore the partial molar volume decreasing along the radial direction considerably reduces the radial stress and the distribution of tangential stress becomes flat. The center point is picked, showing the development of hoop stress. The results show that the hoop stress increases and reaches a maximal value close to the dimensionless time =0.0574. Maximal tensile hoop stress at the center is reduced in an inhomogeneous material. The tensile hoop stress turns into compressive stress over time when elastic modulus and partial molar volume are denoted with E(r) and (r) respectively. The results indicate that the cylindrical composition-gradient electrode with core enriched Ni and edge enriched Mn reduces the max tensile radial stress and tensile hoop stress. It is an efficient way to avoid mechanical fracture in electrode since evidence has accumulated that tensile stress is the lead cause of crack in electrode. The result also provides useful guidance for mitigating the stresses in a cylindrical electrode.

2016, 65 (10): 100301.
doi: 10.7498/aps.65.100301

Abstract +

Entanglement and nonlocality, two most striking features of quantum mechanics, are fundamental resources for quantum information processing. They play an important role in quantum information processing. Therefore, studying the dynamics of quantum nonlocality and entanglement is of importance for both fundamental research and practical applications. In this paper we consider the case that three identical two-level atoms are trapped respectively in the three separated equidistance single-mode cavities, which are placed at the vertices of an equilateral triangle and are coupled by three fibers. Each atom resonantly interacts with cavity via a one-photon hopping. The evolution of the state vector of the system is given by solving the schrodinger equation when the total excitation number of the system equals one. The dynamics of nonlocality in the system is investigated via Mermin-Ardehali-Belinksii-Klyshko (MABK) inequality. By the numerical calculations, the MABK inequality is studied when the initial state vector of three atoms is W state or the initial state vector of three cavities is also W state. The influence of cavity-fiber coupling constant on the MABK inequality is discussed. The evolution curves of the MABK parameters Ba and Bc are plotted. The curves show that Ba and Bc both display periodic oscillations, and their oscillation frequencies all increase with the increase of cavity-fiber coupling constant. Ba and Bc are both larger than 1 when the scaling time gt takes a certain value. The results show that the quantum state of three atoms or that of three cavities displays nonlocality. On the other hand, the nonlocality of three-atom quantum state is strengthened with the increase of cavity-fiber coupling constant.

2016, 65 (10): 100302.
doi: 10.7498/aps.65.100302

Abstract +

Measurement-device-independent quantum key distribution (MDI-QKD) is immune to all detection side-channel attacks, thus when combined with the decoy-state method, it can avoid the actual security loophole caused by quasisingle- photon source simultaneously. A practical weak coherent source is used as a quasi-single-photon source in the current MDI-QKD experiments; it may contain percentage of vacuum-and multi-photon pulses. Moreover, in order to study how the performance of the threshold detector affects the quantum bit error rate (QBER), we introduce the quality factor (the ratio of the dark count rate to the detection efficiency) of the threshold detector. Here, through taking into account the weak coherent source, the quality factor of the threshold detector and the reflectivity of beam splitter, we deduce and evaluate the gain, the probability for successful Bell measurement, incorrect Bell measurement when Alice and Bob send pulses with different photon numbers which have a high probability to appear in weak coherent source, and then we obtain QBER in combination with the probabilities of different photon number states, besides, we also do some simulations. The simulations show how QBER varies with the reflectivity of beam splitter and the quality factor of the threshold detector when the average photon numbers per pulse from Alice and Bob are symmetric. Furthermore, the simulations show how QBER varies with the average photon number per pulse from Alice when average photon number per pulse from Bob is 0.1. Result shows that QBER is affected by the reflectivity of beam splitter, but QBER cannot reach the minimum value in Z basis encoding scheme when the average photon numbers per pulse from Alice and Bob are both 0.1 and the reflectivity of beam splitter is 0.5, which is different from X basis encoding and phase encoding. In addition, QBER increases with the increase of the quality factor of the threshold detector, which means that better performance of the threshold detector will reduce QBER. We show that QBER in Z basis encoding reaches the minimum value when reflectivity of beam splitter is 0.5 and there is large difference between in average photon number per pulse between two sides. In conclusion, for QBER, the effect from the reflectivity of beam splitter is equal to average photon numbers from the two arms only in X basis encoding and phase encoding. Our work will provide a reference for setting up a system with better performance.

2016, 65 (10): 100303.
doi: 10.7498/aps.65.100303

Abstract +

Quantum key distributions, which could make legitimate communication parties Alice and Bob achieve the same random key with unconditional security, will have broad applications in defense, commerce, and communication. The protocol of the continuous variable quantum key distribution (CVQKD) has many advantages, such as easy preparation of the light source, high detector efficiency, and good compatibility with the classic fiber-optic communication systems. In recent years, great progress in the research of CVQKD has been made both theoretically and experimentally. In the protocol, the quadratures of the optical field with Gaussian or Non-Gaussian modulation are employed as the carriers of the key.The quadratures of the pulsed optical quantum states in CVQKD can be detected with a time-domain pulsed homodyne detector. The performance of the detector has great influences on the excess noises and the safe key rate of the quantum communication system. The measurement accuracy, which depends crucially on the common mode rejection ratio and the long-term stability, is the key performance of the detector. In order to improve the accuracy of measurement and avoid saturating the detector, we propose and demonstrate a technique to balance the two output beams of a 50/50 fiber coupler of the homodyne detector automatically. The auto-balance technique, which improves the long-term stability and high common mode rejection ratio, is described in the following.Firstly, the relation between the balance degree and the measurement accuracy is theoretically analyzed in detail. The result shows that a balance degree larger than 10-4 should be reached to ensure a high precision measurement when the intensity of the local oscillator pulse is 108 photons per pulse. Secondly, a fiber-based variable attenuator based on computer-controlled linear stepper motor is designed. The linear stepper motor that is used to drive the fiber coils has a small dimension of 20 cm20 cm28 cm and a minimum step size of 78 nm, and is controlled through the I/O port of a multifunction data acquisition card connected to a computer. The attenuations of the fiber coils of different radii are detected. The precision of attenuation is estimated to be on the order of 10-6 per 100 nm.The principle of the feedback control is described. A method of changing step-size which depends on the balance degree is proposed to fulfill a fast auto-balance process. Using the auto-feedback-control system, a balance degree of about 1.5610-6 can be achieved. The procedure of auto-balance takes about 1 s, and the evolution curves that represent the transformation process from various unbalanced states to the balanced state are presented.The auto-balance apparatus can ensure that the time-domain pulsed homodyne detector run stably in a longterm with a high common mode rejection ratio. The nonlinear and saturation effects due to the drift of the balance point are eliminated. The presented auto-balance time-domain pulsed homodyne detector can be well integrated into the continuous variable quantum key distribution system, and is expected to play an important role in improving the measurement accuracy and reducing the excess noises of the system. We believe that it could also be found to have potential applications in other areas.

2016, 65 (10): 100501.
doi: 10.7498/aps.65.100501

Abstract +

The studies of extended dynamics systems are relevant to the understanding of spatiotemporal patterns observed in diverse fields. One of the well-established models for such complex systems is the coupled map lattices, and several features of pattern formation including synchronization, unsynchronization, traveling waves and clustering synchronization are found. Among the above-mentioned patterns, chaotic synchronization has been intensively investigated in recent years. It has been demonstrated that two or more chaotic systems can be synchronized by linking them with mutual coupling or a common signal or some signals. Over the last decade, a number of theoretical methods have been presented to deal with this problem, such as the methods of master stability functions and eigenvalue analysis. While much effort has been devoted to the networks with different topological structures in continuous systems. The coupled discontinuous maps have been investigated with increasing interest in recent years, they showed that the complete synchronization in coupled discontinuous systems is more complicated than in coupled continuous systems. However, a similar problem of synchronization transition in coupled discontinuous systems is much less known.The synchronization transition in coupled discontinuous map lattices is studied. The average order parameter and maximal Lyapunov exponent are calculated to diagnose the synchronization of coupled piecewise maps. The results indicate that there exist the periodic clusters and the synchronization state, and a new transition style from periodic cluster states to complete synchronization states is found. The periodic cluster states consist of two kinds of periodic orbits: symmetric periodic orbits and asymmetric periodic orbits.Based on the pattern analysis, the common features of the patterns are constructed by the two periodic attractors, and the periodic orbit is an unstable periodic orbit of the isolate map. The discontinuities in a system can divide the phase space into individual zones of different dynamical features. The interactions between the local nonlinearity and the spatial coupling confine orbit into certain spaces and form a dynamic balance between two periodic clusters. The system can reach complete synchronization states when the balance is off. It is shown that synchronization transition of the coupled discontinuous map can exhibit the different processes, which depends on coupling strength. Four transition modes are found in coupled discontinuous map: 1) the transition, from the coexistence of chaotic synchronization and chaotic un-synchronization states to the coexistence of chaotic synchronization, chaotic un-synchronization, symmetric periodic orbits and asymmetric periodic orbits; 2) the transition from the coexistence of chaotic synchronization, chaotic un-synchronization, symmetric periodic orbits and asymmetric periodic orbits to the coexistence of chaotic synchronization, symmetric periodic orbits and asymmetric periodic orbits; 3) the transition from the coexistence of chaotic synchronization, symmetric periodic orbits and asymmetric periodic orbits to the coexistence of chaotic synchronization and symmetric periodic orbits; 4) the transition from the coexistence of chaotic synchronization and symmetric periodic orbits to the chaotic synchronization. Because the local dynamics has discontinuous points, the coupled system shows a riddle basin characteristic in the phase space, and the synchronization transition of coupled piecewise maps looks more complex than continuous system.

2016, 65 (10): 100601.
doi: 10.7498/aps.65.100601

Abstract +

Accurate measurement of absolute distance is crucial for developing the progressive military, aerospace, manufacturing large scientific instruments and other fields. Instead of the traditional phase discrimination scheme in general phase-shift distance measurement, the double polarization modulation range-finding system can simplify the simulation phase circuit, reduce the phase shift noise and improve the accuracy by using interference phase demodulation. The in-depth discussion of theoretical derivation and experimental verification are proposed based on the double polarization modulation range-finding system. The detailed theoretical analysis of optical structure is proposed, and the factors affecting the range accuracy are analyzed based on the theoretical formula of ranging result. Finally, the theoretical formula of range accuracy is obtained, and experimental validation is carried out. In this experiment, the wavelength of laser source is 735 nm, and the phase modulator is 4431 model from the Newport company. The ranging experiment is conducted in the modulation frequency ranges of 0.75-0.85 GHz, 2.7-2.8 GHz, 4.3-4.4 GHz, 6.1-6.2 GHz, and 7.8-7.9 GHz. Experimental results indicate that the measurement accuracy of phase-shift range-finding technology is improved with the increase of modulation frequency. Moreover, the accuracy is proportional to the parameter f/N, in which f is the modulation frequency uncertainty and N is an integer in our algorithm. With the appropriate modulation frequency, the range accuracy of the double polarization modulation range-finding system can reach up to 10-7.

2016, 65 (10): 100701.
doi: 10.7498/aps.65.100701

Abstract +

Bridgman anvil is a useful and effective tool in high-pressure research. However, in this apparatus, the pressure distribution is essentially centrosymmetric. Thus, considerable pressure gradients exist in the gasket and in the sample chamber respectively, and the uniform pressure area is limited. To improve the pressure uniformity in flat face supported anvils, we design a strip face anvil instead of conventional round face anvil and adopt an assortive strip gasket. Principle analysis and a series of pressure calibration experiments are also presented in this paper.The construction of the strip anvil and relevant parts of the apparatus are shown in the diagrams and photos. The relationship between pressure and shearing stress in the strip gasket is investigated by using the model of M. Wakatsuki, which indicates that the pressure distribution should be uniform along the central line of the strip gasket.Pressure calibration experiments are conducted by using strip anvils made of tungsten carbide with a length of 20 mm and width of 5 mm and by using the assortive strip gasket of pyrophyllite. Pressures at different places of the central line are calibrated according to the known phase transitions of bismuth in the same loading process, and the samples are assembled with symmetrical, unsymmerical, and separated local collocations, respectively.Experimental results exhibit that the pressure reaches up to 10 GPa in the central line of the strip gasket, and the pressures are almost equal at least within the range of 12 mm on the central line. The bias errores of oil pressures measured at different places of the central line are all less than 2.0% at 2.55 GPa and 3.6% at 7.7 GPa, indicating only a small pressure gradient along the central line. The main reason for the measuring bias errors lies in the difficulty of the assembly technique. Specifically, the bismuth wire is difficult to adhere to the central line of the anvil during compression. Hence, further improvement of the process is expected in the future.In conclusion, the strip anvil is a unique high-pressure apparatus. The principle analysis and pressure calibration experiments confirm that the pressure is uniform in one-dimensional direction along the central line of the strip anvil. This feature is propitious to the accurate investigation of linear samples under high pressure.

###### ATOMIC AND MOLECULAR PHYSICS

2016, 65 (10): 103101.
doi: 10.7498/aps.65.103101

Abstract +

D-A type copolymer as an organic polymer solar cell electronic material in recent years has attracted wide attention. In order to improve the efficiency of energy conversion, many active layer materials, especially the donor materials, have been designed and synthesized. By inducing the different donor and acceptor units, the absorption spectrum can better match with the solar spectrum and the carrier mobility can increase. In this paper, by using the density functional theory method, we investigate the electronic structures and optical absorption spectra of D-A and D--A copolymers. Benzodithiophene (BDT) as the electron donor unit, and dibenzothiophene (BT) as the electron acceptor unit are used to simulate D-A (PBDT-BX, X = O, S, Se, Te) copolymer systems; and D--A (PBDT-DTBX, X = O, S, Se, Te) structures are constructed with thiophene ring as a bridge between D and A. Firstly, our calculation results indicate that when X is replaced separately by elements O, S, Se and Te in D-A copolymers, the LUMO levels move close to the Fermi level, while the changes of the HOMO energy levels are relatively small, resulting in the band gap decreasing gradually. Then, the analysis of the density of states (DOS) shows that the contribution of LUMO comes from the BT unit and HOMO from the BDT unit. Also the difference in charge density shows that the thiophene ring enhances the charge transfer between BT and BDT. Besides, the studies of the optical absorption spectrum reveal that there appear two strong absorption peaks with the increase of atomic number of X, of which one is at about 4.0 eV and has no obvious change, and the other increases intensively and is red-shifted. Moreover, compared with the D-A structure, the D--A structure has the band gap that will decrease obviously and has a lowest value when X is Te. The optical absorption peak also increases significantly as the atomic number of oxygen group elements increases and peak position is red-shifted. The range of optical absorption peak is mainly from 703.9 to 519.4 nm. According to the absorption spectrum and DOS the optical absorption peak at about 4.0 eV is mainly contributed by the BDT unit. Overall, our findings provide a good understanding of mechanism about the red-shift of optical absorption spectra of copolymers and can serve as guidance for organic polymer design in photovoltaic cell experimentally.

2016, 65 (10): 103201.
doi: 10.7498/aps.65.103201

Abstract +

Rydberg atoms, with large principal quantum number n, have been widely investigated in recent years due to their peculiar properties, such as big sizes, long lifetimes and strong interactions with fields and other Rydberg atoms. Rydberg atoms are very sensitive to external fields due to their large polarizabilities scaling as n7.These make Rydberg atoms an ideal candidate for the quantum information, the many-body interaction, etc.In this work, we investigate the Rydberg atoms using electromagneticlly induced transparency (EIT) in a ladder three-level system. The EIT is a quantum interference effect between two excitation path-ways driven by two laser fields. The main idea is performed in a room temperature cesium vapor cell, where the probe laser frequency is modulated. The ground state (6P1/2), excited state (6P3/2), and Rydberg state (nS1/2) constitute a Rydberg three-level system, in which the probe laser is fixed to the 6S1/2 (F = 4)6P3/2 (F = 5) transition by saturated absorption spectrum technique, whereas the coupling laser is scanned across the 6P3/249S1/2 transition. We detect the demodulated EIT signal with the lock-in amplifier (SR830). The modulated EIT signal shows a two-peak structure. The measured spacing between two peaks increases with the frequency detuning, caused by the modulation amplitude, and half the spacing between the peak-to-peak is nearly 1.67 times the modulation amplitude of the probe laser; the measured result shows that the splitting is independent of the modulation frequency. The experimental results are in agreement with the theoretical calculations. The results in our work can be used for real-time monitoring of the laser-line profiles and the fluctuation of laser frequency.

2016, 65 (10): 103202.
doi: 10.7498/aps.65.103202

Abstract +

The short attosecond (as) pulse is a basic tool for probing the ultra-fast electronic dynamics in matter. High-order harmonic generation (HHG) of atoms exposed to intense laser field is the most promising method of producing the short attosecond pulses. Therefore, the generation of ultra-short attosecond pulses through HHG has been of great interest. How to obtain the ultra-short pulse from HHG has been a hot research subject in recent years. In the present paper, we investigate the characteristic of HHG from atoms with both resonant and non-resonant structure (for short, the general atom) by using numerically solving a one-dimensional time-dependent Schrodinger equation of atom driven by two-color field (infrared (IR) laser + extreme ultraviolet (XUV)). We find that the HHG spectra from resonant atom are obviously different from those of the general atom. For a resonant atom, besides the great increase of the intensity of HHG at some energy (resonant energy + ionized energy), the intensity of HHG at the central frequency of XUV pulse is sensitive to the intensity of XUV pulse. Even the intensity of XUV pulse is weak, the enhancement of HHG spectra from resonant atom is still significant, while the general atom does not has this feature. Only the strength of the XUV pulse is much stronger than that in the case of resonant atom, the spectra of HHG near the center frequency of XUV from atom with non-resonant structure can significantly be enhanced. More importantly, adjusting the time delay of two-color laser pulse makes the width of input XUV pulse compressed obviously in the case of the resonant atom. By performing the time-frequency analysis of Morlet transform, we explain the compression of the attosecond pulse. The reason is that the relation of the input XUV pulse frequency to the resonant frequency of HHG for resonant atom makes the bandwidth of HHG in the region of the center frequency of XUV wider than that of the input attosecond pulse during the emission. Thus, we can obtain shorter pulse by superposing several orders HHG among the enhanced regions. Finally, we propose a way to compress the width of the input XUV pulse by using filter-multi-feedback method. Based on our scheme, the width of the input XUV pulse can be compressed from 200 as to 120 as, thereby offering a new method of obtaining shorter attosecond pulse in experiment.

## EDITOR'S SUGGESTION

2016, 65 (10): 103401.
doi: 10.7498/aps.65.103401

Abstract +

In recent years, the guiding effect of highly charged ions (HCIs) through insulating nanocapillary membrane has received extensive attention. It is found that slow highly charged ions at keV energies can be guided along the capillary even when the title angle of membrane is a few degrees and larger than geometry opening angle of the capillary. Initially, Stolterfoht et al. (2002 Phys. Rev. Lett. 88 133201), according to the incident ions deposit positive charges on the capillary surface in a self-organizing manner, proposed scattering and guiding regions to explain this guiding phenomenon. Hereafter, a detailed experiment and simulation performed by Skog et al. (2008 Phys. Rev. Lett. 101 223202) provided clear evidence that the guiding process is actually attributed to the self-organized charge patches formed on the inner capillary walls. HCIs entering into a capillary may hit the surface, leaving their charge on the inner wall of the capillary. When the capillary axis is tilted with respect to the beam incidence direction, a charge patch is formed in the capillary entrance, simultaneously a repulsive electric field is created. After sufficient charge deposition this field is strong enough to deflect the subsequent ions in the direction of the capillary exit. Therefore, the ions are guided through the capillary. The deflection at the charge patch only occurs at relatively large distances from the capillary wall so that the incident charge state of the ions is kept during the passage through the capillary. Further experimental and theoretical studies on various target materials, such as polyethylene terephthalate (PET), polycarbonate (PC), SiO2, and Al2O3 indeed found that a single or a small number of charge patches near the entrance formed in the charge up process dominate the observed oscillatory variations of the ion emission angle and the final guiding process. Besides, measurements and simulations of the steering of swift ions at MeV energies have shown that the transmission mechanism of the high energy ions in a tapered tube is primarily dominated by multiple random inelastic collisions below the surface and the charge patches are not responsible for the transmission process. However, the studies of the transmission of hundreds keV ions through nanocapillaries are still lacking so far. In this work, we observe the evolution of the angular distribution, charge state distribution, FWHM and transmission rate of 100 keV H+ ions incident on a polycarbonate (PC) membrane at +1 tilt angle. It is found that the transmitted particles are located around the direction along the incident beam, not along the capillary axis, which suggests that the mechanism of hundreds keV (E/q~100 kV) protons through capillaries is significantly different from that for the guiding effect of keV protons. We present a qualitative explanation based on the data: that the 100 keV H+ are transmitted by multiple random inelastic collisions below the surface is attributed to the absence of the deposited charges on the surface of the capillary at the beginning of the experiment. After the equilibrium, several charge patches are formed on the inner wall of the capillary, which suppresses the ions to penetrate into the surface of the capillary, while the H+ is transmitted via specular scattering above the surface (or closest to the surface) assisted by the charge patches, and finally is emitted in the incident direction through twice specular scattering. This finding increases the knowledge of charged ions through nanocapillaries, which is conducible to the applications of nanosized beams produced by capillaries or tapered glass within hundreds keV energies in many scientific fields.

###### ELECTROMAGNETISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS

Broadband circularly polarized high-gain antenna design based on single-layer reflecting metasurface

2016, 65 (10): 104101.
doi: 10.7498/aps.65.104101

Abstract +

According to the Pancharatnam-Berry phase principle, a single-layer reflecting element is proposed for steering the phase of the electromagnetic wave. The structure of the element is composed of metabolic cross wire and copper ground sheet, which are separated by an FR4 dielectric substrate with a thickness of 3 mm. When the incident wave is circularly polarized, different rotary angles of the element are used to achieve the co-polarization reflection with high efficiency in a broadband of 11-16 GHz. In the design of the focusing metasurface, the phase compensation for forming a constant aperture phase is provided by the individual reflected element with a different rotated angle. Remarkably, the size of the element is only 5 mm (0.230), and then it can be more accurate to control the phase of the array. The focusing metasurface is composed of 1515 elements with a focal length of 30 mm at 13.5 GHz. The designed sample is simulated in CST Microwave Studio. The results show that the incident circularly polarized plane wave is well transformed into a spherical wave in the band from 11 to 16 GHz, and the focal length is around 30 mm. For further application, a unidirectional Archimedean spiral antenna is located at the focal point of the metasurface. According to the reversibility principle of electromagnetic wave propagation, the spherical wave radiated by the feed antenna is converted into a plane wave by the reflecting metasurface, so that the antenna gain is remarkably enhanced. Through adjusting the distance between the feed antenna and the focusing metasurface, we find that 28 mm is the best distance. Finally, the feed antenna and the metasurface are fabricated, assembled and measured. Numerical and experimental results are in good agreement with each other, showing that the -1 dB gain bandwidth of the high-gain antenna is 12.5-16 GHz, and in this band the peak gains are all over 19 dBc and the axial ratio is better than 3 dB. In addition, the aperture efficiencies are more than 50% in the band from 12 to 15.5 GHz, especially the efficiency at 13 GHz reaches a highest value of 61%. The good performances indicate that the proposed high-gain antenna has a highly promising application in portable communication systems.

2016, 65 (10): 104201.
doi: 10.7498/aps.65.104201

Abstract +

Optical precursors were first studied by Sommerfield and Brillouin in 1914 to resolve the apparent contradictions between fast light propagation and the theory of relativity. They showed theoretically that the front edge of a step-modulated pulse does not interact with the medium and always travels at c because the dispersive material has a finite response time to the optical pulse. The past experimental studies of precursors in classical pulse propagation were always focused on an opaque medium with single or multiple Lorentz absorption lines. In these cases, the precursor signal cannot be separated from the main pulse or otherwise the main field is absorbed. However, the electromagnetically induced transparency (EIT) technique was successfully used to separate precursors from the main pulse due to the slow-light effect in cold atoms. The EIT refers to the absorption suppression or elimination of a probe field through atomic coherence in a certain medium dressed by a strong coupling field. In this paper, a four-level double-lambda atomic system with two upper states coupled to the excited state is explored to separate optical precursors from a square-modulated laser pulse with the effect of spontaneously generated coherence (SGC). The SGC effect occurs in the process of spontaneous emission, in which the atom decays from closely placed upper levels to a single ground level. The quantum interference between the decay channels takes place, which leads to decay induced transparency, thus enhancing the Kerr nonlinearity and amplification without inversion. With the assistance of spontaneously generated coherence, an EIT window appears with steep normal dispersion when the trigger field is far from resonance. Then we can obtain the optical precursors which are separated from the main pulse due to the slow-light effects in the EIT window. In the absence of SGC, the main pulse is absorbed by an opaque medium with Lorentz absorptive lines, so the slow-light effect could not take place. In addition, we obtain the stacked optical precursors with the input probe field amplitude or phase modulated by designing a series of square pulses. For the amplitude modulation case, the peak power reaches about 4.5 times that of the input pulse. With the phase modulation we obtain a transient pulse with a peak power of 14 times that of the input, as a result of constructive interference between the stacked precursors and main field. We expect these findings to be instructive in devising optical devices for optical communication, detection and medical imaging among other applications.

2016, 65 (10): 104202.
doi: 10.7498/aps.65.104202

Abstract +

In the process of preparing nanosilicon, the crystallization process is an important part to influence and improve the efficiency of nanosilicon luminescence. Thermal annealing, laser annealing, and electron beam irradiation are different ways of crystallizing the nanosilicon. Different photoluminescence (PL) spectra and structures of nanocrystalline silicon are observed for different treatment time of crystallization. The experimental results show that choosing an appropriate crystallization method and parameters is very important for preparing the nanosilicon crystalline structures. High luminous efficiency can be obtained by controlling the parameters properly in the processes of preparing silicon quantum dots (QDs) and quantum surface, especially. It is discovered experimentally that better nanosilicon crystalline structure such as nanosilicon QD structure, better PL luminescence, and the doped localized state luminescence of nanocrystalline silicon can be obtained when the crystallization time is about 20 min. According to the nanosilicon crystallization process under thermal annealing, laser annealing and electron beam irradiation, a physical model of the effect of crystallization time on the nanosilicon localized state luminescence is established in this paper, which can explain the effect of crystallization time on the localized state luminescence of the nanosilicon.

2016, 65 (10): 104203.
doi: 10.7498/aps.65.104203

Abstract +

Owing to damage, thermal issues, and nonlinear optical effects, the output power of fiber laser has been proven to be limited. Beam combining techniques are the attractive solutions in order to achieve high-power high-brightness fiber laser output. Designing such a high-power laser system relies on coherent and incoherent combination of radiation from multiple laser channels into a single beam with enhanced brightness. Spectral beam combination is a promising alternative way that allows each array to be overlapped in near-and far-field without spatial interference, thus relaxing the requirements for linewidth controlling and phase locking of individual array and practically allowing power and brightness to be scaled with the potential to combine a large number of channels. Spectral beam combination implementations can be divided into two subsets: serial and parallel, based on the combining elements. For scaling high power, we pursue spectral beam combining with parallel subsets as an alternative to other beam combination implementation. In the spectral beam combining system based on multi-layer dielectric grating, the combined beam suffers the degradation in beam quality, which is caused by the optical dispersion, and also by the random error due to the misalignment of arrays or the thermal-optic effect of grating in the experimental system. In this paper, we strictly derive the equation of M2 variation caused by the optical dispersion in both single-grating structure and dual-grating structure. And also, we discuss how the laser linewidth, beam size, spectral separation of two adjacent channels, distance between two adjacent channels and the period of grating influence the desired beam quality in detail, separately, in the single-grating structure and the dual-grating structure. The results show that with the value of M2 fixed, the finite beam size gives rise to a laser bandwidth decreasing in single-grating structure combination, whereas the beam size induces a laser bandwidth to increase in dual-grating structure combination. If M2 1.2, the laser bandwidth of dual-grating system can be over several sub-nanometers, rather than several tens of pm as in the single grating design.

2016, 65 (10): 104204.
doi: 10.7498/aps.65.104204

Abstract +

Thermal effect in the gain fiber is one of the main factors which restrict the power improvement of high power fiber amplifiers. Previous studies have shown that the temperature distribution is closely related to the doping concentration along the gain fiber. In order to reduce the maximum temperature of the gain fiber, we propose to use doping concentration varying along the gain fiber as a method to disperse the thermal effect of the fiber laser and improve the laser output power. Based on the rate equation model and thermal conduction model, the thermal distributions and output powers of several different gradient doping gain fibers are simulated in the cases where the output powers are approximately the same. Our study shows that compared with the conventional constant doping gain fiber, linear doping of the rare earth ion along the gain fiber can reduce the maximum temperature of the gain fiber as well as the temperature of the fusion point greatly, thus improving the stabilities of the fusion point and the fiber laser amplifier. In the case of cosinoidal doping, the gain fiber can not only reduce the temperature of the fusion point but also make the temperature have a periodic distribution along the gain fiber, which can suppress the stimulated Brillouin scattering effect effectively. The exponential doping of the gain fiber can also reduce the maximum temperature and the temperature of the fusion point, which is beneficial to the further scaling of the fiber laser output power. At the same time, it can make the gain of the signal light have a uniform distribution along the gain fiber, which suppresses the mode instability effect and improves the output beam quality of the fiber laser. These conclusions also hold true when the pump power changes. Therefore, the gradient doping of the gain fiber proposed in this paper can optimize the temperature distribution along the fiber and improve the stability of the fusion point. Besides, it can improve the beam quality of the output laser and suppress the nonlinear effect and mode instability effect. The results indicate that the gradient doping of the gain fiber is an effective and feasible way to improve the output power of fiber amplifier. Last but not the least, it is possible to produce the gradient doping gain fiber by the laser heated pedestal growth method and the direct nanoparticle deposition technique. The investigation can present a reference for designing the gain fiber in high-power fiber laser systems.

2016, 65 (10): 104205.
doi: 10.7498/aps.65.104205

Abstract +

As the weak ambient energy is hard to be stored directly and rapidly and unable to drive the electronic load into working properly, a high-efficiency energy storage circuit, with quartz crystal serving as a storage element, is presented. When an alternating electric field is applied to it, the quartz crystal will generate mechanical oscillations of a certain frequency. Since the quartz crystal possesses a high quality factor, in the piezoelectric crystal plate there appears a severe mechanical resonance with a small excitation voltage. In the resonant condition, the external weak electrical energy can be converted into mechanical energy stored in the quartz crystal. The principles of quartz crystal energy storage and instantaneous energy discharge are theoretically analyzed. The relationships between the output voltage and the time and between the maximum instantaneous output power and the load in the processes of quartz crystal charging and discharging are deduced, respectively. The storage characteristics of the quartz crystal are investigated experimentally. The experimental results show a good accordance with the theoretical analysis. A quartz crystal of 1 MHz resonant frequency is adopted in this research. When the input voltage amplitude of the energy storage circuit is 100 mV, the optimal matching load is 820 and the maximum instantaneous output power of quartz crystal discharging circuit is 150 W. The storage efficiency and the release efficiency of the quartz crystal can reach up to 77% and 71.4% respectively. These results provide evidence for quartz crystal energy storage in the condition of weak ambient energy.

2016, 65 (10): 104301.
doi: 10.7498/aps.65.104301

Abstract +

The propagations of one-dimensional nonlinear acoustical waves are studied numerically and experimentally in this paper. The finite element method (FEM) is used to simulate the propagations of nonlinear acoustic waves. The FEM equation of one-dimensional nonlinear acoustic wave is derived according to the theory of nonlinear acoustics. A three-dimensional matrix appearing in the nonlinear FEM equation represents the nonlinear part of the nonlinear acoustic equation and indicates the complex propagation characteristics of nonlinear acoustic waves. However, there is no corresponding matrix in the linear FEM equation. The matrix correlates with the nonlinear properties of propagating waves such as wave distortion, high order harmonic wave generation and transformation of energy from basic frequency to high order harmonic frequency, etc. Then, an FEM program is coded to compute the propagations of the one-dimensional nonlinear acoustic waves. The results show that the nonlinear acoustic waves are distorted obviously during propagation. After fast Fourier transform processing the original wave signal, the basic frequency signals and high order harmonic signals both appear in the frequency-region signals. To prove the correctness of the FEM results, nonlinear acoustic experiments in water are carried out under different conditions. In the first experiment, the distance between the transmitting and receiving transducers is kept unchanged, but the transmitting transducer is excited with different energies. So with propagation distance fixed, the influences of different exciting energies on the nonlinear properties of acoustic waves are obtained from this experiment. In the second experiment, with the exciting energy fixed, the influences of different propagation distances on the nonlinear properties of acoustic waves are obtained by changing the distance between the transmitting and receiving transducers. Then the numerical results and the experimental results are compared and analyzed carefully. The result shows that the waveforms and the spectra of simulated nonlinear waves are in good agreement with those of experimental signals. These results prove the correctness of the proposed numerical method. It is also noticed that the propagation properties of basic frequency wave and the second order harmonic waves are different. The amplitude of basic frequency wave decreases gradually, but the amplitude of second order harmonic wave first increases and then decreases after propagating some distance. The amplitude of the second harmonic wave changes with propagation distance and energy of the input source amplitude. The relationship between the amplitude of second harmonic wave and the propagation distance is numerically fitted. We find a fitting equation of the relation between high order harmonic acoustic wave and propagation distance, which also brings clear physical meaning for the high order harmonic waves. Finally, the properties of nonlinear acoustic wave propagation in solid are preliminarily discussed. This study provides theoretical and experimental evidence for the nonlinear acoustic wave traveling in liquid.

2016, 65 (10): 104302.
doi: 10.7498/aps.65.104302

Abstract +

An approach to passive impulsive source range estimation in shallow water is proposed. The approach is based on warping transformation of the energy density function of the received signal. Because of the influence of the sea bottom, it is difficult to find a warping operator adapted to the dispersion characteristics of modes in shallow water. Even though the modes can be separated from each other by using the warping operator adapted to the ideal waveguide, it is impossible to obtain the analytic solutions of the characteristic frequencies, while the energy density function of the received signal is not affected by the sea bottom. Like the received signal and the autocorrelation function of the signal, the frequency spectrum of the warped energy density function of the received signal also owns invariable frequency features. These characteristic frequencies equal the difference in the cut-off frequency between two modes in ideal waveguide, which are easy to calculate with the knowledge of the depth and the average sound speed of the water. What is more, the warping operator transforms mode pairs in energy density function with the same mode number difference into one monotone, which means one characteristic frequency is not unique for one mode pair. In shallow water, the acoustic field is typically composed of a group of modes with close mode numbers. Therefore, the smaller the mode number difference, the more the mode pairs, and the higher the spectral peak of the corresponding monotone is. When the source range is unknown, the approximate relation formula between the extracted characteristic frequency in a supposed source range and the real characteristic frequency is derived, based on which a fast passive source range estimation method is proposed. The proposed method successfully avoids using the guide source and the calculation of replica field, which is necessary in existing passive range estimation algorithms. And applying warping operator to the energy density function of the received signal makes it easy to obtain the analytic solutions of the characteristic frequencies, which is impossible in previous researches. The method is successfully applied to the Yellow Sea impulsive signal data collected by a single hydrophone in November 2011. The mean relative error of range estimation is less than 8%.

2016, 65 (10): 104303.
doi: 10.7498/aps.65.104303

Abstract +

Adaptive beamforming is widely used in the fields such as radar, sonar, wireless communication to estimate the parameters of the signal of interest (SOI) at the output of a sensor array by data-adaptive spatial filtering and interference suppression. The standard Capon beamformer (SCB) is a typical adaptive beamforming approach which provides a superior performance by minimizing the array output power while simultaneously maintaining the array response under the assumption of distortionless direction of arrival (DOA). However, the advantages in performance of SCB are obtainable only when the number of snapshots available for the sample covariance matrix estimation is large enough and the direction of the SOI is known accurately. When applied to practical situations where the aforementioned two requirements are not satisfied, SCB will suffer high sidelobe levels and performance degradation in the parameter estimates due to lack of measurements and mismatch in the steering vector.A sparsity-constrained Capon beamformer (SCCB) arises to alleviate these problems. Unlike SCB, the constraint in SCCB is composed of two parts: the original array output power constraint part and the sparse constraint part (?1 norm constraint, encouraging sparse distribution in the array responses). However, if the sparse constraint in SCCB is set too large compared with the array output power constraint part, the responses in the directions of interferences will be influenced, and a tradeoff between the ability to reduce the sidelobe levels and the ability to reject the interferences must be made. Thus, based on the SCCB, a new robust Capon beamformer utilizing a weighted sparse constraint is proposed in this paper. In the proposed method, the sparse constraint part is replaced by a weighted sparse constraint, which is applied only to the sidelobe regions of the beampattern. By doing so, the number of the non-zero elements in the sidelobe response is minimized, resulting in an enhanced mainlobe region and suppressed sidelobe ones.In sparse recovery, the sparse constraint (the l1 norm constraint) does not necessarily enforce democratic penalization, which means that larger coefficients are penalized more heavily than smaller coefficients. Based on such a consideration, a weighting matrix can be constructed to put larger weights in the interferences directions to discourage their responses, and put smaller weights to maintain the responses in the remaining parts of the sidelobe regions. In this paper, the weighting matrix is obtained by utilizing the orthogonality between the signal subspace and the noise subspace. Since the steering vectors corresponding to the interferences and the SOI span the same space as the signal subspace, the inner products between the steering vectors in the interference directions and the noise subspace will produce zeroes ideally. By taking the reciprocals of these inner products, large values will yield in the interference directions while small values are obtained in other directions in the sidelobe regions. Using these values as the weights to the sparse constraint, a beampattern with deeper nulls, lower sidelobes, and better robustness to steering vector mismatch is obtainable as compared with SCB and SCCB. Besides, the output SINR is also effectively improved. Numerical simulations and a water-tank experiment are conducted to demonstrate the effectiveness of the proposed method.

2016, 65 (10): 104401.
doi: 10.7498/aps.65.104401

Abstract +

Naocrystalline (nc) material shows lower thermal conductivity than its coarse grain counterpart, which restricts its engineering applications. In order to study the effects of grain size and grain boundary on the thermal conductivity of nc material, nc copper is prepared by the high pressure sintering method. The pure nc Cu powder is used as the starting material, and the high pressure sintering experiment is carried out under a DS614 MN cubic press. Prior to the high pressure sintering experiment, the Cu powders are first pre-compressed into cylinders, then they are compressed under 5 GPa at temperatures ranging from 700 to 900 ℃ for 30 min. The grain size and micro-structural characteristics are investigated by the scanning electron microscope (SEM) and X-ray diffraction (XRD). The results show that the sintered Cu bulk material can achieve nearly full densification with a relative density of 99.98% and the grain growth of the Cu particles is effectively inhibited. The thermal conductivity measurement is performed by NETZSCH LFA-427 at 300 K and 45% RH. The test results show that the thermal conductivity of nc copper is lower than that of its coarse grain counterpart, and the thermal conductivity increases with grain size increasing. For example, as the grain size increases from 390 to 715 nm, the corresponding thermal conductivity increases from 200.63 to 233.37 Wm-1K-1, which are 53.4% and 60.6% of the thermal conductivity of the coarse grain copper, respectively. For a better understanding of the effects of grain boundary and size on the thermal conductivity of nc material, a simple modified model, with special emphasis on the contributions of electron and phonon conduction, is presented by incorporating the concept of the Kapitza resistance into an effective medium approach. The theoretical calculations are in good agreement with our experimental results. The combination of experimental results and theoretical calculations concludes that the thermal conductivity of nc material is weakened mainly by two factors: the grain boundary-electron (phonon) scattering on the grain boundary and the electron (phonon)-electron (phonon) scattering in the grain interior. That is to say, the thermal resistance of nc material can be divided into two parts: one is the intragranular thermal resistance from the grain, the other is the intergranular thermal resistance from the grain boundaries. As is well known, when the grain size decreases to a nano-range, the volume fraction of the grain boundary presents a sharp increase, and the intergranular thermal resistance from the grain boundaries becomes more important.

2016, 65 (10): 104501.
doi: 10.7498/aps.65.104501

Abstract +

As a typical energy dissipation system, granular material acts as a buffer under the action of impact load, with absorbing and dissipating energy effectively through the sliding friction and viscous contacts between particles. In this paper we study the buffer capacity of granular material under impact load, by the discrete element method (DEM). The spherical elements are filled randomly into a rigid cylinder under the action of gravity. A spherical projectile with a certain initial velocity drops into the granular bed from a given height. The impact loads on the projectile and the rigid bottom plate of cylinder are both obtained with DEM simulations. The simulated impact loads on the bottom plate are compared well with the physical experiment data. The influences of granular thickness, sliding friction and initial concentration on buffer capacity are investigated under the impact of spherical projectile. The DEM results show that granular thickness H is a key factor for buffer capacity. In the DEM simulations, the impact load on bottom plate presents unique characteristics under various granular thickness values. With granular thickness increasing from zero, a transition from one peak to two peaks takes place, then the two peaks return to one peak in the time curve of impact load. The evolution of impact load peak with its temporal interval is discussed. A critical thickness Hc is obtained. The impact force decreases with the increase of granular thickness when H Hc, but is independent of the granular thickness when H Hc. Moreover, the impact forces are simulated under various sliding friction coefficients and initial concentrations. It is found that the smooth and loose granular material has more effective buffer capacity. Finally, the spatial structures of force chains and the distribution of impact forces on bottom plate are discussed to reveal the mechanism of buffer properties of granular material on a micro scale.

2016, 65 (10): 104701.
doi: 10.7498/aps.65.104701

Abstract +

In the frame of the Harrison bonded-orbital method, the variations of the force constant, the Young modulus, the torsional modulus and the phonon frequency with temperature are given through the relevant theory or method of the solid state physics with considering the non-harmonic effect and the short-range interaction of atoms. Results show that the force constant, the Young modulus, the torsional modulus, the phonon frequency and the Poissons coefficient all vary with temperature. The results show that the first three quantities increase with temperature but not very much; the phonon frequency increases with temperature rapidly; the Poissons coefficient decreases fast with the increase of temperature. There are transverse vibrations along the direction perpendicular to the bond-length direction and the longitudinal vibrations along the bond-length direction, in which the longitudinal vibrations are dominant. The nonharmonic effect of the longitudinal vibration is much larger than that of the transverse vibration. The first and the second non-harmonic coefficient of the transverse vibration are both much less than those of the longitudinal vibration, where 0/0 8.477 and 2/2 156. The above five physical quantities are constant at different temperatures if the first and second nonhamonic effects are omitted, which does not conform to the experimental results. After the first and second nonhamonic effects are considered, they all increase with temperature and results are in good agreement with experimental data. The anharmonic effect increases with temperature.

2016, 65 (10): 104702.
doi: 10.7498/aps.65.104702

Abstract +

Ejecta mixing takes place at the interface between metal and gas under shock loading, i.e., the transport process of ejecta from metal surface happens in gas. Ejecta production and transport processes in gas are the focuses and key problems of shock wave physics at present. So far, extensive investigations have been devoted mainly to the ejecta formation from metal surface under shock-loaded conditions, and many experimental measurement techniques have been developed, such as the Asay foil, high-speed camera and holography technique. As a newly developed instrument, photon Doppler velocitymetry (PDV) which allows the simultaneous detection of velocities of multiple particles has been widely used in the dynamic impact areas, especially in micro-jetting and ejecta mixing experiments. Although PDV spectrogram includes abundant information about ejecta particles, it seems to be too hard to obtain the particle velocity history, which embarrasses the analysis and application of PDV spectrogram. In this paper, the equation of particle motion including the effects of aerodynamic damping force, pressure gradient force, and additional mass force is established, and the analytical solutions of the particle position and velocity are derived in the conditions of planar constant flow, constant flow, and constant acceleration flow. According to the analytical solutions, the characteristics of particle movement are analyzed. A simplified formulation of the relaxation time of the particle velocity, which reflects the particle decelerated speed, is given. And it is found that the relaxation time is proportional to the four-thirds power of particle diameter. Based on the characteristics of particle motion in the planar constant flow, a new method is proposed to analyze the spectrogram of PDV. The fastest velocity of particle in the mixing zone is obtained by extracting the upper part of PDV spectrogram. By integrating the fastest velocity, the time evolution of the head of mixing zone is deduced approximately. The thickness of the mixing zone can be obtained by subtracting the free surface position from the head of mixing zone. The relaxation time of particle velocity is inferred by the exponential fitting of the fastest velocity based on the motion equation of the particle in the planar constant flow. Furthermore, the equivalent diameter of the mixing zone head can also be obtained through the relaxation time. Based on the above methods, the spectrograms of various ejection mixing experiments under different shock-loaded conditions and gas environments are analyzed. The time evolutions of the mixing zone and equivalent diameter are presented, and the effects of shock loading strength and post-shock gas temperature on the mixing zone are analyzed. It is found that the deduced equivalent diameter in gas is smaller than that in vacuum, validating the pneumatic breakup of liquid metal particles in gas.

2016, 65 (10): 104703.
doi: 10.7498/aps.65.104703

Abstract +

Exploring the freezing process and its potential mechanism of the droplets impacting on a solid surface is desperately desired, owing to its anti-icing applications in aircraft, cable, radar, etc. On the controllable low temperature test equipment, the freezing dynamic behaviors of droplets impacting on three cold plates, made of copper, aluminum and silicon, are recorded by a high-speed camera in this paper, and characterized by the droplet spreading diameter, oscillation and freezing time. Here, the freezing behavior of droplets is predicated by observing the color change of the droplet. Through the experimental exploration and theoretical analysis, we reveal the effects of the impacting speed, surface temperature and thermal conductivity of material on the freezing dynamics of the droplet. We demonstrate that a cold surface shrinks the maximum spreading diameter of droplet compared with the surface at ambient temperature; the lower the surface temperature, the shorter the freezing time would be and the smaller the maximum spreading diameter would be; the maximum spreading diameter increases with increasing Weber number, whereas the oscillation and freezing time decrease. Meanwhile, the higher the material thermal conductivity, the shorter the freezing time would be, and the bigger the rising slope of the maximum spreading diameter with increasing Weber number will be. A function to predict the freezing time is derived from thermodynamic condition. The calculated values are in good agreement with the experimental data, with the maximum relative error of less than 5.3%.

2016, 65 (10): 104704.
doi: 10.7498/aps.65.104704

Abstract +

In most of researches about the droplet spreading on a substrate, one adopts aprecursor layer to relieve the stress singularity near the contact line without considering wall properties, which, however, is inapplicable for studying the relationship of the wettability with wall temperature. In this paper, the spreading of a heated droplet on the solid substrate, under the action of the three-phase contact line, is simulated. The influences of the wall temperature on wettability and droplet spreading are examined from the viewpoint of equilibrium contact angle. The simulated results show that when the wall temperature is uniform, the evolution of droplet spreading is dominated only by the gravity, illustrating symmetrical spreading characteristics. When the temperature gradient is applied to the wall, the combination of thermocapillary force and gravity drives the droplet into spreading, therefore the main part of the droplet migrates toward the low temperature region due to the Marangoni effect. The left contact line continually moves toward the left side while the right contact line first moves toward the right side, then turns to the left side after the receding time. The spreading range of the droplet is changed notably because of different travelling speeds of the contact line on both sides. With the increase of the temperature gradient, the Marangoni effect is promoted, resulting in a faster migration toward the low temperature region. A thin film is formed between the contact line in the hotter region and the bulk of the droplet, where the gravity and thermocapillary force dominate the spreading successively. The present simulation shows that the surface wettability is not only dependent on its chemical composition and geometrical morphology, but also closely related to wall temperature. When the sensitivities of the liquid-solid, liquid-gas and solid-gas interfacial tensions to temperature are all identical, the equilibrium contact angle between the droplet and the wall keeps constant, leading to a uniform wettability on the wall. When the liquid-solid interfacial tension or the liquid-gas interfacial tension is more sensitive to temperature than the other two interfaces, the equilibrium contact angle increases and the wettability tends to be worse, presenting a more hydrophobic substrate, which decelerates the spreading of the droplet with the contact line moving to the colder region. As the solid-gas interfacial tension is more sensitive to temperature than the other two interfaces, the equilibrium contact angle tends to lessen, and the contact line feels a more hydrophilic substrate (the droplet wets perfectly when the equilibrium contact angle decreases to zero), hence the spreading is enhanced. The present results indicate that the equilibrium contact angle plays a key role in the evolution of a heated droplet on a horizontal plate. The simulation conclusions can provide a theoretical basis for relevant experimental findings, which promotes the understanding of the relationship between wall temperature and its wettability.

2016, 65 (10): 104705.
doi: 10.7498/aps.65.104705

Abstract +

The flow properties of liquid in microchannel have received more attention for their wide applications in different fields. Up to now, little work has focused on the flow behaviors of liquid metals. Recently, liquid lithium (Li) has been considered as one of the candidate plasma-facing materials (PFMs) because of its excellent properties in fusion reactor applications. Considering an accident condition, liquid Li may contact Cu components and erode them, which may cause a serious disaster. The study of the flow properites of liquid Li in Cu microchannel is crucial for the safe application of liquid Li working as a PFM. With the method of non-equilibrium molecular dynamics simulations, in this paper we investigate the flow behavior of liquid Li flowing in Cu microchannels. The density and velocity distributions of Li atoms are obtained. The influence of the dimension of Cu microchannel on the flowing behavior of liquid Li is studied. Comparative analyses are made in three different fluid-solid interfaces, i.e., Li-Cu(100), Li-Cu(110) and Li-Cu(111), respectively. Results show that the density distributions of liquid Li near the interface present an orderly stratified structure. Affected by a larger surface density, a more obviously stratification is found when Li atoms are near the fluid-solid interfaces of Li-Cu(100) and Li-Cu(111) and a wider vacuum gap appears between Li atoms and Cu(111) interface. When Li atoms are near the Li-Cu(110) interface, a lower stratification can be found and an alloy layer appears at Li-Cu(110) interface. Because of its lower surface density, Li atoms spread into the bulk Cu more easily. However, the density distributions have little difference when Li atoms are close to the same fluid-solid interface but with different flow directions. The velocity of Li atoms in microchannel has a parabolic distribution. Because there exists a wider vacuum gap and stratified structure, the Li atoms closed to the Li-Cu (111) interface have the largest velocity. Closed to the Li-Cu (110) interface, Li atoms have the smallest velocity because of the alloy layer and the lower stratified structure. Owing to the diversity of the atomic configurations of Cu (110) face, the liquid Li atoms flow with diverse velocities in different directions on the Li-Cu (110) interface. It is also found that the magnitude of flowing velocity of liquid Li is proportional to the square of microchannel dimension and increases with it. When liquid Li is flowing on the Li-Cu(100) interface, the simulation result reveals that the relationship between microchannel dimension and the largest velocity of Li atoms is in good agreement with Navier-Stokes theory result. It is noteweathy that the present result is smaller than the theoretical result when a negative slip occurs at the Li-Cu(110) interface. In contrast, the result is greater than the theoretical result in the presence of a positive slip at Li-Cu(111) interface.

###### PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES

2016, 65 (10): 105201.
doi: 10.7498/aps.65.105201

Abstract +

Graphene is a one-atom-thick planar sheet of sp2-bonded carbon atoms that are densely packed in a honeycomb crystal lattice. Graphene has been found to support plasmons in a wide range from infrared to terahertz. The confinement of plasmons in graphene is stronger than that on metallic surface. Moreover, the plasmon properties can be dynamically adjusted by doping or grating graphene. In this study, a composite structure comprised of graphene and subwavelength grating is proposed. Highly confined plasmons in graphene are excited by using a diffraction grating with guided mode resonance effect. The wave vector of plasmonic wave in graphene is far larger than that of light in vacuum. To excite plasmons in graphene with a freespace optical wave, their large difference in wave vector must be overcome. Optical gratings are widely used to compensate for wave vector mismatches. A diffraction wave generated by the grating structure can overcome the large wave vector difference and excite surface plasmons. The guided-mode resonance can greatly enhance the intensity of the diffraction field and the coupling efficiency between graphene and incident light. When the phase matching between illuminating wave and a guide mode supported by grating is achieved, guided-mode resonance effect occurs. A nearly 100% diffraction efficiency peak in the reflection or transmission spectrum occurs at a certain wavelength. In this study, the influences of graphene and grating structure on the local characteristics (the surface electric field Ex/Ein, quality factor Q, and effective mode area Seff) of surface plasmons are investigated. The effects of the structural parameters (the thickness of the buffer layer T2, the grating period p, the carrier mobility , and the Fermi level EF) on localization properties are analyzed by the finite element method (COMSOL). The results reveal that the localizations of the surface plasmons in the graphene surface is significantly improved at the certain parameters. 1) The increase of T2 will reduce the intensity of electric field on graphene (Ex/Ein), but the quality factor will obtain a certain increase. The excition of highly confined SPPs needs to improve Q and keep the intensity of Ex/Ein, so in this study T2 = 10 nm. 2) By adjusting the quality factor of SPPs can be improved significantly without changing the resonance frequency ( = 0.7 m2(Vs), Qmax = 1793). 3) Small changes in p and EF will make the resonance peak shift obviously, and the electric field on graphene is greatly enhanced (p = 235 nm, Ex/Ein = 3154; EF = 0.72 eV, and Ex/Ein = 3968). Strong localization leads to strong light-matter interaction, and thus the proposed structure has the potential to be used as sensors with high sensitivity and high-efficiency nonlinear optical devices, greatly expanding the application of graphene in nano optics.

###### CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES

2016, 65 (10): 106101.
doi: 10.7498/aps.65.106101

Abstract +

Cement is a widely used construction material in the world. Calcium-silicate-hydrate (C-S-H) is the main component of aged cement (concrete). The quality and durability of concrete are strongly affected by the dynamics of water confined in it. Dynamics of the confined water can be studied experimentally by the quasi-elastic neutron scattering (QENS). In this paper, the jump-diffusion and rotation-diffusion model (JRM) is used to fit the QENS spectra of C-S-H paste samples at different measured temperatures for the whole scattering vector Q-range from 0.3 to 1.7 -1. Five important parameters are extracted to describe the dynamics of water confined in C-S-H samples: the index of immobile water C, the rotational diffusion constant Dr, the mean square displacement (MSD) u2 , the self-diffusion coefficient Dt, and the average residence time between jumps 0. Here, all the C-S-H samples, each with a 30% weight ratio of water to cement, are measured at temperatures ranging from 230 to 280 K. The fitted parameters can quantitatively describe the dynamics of water confined on different scales of C-S-H gel pores. The results show that the QENS spectra are fitted very well not only for small Q (Q 1 -1) but also for large Q (Q 1 -1). The obtained rotational diffusion constant is Q dependent. Thus the contribution of the water molecule rotation to a QENS spectrum increases with the value of Q increasing. The mean square displacement (MSD) u2 decreases with the increase of Q, which can be used to distinguish the confined water, ultra-confined water, and bound water contained in C-S-H samples. When Q is small, the fitted Dt and 0 vary with the measured temperature. Dt has a maximum value at 230 K and 0 has a peak at 240 K. These indicate that the dynamics of the confined water shows different behaviors at 230 K and 240 K. There are crossover or critical phenomena for water confined in C-S-H sample at low temperature.

2016, 65 (10): 106201.
doi: 10.7498/aps.65.106201

Abstract +

Electron transport properties of the (GaAs)n(n=1-4) linear atomic chains, which are sandwiched between two infinite Au(100)-33 leads, are investigated with a combination of density functional theory and non-equilibrium Greens function method from first principle. We simulate the Au-(GaAs)n-Au nanoscale junctions breaking process, optimize the geometric structures of four kinds of junctions, calculate the cohesion energies and equilibrium conductances of junctions at different distances. The calculation results show that there is a stable structure for each nanoscale junction. The average bond-lengths of Ga-As in each chain at equilibrium positions for stable structure are 0.220 nm, 0.224 nm, 0.223 nm, 0.223 nm, respectively. The corresponding equilibrium conductances are 2.328G0, 1.167G0, 0.639G0, and 1.237G0, respectively. It means that each of all the junctions has a good conductivity. We calculate the transmission spectra of the all the chains. With the increase of atomic number in the (GaAs)n (n=1-4) chains, there appears no oscillation phenomenon for the equilibrium conductance. We calculate the projected densities of states of all nanoscale junctions at equilibrium positions, and the results show that electronic transport channel is mainly contributed by the px and py orbital electrons of Ga and As atoms. In the voltage range of 0-2 V, we calculate the current-voltage characteristics of junctions at equilibrium positions. With the increase of external bias, the current increases, and the I-V curves of junctions show linear characteristics for the (GaAs)n (n=1-3) atomic chains. However, there appears a negative differential resistance phenomenon in each of the voltage ranges of 0.6-0.7 V and 0.8-0.9 V for the (GaAs)4 linear atomic chain.

2016, 65 (10): 106401.
doi: 10.7498/aps.65.106401

Abstract +

Equation of state of detonation products possesses various types of mathematical expressions which describe the relation between pressure and volume. Jones-Wilkin-Lee (JWL) equation of state is a widely used equation of state of detonation products because of its simplicity in hydrodynamic calculations. The JWL equation of state may accurately describe the process of expansion drive of detonation products. The JWL equation of state contains parameters, and describe the relation among the volume, energy and pressure of detonation products. These parameters may be determined by detonation experimental data and numerical method. Traditional numerical method is adjusting parameters based on experimental data and numerical experience. Obviously, artificial ingredient may affect the calibrating result in traditional method. This paper uses the Bayesian method to determine the unknown (uncertain) parameter of JWL equation of state for detonation products. The method can calibrate the uncertain parameters based on the known parameter information, the experimental and simulating data. The results of the paper are consistent with those in the reference papers. By theoretical analysis the calibration result accords with the physical signification of the parameters of JWL equation of state. The epistemic uncertainty is slightly reduced. The calibration result collects all the parameter information in the prior parameter information, experimental data and numerical results. The experimental data are totally included in a 90% confidence interval of simulation. The numerical result shows that this method can be used to study the uncertain parameter of JWL equation of state for some sample explosives. Especially, the method reduces the artificial ingredient in the parameter calibration.

2016, 65 (10): 106402.
doi: 10.7498/aps.65.106402

Abstract +

The liquid phase separation of small Fe-Sn-Si/Ge alloy droplets under reduced-gravity condition is investigated experimentally by free fall technique and theoretically by lattice Boltzmann method. In the drop tube experiments, the Fe-Sn-Si/Ge monotectic alloys are heated by induction heating in an ultrahigh vacuum chamber and further overheated to 200 K above their liquid temperatures for a few seconds. Finally, the molten alloy melt is ejected out from the small orifice of a quartz tube by high pressure jetting gas of He and dispersed into numerous tiny droplets, which are rapidly solidified during free fall in a protecting He gas environment. These droplets benefit from the combined advantages of high undercooling, containerless state and rapid cooling, which can provide an efficient way to study the liquid phase separation of high-temperature alloys in microgravity. In order to efficiently reproduce the dynamic process of phase separation inside drop tube equipment, the effects of surface segregation and Marangoni convection are introduced into the interaction potential of different liquids within lattice Boltzmann theory. Based on this modified model, the dynamic mechanism of phase separation can be sufficiently analyzed and the phase separation patterns can be realistically simulated. Experimental results demonstrate that conspicuous liquid phase separations have taken place for both Fe-Sn-Si and Fe-Sn-Ge alloy droplets and the corresponding morphologies are mainly characterized by core-shell and dispersed structures. The phase separation process can be modulated by the third-element addition. As the Si element of Fe-Sn-Si alloy is replaced by the Ge element with the same fraction, the distribution order of Fe-rich and Sn-rich zones is reversed within core-shell structure. A core-shell structure composed of a Fe-rich core and a Sn-rich shell is frequently observed in Fe-Sn-Si alloy droplets whereas the Fe-Sn-Ge alloy droplets tend to form a core-shell structure consisting of a Sn-rich core and a Fe-rich shell. Theoretical calculations show that the droplet cooling rate is closely related to droplet size: a smaller alloy droplet has a higher cooling rate. The liquid L2(Sn) phase always nucleates preferentially and forms tiny globules prior to solid Fe phase. Stokes motion can be greatly weakened in this experiment and the Marangoni migration dominates the globule movement in the process of liquid phase separation. Furthermore, the intensity of Marangoni convection within Fe-Sn-Ge alloy droplets is significantly stronger than that inside Fe-Sn-Si alloy droplets. Numerical simulations reveal that the cooling rate, Marangoni convection and surface segregation play the important roles in determining the selection of core-shell configurations and the formation of dispersed structures. Ultrahigh cooling rate contributes to forming the dispersed structures. When the Marangoni convection proceeds more drastically than the surface segregation, the minor liquid phase with a smaller surface free energy migrates to droplet center and occupies the interior of droplet, otherwise most of the minor phases appear around the periphery of droplet.

2016, 65 (10): 106403.
doi: 10.7498/aps.65.106403

Abstract +

Grain size has a significant influence on the performances of materials. Cooling rate is a key process parameter for controlling the size of crystal grain. Real-time observations of crystallization process on an atomic scale under different cooling rates are helpful for an in-depth understanding of this scientific issue. However, it is very difficult to observe directly the crystallization process on an atomic scale because it is small in size and fast in motion. Over last decades, colloidal suspension has attracted many researches attention as a model system of condensed matter to investigate phase transition kinetics at a particle scale level because colloidal particles are micrometer-sized and their thermal motions can be directly visualized and measured with an optical microscope. Thermo-sensitive poly-N-isopropylacrylamide (PNIPAM) colloidal suspension is one of the model systems and its phase transition can be easily controlled by temperature. In this paper, the PNIPAM colloidal system is used to make the real-time observation of the influence of the cooling rate on crystal grain size. Firstly, the crystal nucleation and growth process of PNIPAM colloidal suspension at a cooling rate of 30.0 ℃/h is observed with a high-resolution transmission microscope. It is found that liquid-solid phase transition of the PNIPAM colloidal suspension begins from a sudden transient nucleation, followed by a rapid grain growth as temperature decreases. The variation of crystal phase fraction with temperature undergoes three stages: slow, rapid and slow. In the initial stage, nuclei are limited and the growth driving force is low, therefore the crystal phase fraction changes slowly. In the middle stage, as temperature decreases, the growth driving force further increases and the crystal phase fraction increases rapidly. In the final stage, the crystal grains begin to adjoin with each other and the left liquid volume becomes less and less, so the crystal phase fraction increases in a slow mode again. Secondly, the PNIPAM colloidal crystal under different cooling rates from 0.5 ℃/h to 30.0 ℃/h is observed with Bragg diffraction technique. The grain size of PNIPAM crystal is also measured. It is found that the size of PNIPAM colloidal crystal grain decreases with the increase of cooling rate and the relationship between the grain size and the cooling rate obeys a power-law formula, which is also used to well describe the effect of cooling rate on grain size in metallic system. This suggests that the crystallization behavior of PNIPAM colloidal system under continuous cooling is similar to those of metallic systems. However, the fitted power-law pre-factor of PNIPAM colloidal system is very different from those of the metallic systems because the sizes and motions of PNIPAM particles are much larger and slower than those of atoms, respectively.

2016, 65 (10): 106701.
doi: 10.7498/aps.65.106701

Abstract +

Porous media are widely used in the production and living, and also in science and technology. With the development of energy, chemical industry, metallurgy, atomic energy and also with the progress of the modern industrial and agricultural production technology, a large number of heat and mass transfer problems in porous media gradually appear. Further promoting the development of the discipline about the formation and development of porous media becomes one of hot research points in the modern science and technology. It is expected that the accurate experimental picture and data can be obtained through the experiment, and the fluid flow picture and experimental data are analyzed in depth by using the corresponding software, so that the reliable data are obtained and the theory is supported intuitively, making the research of porous media more perfect. The experiment combined with particle image velocimetry technology and refractive index matching technique is conducted to test the transformation process of liquid flow in a random ball porous medium filled bed, and to extract the data. The extracted data are processed by using Tecplot software, and the transformation process of liquid flow mechanism is obtained. Experimental solid phase is a 25 mm-diameter crystal glass ball stacked bed, and liquid phase is the matching liquid prepared with the mixture of the 65% benzyl alcohol and 35% anhydrous ethanol. The refractive indexes of liquid phase and solid phase are both 1.477, which can successfully eliminate the laser light bending caused by the nismaching of refractive indexes. The flow field diagram in the pebble bed with Reynolds number Re in a range 4.7 Re 1000 is obtained experimentally. The comparisons of variations of flow field and flow lines among the different Reynolds numbers reveal that with the increase of Reynolds number, flow lines become more and more disorder: When the Reynolds number Rep exceeds 220, stable swirl flow inside the bed changes suddenly, and manifests a random feature in location and configuration, which forebodes its entrance into stable turbulence phase.

###### CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES

2016, 65 (10): 107101.
doi: 10.7498/aps.65.107101

Abstract +

By using first-principle with pseudopotential method based on the density functional perturbation theory, in this paper we calculate the electronic properties of wurtzite 6H-SiC crystal under the strong laser irradiation and analyze the band structure and the density of states. Calculations are performed in the ABINIT code with using the generalized gradient approximation for the exchange-correlation energy. And the input variable tphysel is used to set up a physical temperature of electrons Te. The value of Te is set to simulate the corresponding electron temperature of the crystal when irradiated by intensive laser within an ultrafast time. The highly symmetric points selected in the Brillouin zone are along -A-H-K--M-L-H in the energy band calculations. After testing, we can always obtain a good convergence of the total energy when choosing 18 Hartree cut-off energy and 333 k-point grid. By optimizing the structure and then using the optimized equilibrium lattice constant, the structural parameters and the corresponding electronic properties of 6H-SiC in the different electron-temperature conditions are studied. First of all, when the electron temperature stays in a range between 0 eV and 5.0 eV, we choose 23 groups of different electron temperatures to respectively test the values of equilibrium lattice parameters a and c of 6H-SiC. Within a temperature range between 0 eV and 4.25 eV, we continue to test 20 groups of the electrical properties of 6H-SiC under different electron temperatures, calculating the forbidden bandwidths at different electron temperatures and analyzing the changes of the bottom of conduction band and the top of valence band as the electron temperature goes up. Meanwhile, taking for sample two groups of the band structures in ranges of 0-2 eV and 3-4 eV, we comparatively analyze the changes of the energy and position of the bottom of conduction band and the top of valence band with electron temperature. The calculation results indicate that the equilibrium lattice parameters a and c of 6H-SiC gradually increase as electron temperature Te goes up. With the electron temperature going up, the top of valence band still stays there, while the bottom of conduction band shifts to the location between M and L point as electron temperature increases, leading to the fact that 6H-SiC is still an indirect band-gap semiconductor in a range of 0-3.87 eV, and as electron temperature reaches 3.89 eV and even more, the crystal turns into a direct band-gap semiconductor. With Te rising constantly, the bottom of the conduction band and the top of valence band both move in the direction of high energy or low energy. When Te is in excess of 4.25 eV, the top of valence band crosses the Fermi level. When Te varies in a range of 0-2.75 eV, the forbidden bandwidth increases with temperature rising, and when Te varies in a range of 2.75-3 eV, the forbidden bandwidth decreases slowly, and when Te varies in a range of 3-4.25 eV, the forbidden bandwidth quickly reduces. This variation shows that the metallic character of 6H-SiC crystal increases with electron temperature Te rising. The total densities of states (DOS) are calculated at Te = 0 eV and 5 eV. The DOS figures indicate that 6H-SiC is a semiconductor and its energy gap equals 2.1 eV. At Te = 5 eV, the gap disappears, presenting metallic properties. This result shows that the crystal covalent bonds are weakened and metallic bonds are enhanced with temperature increasing and the crystal experiences the process of melting, entering into metallic state.

2016, 65 (10): 107201.
doi: 10.7498/aps.65.107201

Abstract +

Thermoelectric materials can generate electricity by harnessing the temperature gradient and lowering the temperature through applying electromotive force. Lead chalcogenides based materials, especially PbTe-based ones, have shown extremely high thermoelectric performance. PbSe has a similar crystal structure and band structure to PbTe. Compared with the commonly-used PbTe, PbSe possesses a high melting point and has an abundant reserve of Se, making it attractive to high temperature thermoelectric applications. It has been theoretically proposed that Mn-doping in lead chalcogenide should be able to lower the temperature of band degeneracy, and experimental evidences have been represented in Mn-PbTe. However, such an experimental study as well as the investigations of influences of Mn on microstructure, mechanical, electrical and thermal properties has not been conducted in Mn-PbSe. In this work, Pb0.98-xMnxNa0.02Se (0 x 0.12) materials are prepared by the melting-quenching techniques combined with rapid hot-press sintering. Effects of Mn doping on the microstructures, mechanical and thermoelectric properties of PbSe samples are systematically studied. The refined lattice parameters from X-ray powder diffraction patterns show that the solubility of Mn in the matrix is in a range from 0 to 0.04. The back-scattered electron images and elemental maps reveal that the MnSe-rich impurity phases exist in the PbSe matrix, which makes the PbSe-MnSe system a nano-composite system. Pb0.96Mn0.02Na0.02Se has also such microstructures, implying that the solubility of Mn should be below 0.02. Cubic-phase MnSe-rich precipitates have the sizes ranging from 50 nanometers to 1-5 micrometers. They are well dispersed in the PbSe-rich matrix, as round or layered microstructures. The mechanical properties of the nanocomposites can be determined by micro-hardness measurements. Interestingly, the average Vickers hardness values of the PbSe-MnSe nanocomposites are significantly improved, which are 16.6% and 51.6% harder respectively in x= 0.02 and 0.06 samples than those of pristine PbSe. Smaller Mn content can optimize the figure of merit ZT due to the band convergence and additional phonon scattering by precipitates, while higher Mn content has little influence on ZT because of the saturated Seebeck coefficient and anomalous increase in lattice thermal conductivity. As a result, the highest figure of merit is 0.52 at 712 K, which is achieved in the Pb0.96Mn0.02Na0.02Se sample. By further adjusting the Na content from 2% to 0.7%, the carrier concentration is optimized. Thus, the Seebeck coefficient and power factor become higher. A figure of merit of 0.65 is achieved at 710 K in the PbSe-MnSe nano-composite with a nominal composition of Pb0.973Mn0.02Na0.007Se. We suggest that further optimizing the electrical properties may achieve a higher thermoelectric performance in the PbSe-MnSe system.

2016, 65 (10): 107301.
doi: 10.7498/aps.65.107301

Abstract +

Perovskite oxide heterostructure possesses attractive magnetic, optical and electric properties, such as superconducting interface between two insulators, two-dimensional electron gas, positive giant magnetoresistance, photoelectric response characteristic, magnetocaloric effect, and coexistent different magnetic structures. Especially for the photoelectric response behaviors of A1-xAxMnO3 (A=La, Pr etc.; A = Sr, Ca etc.) perovskite manganese oxide heterostructure, one has made a systematic study on the photoelectric conversion efficiency, the photovoltaic response speed, and the in-plane lateral photovoltage. Besides A1-xAxMnO3 structure, manganese oxides can also exhibit the double layered perovskite structure A2-2xA1+2xMn2O7. Double layered perovskite structure can be regarded as the layers of perovskite and rock salt which are alternately stacked. This double layered perovskite manganese oxide (such as La2-2xSr1+2xMn2O7) is a natural structure of the tunnel structure: ferromagnetic metal layer-insulating layer-ferromagnetic metal layer. Double layered perovskite manganese oxide has not only the characteristics of giant magnetoresistance, but also the novel physical properties, such as persistent photoconductivity, etc. However, there are few reports on the physical properties of the double layered perovskite manganite oxides, heterostructures, especially the photovoltaic properties. In this work, the La1.3Sr1.7Mn2O7 (LSMO) film is deposited on an n-type SrTiO3-Nb (NSTO) single crystal substrate by a pulsed laser deposition method. Additionally, we study the transporting properties of LSMO/NSTO heterostructure and its photovoltaic effect. The heterostructure exhibits benign rectifying and palpable photovoltaic effect. Under the 532 nm laser irradiation, the photovoltage first increases and then decreases with temperature rising. The maximal photovoltage reaches 400 mV at 150 K which is consistent with the metal-insulator transition temperature of LSMO film. It is indicated that the photovoltaic effect of the heterostructure is regulated by the inner transporting characteristics of LSMO film. The dynamical process of the heterostructure, photovoltaic response, is analyzed. Meanwhile, by analyzing the relationship between the photovoltage and time, it is found that the rising edge fits to the first order exponential function, which is related to the migration of carriers. While the falling edge of second-order exponential function indicates that the compound of carriers has two different physical processes: 1 corresponds to the neutralization process of the carriers aggregated on both junction sides through the external circuit, and 2 corresponds to the annihilation process of non-equilibrium carriers. The carrier lifetime of our p-n junction is longer, on the order of ms, than those of other manganese oxides p-n junctions. Remarkably, the time constants of both the rising edge and falling edge first increase and then decrease as temperature increases, and the maximum values occur at the metal-insulator transition temperature of LSMO film.

2016, 65 (10): 107501.
doi: 10.7498/aps.65.107501

Abstract +

The compounds doped with or containing Cr2+ ions are extensively used as optoelectronic and nonlinear optical materials, because they have special optical, magnetic and electric properties. These properties are very closely related to the absorption spectra and zero-field-splitting. The studies of the absorption spectra and zero-field-splitting are very important for realizing the doped microscopic mechanism and understanding the interaction between impurity ions and host crystals, and they may be useful to material designers. The concept of the standard basis adapted to the double group chain is adopted in the strong-field scheme by the crystal field theory. This concept emphasizes the standardization of the basis of the whole 3d4 configuration space including all spin states. Thus, the basis functions can be constructed according to each irreducible representation of the double group and each basis function has a certain expression. Each standard basis adapted to the double group chain can be built from the former by a linear transformation, which forms a basis chain. Thus, the complete energy matrix including spin singlet is constructed for Cr2+ ion in tetragonal symmetry environment in the strong-field-representation by the crystal field theory. The fine structures of absorption spectra and the spin-singlet contributions to zero-field-splitting parameters for BaCrSi4O10 and AgGaSe2:Cr2+ are studied by diagonalizing the complete energy matrix. The fine structures for the two systems and the zero-field-splitting parameters for BaCrSi4O10 are given theoretically for the first time. The fine structures are assigned by the irreducible representation of the group. The results show that the spin-singlet contribution to D is negligible, but the contributions to a and F are important. The contributions arise from the interaction of the spin quintuplets with both spin triplets and spin singlets via spin-orbit coupling. However, the selection rule of spin-orbit coupling shows that the spin singlets do not affect the quintuplets directly but indirectly via the spin triplets. Thus, all spin states should be considered to obtain more accurate zero-field-splitting values.

2016, 65 (10): 107601.
doi: 10.7498/aps.65.107601

Abstract +

Multi-exponential inversion algorithm of nuclear magnetic resonance (NMR) T2 spectrum is an important mathematical tool for the NMR relaxation study of complicated samples. The popular algorithm usually obtains the T2 spectrum by linear fitting under the prescribed distribution of T2. When the T2 spectrum is dispersed, such a procedure is inaccurate because of the lack of adaptive prescription and the limit of linear method. Nonlinear fitting method does not fix the T2 distribution, and it provides the positions and the weights of T2 simultaneously via the nonlinear fitting of multi-exponential function. In this case, the problem of multi-exponential inversion is transformed into a nonlinear optimization problem with non-negative constraints. The optimization objective function is the residual sum of squares (or residual sum of squares with regularization). The nonlinear optimization problem can usually be solved by Levenberg-Marquardt algorithm and evolutionary algorithm. But the results of Levenberg-Marquardt algorithm are dependent on initial values, and the calculation of evolutionary algorithm is complicated. We provide an optimal model for the nonlinear fitting in the inversion of dispersed T2 spectrum based on the linear regression and least-squares. The key idea is that the optimal weights of T2 can be calculated by least square when the positions of T2 are fixed, although the positions of T2 are adjusted adaptively. So we can relate the positions to weights appropriately to improve the popular nonlinear fitting algorithms. Such an improvement can reduce the searching inversion parameters, speed up its convergence and reduce the dependence on initial value. Incorporating it into the Levenberg-Marquardt algorithm or evolutionary algorithm can improve the inversion accuracy and make the algorithm more robust. The validity of our improvement is demonstrated by the inversions of simulation data and practical NMR data by combining Levenberg- Marquardt algorithm and differential evolution algorithm with our improvement. The inversion results of simulation data show that for dispersed T2 spectrum, the algorithm using this improvement can obtain more accurate T2 spectrum than previous ones, especially in the case of low signal-to-noise ratio (SNR) cases. The inversion results also indicate that the improvement can reduce the dependence on initial value of Levenberg-Marquardt algorithm, and can accelerate the convergence of differential evolution algorithm. The inversion results of practical NMR data show that the algorithm using the improvement can obtain more accurate T2 spectrum than the widely used CONTIN program in the case of low signal-to-noise ratio (SNR). The inversion results of oil-water mixture sample NMR data also demonstrate that the relaxation time T2 is independent of dispersion degree of immiscible system components.

## EDITOR'S SUGGESTION

2016, 65 (10): 107701.
doi: 10.7498/aps.65.107701

Abstract +

It is an urgent and significant issue to investigate the influence factors of functional devices and then improve, modify or control their performances, which has important significance for the practical application and electronic industry. Based on first principle and quantum transport calculations, the effects of compressive strain on the current transport and relative electrical properties (such as the electrostatic potential energy, built-in electric field, charge density and polarization, etc.) in gallium nitride (GaN) tunnel junctions are investigated. It is found that there are potential energy drop, built-in electric field and spontaneous polarization in the GaN barrier of the tunnel junction due to the non-centrosymmetric structure of GaN. Furthermore, results also show that all these electrical properties can be adjusted by compressive strain. With the increase of the applied in-plane compressive strain, the piezocharge density in the GaN barrier of the tunnel junction gradually increases. Accordingly, the potential energy drop throughout the GaN barrier gradually flattens and the built-in electric field decreases. Meanwhile, the average polarization of the barrier is weakened and even reversed. These strain-dependent evolutions of the electric properties also provide an atomic level insight into the microscopic piezoelectricity of the GaN tunnel junction. In addition, it is inspiring to see that the current transport as well as the tunneling resistance of the GaN tunnel junction can be well tuned by the compressive strain. When the applied compressive strain decreases, the tunneling current of the junction increases and the tunneling resistance decreases. This strain control ability on the tunnel junctions current and resistance becomes more powerful at large bias voltages. At a bias voltage of -1.0 V, the tunneling resistance can increase up to 4 times by a -5% compressive strain, which also reveals the intrinsic giant piezoelectric resistance effect in the GaN tunnel junction. This study exhibits the potential applications of GaN tunnel junctions in tunable electronic devices and also implies the promising prospect of strain engineering in the field of exploiting tunable devices.

###### INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY

## EDITOR'S SUGGESTION

2016, 65 (10): 108501.
doi: 10.7498/aps.65.108501

Abstract +

In recent decades, infrared (IR) detection technology has been widely used in many fields such as weather monitoring, environmental protection, medical diagnostics, security protection, etc. With the progress and mature of the technologies, more attention has been paid to the imaging detections of weak IR signals. So the higher efficiency of the device is required. Moreover the next-generation IR photodetection technology focuses on large-scale, high-speed and low-dark-current imaging. The mechanical bonding between infrared detector chip and silicon readout circuit inevitably causes a thermal mismatch problem. Up-conversion IR photodetectors can solve the problem about the performance deterioration of photodetector and the thermal mismatch with silicon-based readout circuit, hence they have great advantages in realizing large-format focal plane array detection.However, the poor light extraction efficiency due to total reflection severely restricts the overall efficiency of the up-conversion device, which has become one of the bottlenecks to improve the device efficiency. In this paper, surface microstructures with micro-pillar morphology are designed and fabricated on quantum-cascade up-conversion IR photodetectors. The effect on the up-conversion efficiency is investigated by enhancing the light extraction efficiency.Firstly, by the optical ray retracing method, the influence of surface microstructure on light extraction efficiency is studied when considering different morphology parameters, and optimized surface microstructure is designed to possess a pillar base length of 150 nm, height of 105 nm and side wall angle of 75.Then based on the results of simulation, up-conversion IR photodetectors with surface microstructures are fabricated using polystyrene nanospheres as mask. The self-assembled monolayer nanospheres are first etched to a proper size by using O2 plasma, then the patterns are transferred to SiNx film, which acts as an ICP dry etching mask of the micro-pillars. Finally, the up-conversion device and a silicon detector are together loaded on a cold finger of a cryogenic dewar. The characteristics of the up-converter and up-conversion system are evaluated using a blackbody source.The experimental results show that the devices with and without surface microstructure exhibit similar IR responses and dark currents, while the emission of device with microstructure is obviously increased. Taking into consideration other factors related to external quantum efficiency, the light extraction efficiency of the device with micro-pillar structure on surface can be increased by up to 130%. Therefore it can be concluded that this method is an efficient way to improve the efficiency of up-conversion IR photodetector. The finding in this paper can also be applied to other semiconductor device with light extraction efficiency.

2016, 65 (10): 108502.
doi: 10.7498/aps.65.108502

Abstract +

We report on a three-color InAs/GaAs quantum dot infrared photodetector grown by molecular beam epitaxy. The InAs quantum dots with AlGaAs inserting layers are formed on an InGaAs quantum well layer as an absorber region. The detector is an nin-type device, and three photocurret peaks at 6.3, 10.2 and 11 m are observed at 77 K, respectively. Each peak is designated and the physical mechanism is discussed. The dependences of the intensities of the three peaks on the applied bias voltage are different for the positive and the negative bias conditions due to the asymmetric structure of the absorber region. The peak arising from the transition between the quantum dot ground state and a continuum state becomes weaker when the bias voltage is larger than a certain value. The physical reason is attributed to the decrease of the wavefunction overlap between the two quantum states.

2016, 65 (10): 108701.
doi: 10.7498/aps.65.108701

Abstract +

Autism spectrum disorder is a kind of mental disease which involves the disorders of the perception, emotion, memory, language, intelligence, thinking, action, etc. The aim of this paper is to investigate the brain activity characteristics of the children with autism during complex environments by analyzing electroencephalogram (EEG) signals from the neuroergonomics perspective. The virtual driving environment as a complex multi-task source is used to organically connect brain systems with human motion control. The 14-channel EEG signals are obtained including the EEG baseline signals on a resting state (about 3 min) and the EEG activity signals during driving (about 5 min). The method of the shift average sample entropy is proposed to deal with EEG signals in the resting and the virtual driving environments. Considering the highly complex hyper-dimensional characteristics of EEG signals, the different embedding dimensions (such as 2 and 6 dimensions) are analyzed in the sample entropy estimation. The results show that the average sample entropy values of autism spectrum disorder (ASD) subjects are lower than those of healthy subjects during resting and driving, respectively, especially in the prefrontal lobe, temporal lobe, parietal lobe and occipital lobe during resting and in temporal lobe and occipital lobe during driving. It indicates that ASD children lack the ability to adapt easily their behaviors. Meanwhile, like healthy subjects, the average sample entropy values of ASD subjects during driving are higher than those during resting as a whole. Moreover, the EEG activity signals of ASD are obviously higher than the EEG baseline signals in prefrontal lobe, frontal lobe, frontal central lobe and temporal lobe regions in 95% significant level. And for healthy subjects, the activity signals are significantly higher than the baseline signals only in parietal lobe region. Furthermore, the brain activities of ASD subjects during driving come closer to those of healthy subjects during resting. It suggests that the virtual driving environment may be helpful for the treatment of ASD individuals. In addition, the ASD and healthy subjects have a certain right hemisphere dominance in the whole region except in the parietal lobe region. In the parietal lobe region, they have some left hemisphere dominance, especially during driving. And for ASD subjects, there is the significant right hemisphere dominance in the temporal lobe in 95% confidence level no matter whether in the resting state or in the driving state. The results show that it is suitable for the shift average sample entropy analysis to study the brain activities of ASD individuals. This study will provide a new research method for the further research on the mechanism of autism and its diagnosis, evaluation and intervention.

2016, 65 (10): 108801.
doi: 10.7498/aps.65.108801

Abstract +

At present, solar cells are the main sources for spacecrafts. For a long time the bulk of the space power installations has been the solar arrays based on single junction silicon and gallium arsenide solar cells. In recent years a trend has been the active use of triple-junction GaAs solar cell with higher efficiency instead of single junction solar cells. One of the most important characteristics of solar cells used in spacecrafts is the resistance to radiation damages caused by high energy particles of the near-Earth space. According to the spectral response of triple-junction GaAs solar cell and the damage characteristics of the current under the condition of electron irradiation, the physical mechanism of cell attenuation can be determined: the current degradation originates mainly from the GaInAs subcells. These damages form additional centers of nonradiative recombination, which results in the reduction of the minority charge carrier diffusion lengths and in degradation of the solar cells photocurrent.The radiation damage caused by the electron irradiation will shorten the diffusion length of the base region and affect the collection of photo generated carriers. The ways of improving absorption of long wavelength light in GaInAs subcells with a thin base in using the distributed Bragg reflector can be investigated by the mathematical simulation method based on calculating the light propagation in a multilayer structure by means of the TFCalc software which can design optical structure. To estimate the validity of these methods for solar cells structures with distributed Bragg reflector, the spectral dependences of the photoresponse and the reflection coefficient with different base thickness values are calculated and compared with experimental results. Based on the physical mechanism of the degradation, the thickness of middle subcell base layer is reduced, and an appropriate structure of the distributed Bragg reflector is simulated by the TFCalc software. As a result, the new structure solar cells are that the thickness of the base layer is 1.5 m compared with the different middle subcell thickness values, and the distributed Bragg reflector structure with 15 paris of the Al0.1Ga0.9As/Al0.9Ga0.1As with 850 nm central wavelength is embedded in the middle subcell of the base layer, the distributed Bragg reflector has a highest reflectivity of more than 97% in the actual test, and a bandwidth of 94 nm, which can satisfy design requirement. After irradiating the new structure of solar cells, the decay of its short-circuited current is reduced by 50% compared with that of the original structure, and the remaining efficiency factor is increased by 2.3%.