## Vol. 66, No. 19 (2017)

##### 2017-10-05

###### GENERAL

2017, 66 (19): 190702.
doi: 10.7498/aps.66.190702

Abstract +

Fusion power offers the prospect of a safe and clean sustainable energy source, and is of increasing importance for meeting the world energy demand and curbing CO2 emissions. For an indirect-driven inertial confinement cryogenic target, the D-T ice layer inside the capsule should have a uniformity more than 99% and an inner surface roughness less than a root mean square value of 1 m to avoid Rayleigh-Taylor instabilities. And this highly smooth ice layer required for ignition is considered to be affected by the thermal environment around the fuel capsule. In the present study, a numerical investigation is conducted to examine the static and dynamic characteristics of the thermal environment outside the fuel capsule. Numerical model is proposed and verified by a simplified cryogenic target, and the calculated temperature distribution around the capsule shows to be in good agreement with the experimental data. Based on the established model, the propagation of periodic disturbance of cooling wall temperature in the hohlraum is investigated, and the relations between the temperature disturbance on the cooling wall and the temperature distribution around the capsule surface are obtained. The effects of disturbance amplitude, the disturbance period, and the hohlraum gas composition on the propagation process are investigated separately. The results indicate that for stable cooling temperature, the thermal environment around the capsule shows certain dependence on the gas filled in the hohlraum. The temperature uniformity of the capsule outer surface deteriorates with the increase of fill gas pressure but can be improved by increasing the He content of the filling gas mixture. At an oscillating cooling temperature, the attenuation of amplitude is significant when the periodic disturbance propagates from the cooling rings to the hohlraum and to the capsule surface. For the sine wave form disturbance investigated in the present study, shorter disturbance period results in larger attenuation of the disturbance amplitude. Higher gas pressure leads to smaller amplitude of average temperature on the capsule outer surface. The propagation process of cooling temperature disturbance also demonstrates dependence on the filling gas composition. The higher fraction of H2 in the He-H2 mixture helps to attenuate the disturbance amplitude and suppress the propagation of the temperature disturbance. However, the temperature uniformity around the capsule exhibits different characteristics from cooling temperature disturbance. Under the oscillating cooling conditions, moderate period, lower amplitude, lower pressure and higher fraction of He in the He-H2 mixture help to improve the temperature uniformity around the capsule. The results are of guiding significance for determining the controlling scheme in experiment and further design option for the cryogenic target.

2017, 66 (19): 190703.
doi: 10.7498/aps.66.190703

Abstract +

Raman spectroscopy is a powerful diagnostic method for gas analysis due to its advantages like non-invasiveness and fast speed. However, its applications are greatly restricted because of the weak signal level caused by small scattering cross section. In order to enhance the Raman signal level and improve the detection sensitivity, a sample cell of confocal cavity is designed and the enhanced Raman signal of ambient air based on this cavity is demonstrated experimentally. The confocal cavity is constructed with a pair of plano-concave reflectors with a curvature radius of 150 mm and reflectivity of 92%. This low reflectivity design not only allows for bandwidth matching with the line-width of excitation laser but also makes the resonant condition satisfied easily. The measured output power of the confocal cavity is over 42 mW in resonant condition, which gives a coupling efficiency of 87.5% when divided with the input power 48 mW. The high coupling efficiency enables the output power efficiently to reach 11 times that for the intra-cavity laser power in one direction. Raman scattering of ambient air is tested to verify the performance of the confocal cavity. In our experiments, the Raman signals are collected in a forward scattering configuration by an imaging Raman spectrometer which is connected to a CCD camera. Strong Raman signals of O2 and N2, even H2O are observed with 1 s exposure time in resonant condition, and rotational lines (O-branch and S-branch) of O2 and N2 are also clearly detected when exposure time is set to be 10 s. Compared with the results obtained without confocal cavity, the Raman signal level is enhanced 17 times and the signal-to-noise ratio is improved twice. In addition, a limit of detection (3) at a magnitude of 200 ppm for CO2 in ambient air is achieved for the resonant confocal cavity. These results indicate that the system can significantly enhance the spontaneous Raman scattering signal level and improve the detection sensitivity. Furthermore, the confocal cavity is applicable to the Raman analyses of other gas samples.

2017, 66 (19): 190704.
doi: 10.7498/aps.66.190704

Abstract +

A birefringent Fourier transform imaging spectrometer with a new lateral shearing interferometer is presented. The interferometer includes a Wollaston prism and a retroreflector. It splits an incident light beam into two shearing parallel parts to obtain interference fringe patterns of an imaging target, which is well established as an aid in reducing problems associated with optical alignment and manufacturing precision. The proposed method provides a direct technology for robust and inexpensive spectrometers to measure spectral signatures. Formulas for the optical path difference (OPD) produced by the proposed birefringent interferometer are derived by the ray-tracing method. Two experiments are carried out to demonstrate the accuracy of the formulas for OPD in the inner scanning mode and window scanning mode, respectively. A laser of wavelength 650 nm is used as a source of the experimental setup. The experimental estimations of the OPD and a reference OPD curve obtained with theoretical analysis are used for comparison. The match between the two curves is highly consistent, for the maximum deviation of the experimental OPD is less than /4. For the further verification of the imaging performance of the proposed method, another experiment is performed. A scene illuminated by an incandescent lamp is used as an imaging target. The temporal rotating of the retroreflector produces a series of time sequential interferograms, where the target is fixed and fringe patterns move. Performing nonuniform fast Fourier transform of the interferogram data produces a spectral data cube (i.e., the spectral images of the target).A series of recovered spectral images whose center wavelengths range from 450 to 650 nm is presented.In this paper, the principle of the instrument is described, and the OPD distribution formula is obtained and analyzed. The performance of the system is demonstrated through a numerical simulation and three experiments. This work will provide an important theoretical basis and the practical instruction for designing a new type of birefringent Fourier transform spectrometer based on Wollaston prism and its engineering applications.

2017, 66 (19): 190501.
doi: 10.7498/aps.66.190501

Abstract +

The permanent magnet synchronous generator (PMSG) for wind turbine system operating under inevitable stochastic disturbance from wind power is a nonlinear stochastic dynamical system. With the random interaction and nonlinearity, the intense nonlinear stochastic oscillation is likely to happen in such a system, causing the system to be unstable or even collapse. However, the PMSG is usually considered as a deterministic system when analyzing its nonlinear dynamic behaviors in the past researches. Such a simplification can lead to wrong predictions for the system stability and reliability. This paper aims to discuss the effect of the stochastic disturbance on the nonlinear dynamic behavior of the PMSG. Based on the derived PMSG model considering the stochastic disturbance from the input mechanical torque, the evolution of the system global structure with the stochastic intensity is investigated using the generalized cell mapping digraph method. Meanwhile, the occurrence process and development process of the stochastic bifurcation are illustrated. Based on this global analysis, the intrinsic mechanism for the effect of the stochastic disturbance on the operating performances of the PMSG is revealed. Finally, the numerical simulations based on the Euler-Maruyama algorithm are carried out to validate the results of the theoretical analysis. The results present that as the intensity of the stochastic disturbance increases, two kinds of stochastic bifurcations can be observed in the PMSG system according to the definition of a sudden change in characteristic of the stochastic attractor. One is the stochastic interior crisis that occurs when a stochastic attractor collides with a stochastic saddle in its attraction basin interior, leading to the abrupt increase of the attractor and the disappearance of the saddle. This kind of bifurcation results in the intense stochastic oscillation and instability of the PMSG system. Another stochastic bifurcation is the stochastic boundary crisis which occurs when a stochastic attractor collides with the boundary of its attraction basin and results in the disappearance of the attractor. This sudden change of the number of stochastic attractors induces the stable solution set to vanish and thus the PMSG system to collapse. In a word, even the stochastic disturbance with small intensity may lead to the complete destruction of the stable structure of the PMSG, inducing the system to suffer a strong disordered oscillation or the operation to collapse. The results of this paper can provide significant theoretic reference for both practically operating and designing the PMSG for wind turbine systems.

2017, 66 (19): 190701.
doi: 10.7498/aps.66.190701

Abstract +

Entropy force is fairly ubiquitous in nature, but it is not practically beneficial for most cases, thus how to reduce the entropic force of the system is very important. In this paper, by employing the overdamped Langevin dynamics simulations, we explore the entropy force between two large nanoparticles (or two nanorods) immersed in a self-propelled system. Self-propelled particles can be regarded as active matter, and the active matter is an interesting subject which has been studied theoretically and experimentally over the past few years. A great many biological and physical systems can be referred to as active matter systems, including molecular motors, swimming bacteria, self-propelled colloids, motile cells, and macroscopic animals. Active matter obtains energy from an external system under non-equilibrium conditions, and active particles with suitably designed constructions are able to convert energy input into the desired control of function, which has wide potential applications in a diversity of fields, such as drug delivery in medicine. Self-propelled particles without angular velocity would gather around the nanoparticles (or nanorods) under the effect of entropy force, which can induce large entropy force between nanoparticles. The interaction force between two nanoparticles is large enough, owing to the asymmetry of the system, and entropy force also depends on the distance between two nanoparticles (or two nanorods). For the case of self-propelled particles with an angular velocity, the entropic effect is weak, and the larger the angular velocity, the weaker the entropic force is. Moreover, nanoparticles will no longer assemble together because of their weak entropic forces. Meanwhile, the entropy force between two nanorods can be tuned from a long repulsion into a long range attraction by changing the distance between two nanorods. The present investigation can help us understand the entropy forces in non-equilibrium systems.

###### ATOMIC AND MOLECULAR PHYSICS

2017, 66 (19): 193301.
doi: 10.7498/aps.66.193301

Abstract +

We study the photodissociation of Br2 in a wavelength range from 360 nm to 610 nm in the near-visible UV continuum band based on the calculation of time-dependent quantum wave packet including the rotational degree of freedom. We calculate four representative samples of two-dimensional (2D) slice images taken from photolysis of Br2 molecules, in which the different rings in the 2D slice images are corresponding to the different photodissiation channels. The radius of each 2D slice image ring is positively related to kinetic energy of photofragment. The maximum photofragment flux perpendicular or parallel to the photolysis polarization is also related to photodissiation channel. Furthermore, we calculate the total kinetic energy distribution P(E) and the P(E) distribution from the respective electronic excited states A, B and C in the wavelength range of 360-610 nm, from which we find that the wavelengths corresponding to the maximum dissociation probability from respective electronic excited states A, B and C are 510 nm, 469 nm, and 388 nm, respectively. As is well known, not only the total dissociation probability, but also the respective dissociation probability of electronic excited states is dependent on the laser wavelength. We also calculate the dissociation probabilities from electronic excited states A, B and C, respectively. We find that the dissociation probability of electronic excited state A is not significant when 480 nm and that the peak intensity of the dissociation probability to the A state is about 13.0\% of that to the C state, while that to the B state is about 43.4\%. In addition, because the electronic excited states A and C are related to the photodissociation channel Br + Br, and the electronic excited state B is corresponding to the photodissociation channel Br + Br*, the images which reveal the involvement of more than one product channel can be analyzed by the respective channel branching ratios. At the short wavelength ( 400 nm) the branching ratio (Br*/(Br+Br*)) is small, even near to zero, which reflects that electronic state C transition gives rise to many Br + Br over Br + Br*. However, within the wavelength range (=440-500 nm) Br + Br* photofragments are excess of Br + Br, so the electronic state B transition is dominant. At longer wavelength ( 530 nm) the branching ratio (Br*/(Br+Br*)) is also low, near to zero, indicating the prevalence of electronic state A transition. Ignoring the dissociation from electronic state C, the maximum dissociation probability 469 nm is consistent with branching ratio maximum 462 nm. Because the electronic excited state C is related to the photodissociation channel Br + Br, the branching ratio will be reduced. So the maximum wavelength of branching ratio is blue shifted.

2017, 66 (19): 193701.
doi: 10.7498/aps.66.193701

Abstract +

The spectra of Rydberg atoms are of great significance for studying the energy levels of Rydberg atoms and the interaction between neutral atoms, especially, the high-precision spectra of Rydberg atoms can be used to measure the energy level shifts of Rydberg atoms resulting from the dipole-dipole interactions in room-temperature vapor cells. In this paper we report the preparation of cesium Rydberg states based on the cascaded two-photon excitation of 509 nm laser and 852 nm laser in opposite, and the measurements of the fine structure of cesium Rydberg states. In this experiment, the 509 nm laser is generated by the cavity-enhanced second-harmonic generation from 1018 nm laser with a periodically-poled KTP crystal and has a maximum power of about 1 W, and the 852 nm probe laser is provided by an external-cavity diode laser with a maximum output power of 5 mW and a typical linewidth of 1 MHz. By scanning the frequency of 509 nm coupling laser, it is presented that the Doppler-free spectra based on electromagnetically-induced transparency (EIT) of 509 nm coupling laser and 852 nm probe laser. The velocity-selective EIT spectra are used to study the spectral splitting of 6S1/26P3/257S(D) ladder-type system of cesium Rydberg atoms in a room-temperature vapor cell. The powers of 852 nm probe laser and 509 nm coupling laser are 0.3 upW and 200 mW, respectively. Their waist radii are both approximately 50 m. The intervals of hyperfine splitting of the intermediate state 6P3/2(F'=3, 4, 5) and fine splitting of 57D3/2 and 57D5/2 Rydberg states are measured by a frequency calibrating. Concretely, the velocity-selective spectrum with a radio frequency (RF) modulation of 30 MHz is used as a reference to calibrate the Rydberg fine-structure states in the hot vapor cell, where the RF frequency precision is smaller than a hertz on long time scales and the EIT linewidth is smaller than 13 MHz. The experimental value of the fine structure splitting of 57D3/2 and 57D5/2 Rydberg states is (354.72.5) MHz, that is in consistence with the value of 346.8 MHz calculated by Rydberg-Ritz equation and quantum defects of 57D3/2 and 57D5/2 Rydberg states. The experimental values of hyperfine splitting of intermediate state 6P3/2(F'=3, 4, 5) are also coincident with the theoretical calculated values. The dominant discrepancy existing between the experimental and calculated results may arise from the nonlinear correspondence of the PZT while the 509 nm wavelength cavity is scanned, and the measurement accuracy influenced by the spectral linewidth. The velocity-selective spectroscopy technique can also be used to measure the energy level shifts caused by the interactions of Rydberg atoms.

2017, 66 (19): 193201.
doi: 10.7498/aps.66.193201

Abstract +

Photoexcitation and photoionization of atoms, the central part of atom vapor laser isotope separation (AVLIS), relate to the ionization yield and isotope selectivity directly. Doppler broadening of absorption lines is one of the parameters that influence the photoexcitation and photoionization process of atoms. When evaporation temperature is high or beam equipment is absent, Doppler broadening has obvious influence on the ionization yield because most of atoms are non-resonantly excited. In this paper, we study the influences of Doppler broadening of absorption lines on a multi-step photoexcitation and photoionization process of atoms according to the facts of AVLIS. A Doppler-broadened three-level atom system with two resonant lasers is investigated. The interaction between laser field and atoms is described by a density matrix, which is calculated by fourth-order Runge-Kutta numerical method with variable steps. The results show that the ionization yield of atoms decreases with the increase of Doppler broadening of absorption lines under the same laser parameters. At a constant laser power, the ionization yield reaches its maximum value at the best laser bandwidths, which is different from that obtained without Doppler broadening, as reported in published papers. Meanwhile, the best laser bandwidth increases with the increase of Rabi frequency and Doppler broadening when other parameters are constant. Moreover, the best bandwidth of the second laser is smaller than that of the first laser in a multi-step photoionization process of atoms. Therefore, lasers should work at the best bandwidths in AVLIS for highest ionization yield. It is advantageous to make laser bandwidths slightly bigger than the best bandwidths technically for smaller fluctuation of ionization yield, owing to incoercible stochastic volatility in laser bandwidths. The ionization yield increases with the decrease of Doppler broadening, especially at the best laser bandwidths. Therefore, it is necessary to reduce Doppler broadening of atom vapor in laser ionization zone.

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

2017, 66 (19): 194201.
doi: 10.7498/aps.66.194201

Abstract +

Scattering introduced by optical surface fabrication errors could degrade optical performance severely. Therefore, the optical designers are required to provide a roughness index for describing the specific surface or even all surfaces to ensure the final imaging performance. The surface root-mean-square (RMS) roughness is a common index to quantify surface topography. And there are also some available methods to acquire the surface RMS roughness based on bidirectional scattering distribution function theory or the angle spread function theory. However, the influence of the optical surface scattering on the optical system cannot be accurately revealed by the surface RMS roughness determined by these methods. On the one hand, the RMS roughness corresponds to an excessively wide spatial frequency range from 0 to 1/, where is the wavelength of the light. Consequently, it is difficult to measure the RMS roughness during manufacture. On the other hand, what really worsens the stray light performance of the system is only the surface profile located within a certain subinterval of the aforementioned frequency range, to put it in another way, the surface RMS roughness identified by the methods above is incompetent to quantify the amount of the energy that is surfacescattered to the detector. To address the issues above, in this paper we propose a novel approach to identifying the surface roughness. This method seeks to deduce the relation between optical surface RMS roughness and the stray light requirement of the system by dint of partial integrated scattering (PIS). In contrast to total integrated scattering, PIS counts the scattering light energy that could reach the detector. Hence, the RMS roughness identified in this way corresponds to the effective spatial frequency range that contributes to the stray light in the system. Firstly, the effective frequency range concerned with the system stray light level is identified through the analysis of the propagation path of the scattered light. Then, the surface RMS roughness would be measured within the established range according to the stray light requirement of the system and used to control the surface roughness as the roughness index during the optical manufacture process. The method not only considers the scattering as the surface characteristic, but also takes into account the influence of scattering on the system. Taking the solar magnetic field telescope (MFT) for example, the validity of the method is verified by comparing with the traditional methods. As manifested in the outcome, the effective frequency range of primary mirror is from 0 to 18 mm-1, and the surface RMS roughness identified in such a new way can stage the stray light performance of MFT in a more precise manner, which is more reliable to serve as a surface roughness index.

## EDITOR'S SUGGESTION

2017, 66 (19): 194202.
doi: 10.7498/aps.66.194202

Abstract +

High power fiber lasers and amplifiers are widely used in the scientific and industrial field. In order to meet the requirements for high output powers the effective area of fibers becomes larger and larger to reduce optical nonlinearities. With the increase of effective area, the number of high-order modes will increase. In the case of high output power, the spectral shift and broadening of the optical fiber will also affect the modal number and content. The number and content of fiber modes affect the pointing stablity and quality of the laser beam. The M2-parameter is commonly used to define the quality of the laser beam, but a small M2 number is not guaranteed for single mode operation. Therefore, the relationship between wavelength and transmission mode in fiber transmission is studied in this paper. We use the spatial and spectral Fourier transform (F2) method to establish a theoretical-experimental method of describing the relationship between wavelength and mode. This method can directly give out the modal content of optical fibers without any priori parameter such as the properties of fiber and requirement for setup accuracy. On the one hand, the theoretical modeling of wavelength affects modal content. In the simulation, the sources with the same wavelength bandwidth and different central wavelengths are used to test the fiber. The results show that the modal content and number of the fiber change with the wavelength bandwidth and center wavelength. The mode components of the corresponding optical fiber will change after changing the central wavelength. As the spectral width of the light source increases, the number of high-order modes increases. On the other hand, in order to further verify the relationship between wavelength and mode of fiber, the F2 method is used to measure the optical fiber modal content with different wavelengths. The final experimental results are in agreement with the theoretical results. The experimental and simulation results show that the mode field distribution of each mode varies with wavelength:the longer the wavelength, the larger the mode field is. The beam quality has little change with the wavelength except for those positions with frequency near the cutoff frequency, and the power ratio of each mode relates to the wavelength.

2017, 66 (19): 194203.
doi: 10.7498/aps.66.194203

Abstract +

In this paper, a continuous-wave all-solid-state Nd:YVO4 self-Raman laser in-band pumped by a wavelength-locked laser diode at 878.9 nm is theoretically investigated in detail. Considering the thermal lens effect in the laser crystal, cavity mode parameters are calculated for several output couplers with different radii of curvature, by employing the standard ABCD matrix approach and equivalent G parameter method. The influence of cavity structure on the output characteristic of the Raman laser is investigated by analyzing mode matching between the pump and the fundamental beams, as well as the fundamental intensities in the Raman crystal. This provides theoretical explanations for the experimental results, and based on the analysis above, laser cavity is optimized. Finally, a highest Raman output of 5.3 W is obtained at 1175 nm, corresponding to a diode-to-Stokes optical conversion efficiency of 20%.

2017, 66 (19): 194302.
doi: 10.7498/aps.66.194302

Abstract +

Owing to the decomposition of organic material and other reasons, the actual marine sediment contains gas bubbles, and the existence of gas bubbles will significantly affect the low-frequency acoustic characteristics of sediment. Therefore, it is significant to investigate the effect of gas bubbles on the low-frequency sound velocity in the sediment. Considering the uncontrollable environmental factors of field experiment, an experiment platform for obtaining acoustic characteristics of a large-scale gas-bearing unsaturated sandy sediment is constructed in the indoor water tank. Considering the long wavelength of low-frequency acoustic wave and the multipath interference in water tank, the transmitted acoustic signals are received by hydrophones which are buried in the unsaturated sediment. The sound velocity data (79-142 m/s) in the gas-bearing unsaturated sediment are acquired by using a multi-hydrophone inversion method in the bounded space for the first time in a 300-3000 Hz range, and the sound velocity data (112-121 m/s) are also acquired by using a double-hydrophone method in the same frequency range. The refraction experiments at different horizontal distances between the source and the hydrophones are conducted, which verifies the reliability of sound velocity data acquired by using the multi-hydrophone inversion method and the double-hydrophone method. At the acoustic frequency well below the resonance frequency of the largest bubble in the sediment, the pore water and the gas bubbles are regarded as an effective uniform fluid based on effective medium theory. On this basis, the density and the bulk elastic modulus of pore water in the effective density fluid model are replaced by the effective density and the effective bulk modulus of the effective uniform fluid, then a corrected effective density fluid model is proposed in gas-bearing unsaturated sediment. The numerical analysis indicates that when the gas bubble volume fraction is small (1%), a small increase in the gas bubble will cause a significant decrease in the effective bulk elastic modulus of sediment, but the density of pore water is much greater than the density of gas bubbles, the presence of a small number of gas bubbles hardly changes the density of pore fluid and certainly does not change the density of sediment, which results in a significant decrease at a low-frequency sound velocity in the gas-bearing unsaturated sediment. Furthermore, with the increase of gas bubble volume fraction, the sound velocity predicted by the corrected model gradually decreases, and the decreasing trend gradually becomes gentle. The corrected model reveals the effect of gas bubbles on the low-frequency acoustic characteristic of sediment. By analyzing the sensitivity of the predicted sound velocity to parameters of the model, the gas bubble volume fractions (1.07%-2.81%) of different areas are acquired by inversion according to the measured sound velocity distribution and the corrected model. In the future, it will provide a new method of obtaining the volume fraction and the distribution of gas bubbles in the sediment.

2017, 66 (19): 194303.
doi: 10.7498/aps.66.194303

Abstract +

When ultrasound propagates in a liquid alloy, nonlinear effect takes place such as cavitation effect and acoustic streaming, which accelerates the solute and thermal transportation during alloy solidification, and consequently, improves the solidification microstructures and mechanical properties of the metallic alloy. Therefore, it is significant to investigate the ultrasound propagation characteristics in liquid. Here, by choosing water as a model transparent material, the acoustic fields and flow fields induced by 20 and 490 kHz ultrasounds are investigated by numerical simulation, and the effects of frequency and ultrasonic horn radius are studied. Firstly, the simulation results demonstrate that the sound pressure under 20 kHz ultrasound decreases obviously along the ultrasonic propagation direction, and the maximum of sound pressure value is equal to the initial pressure. In this case, the cavitation effect only occurs in the region close to the ultrasonic horn. By contrast, when the ultrasonic frequency increases to 490 kHz, the sound pressure is higher than that of 20 kHz ultrasound, and displays periodical vibration characteristic along the wave propagation direction. The cavitation volume correspondingly expands to a large extent with a regular striped distribution. It can also be found that increasing the ultrasonic horn radius under 20 and 490 kHz ultrasounds can effectively promote the sound pressure level in water, and hence leads to the remarkable enlargement of cavitation volume. Secondly, the calculated results of flow field indicate that the streamlines in water are similar under the two ultrasounds with different frequencies. A jet produced by the center of horn spreads down and divergences to both sides after reaching the bottom. For both frequencies as the horn radius increases, the radius of jet increases and the average velocity in water first increases and then decreases, whose maximum value appears when the horn radius is 40 mm. Meanwhile, the average velocity under 20 kHz ultrasound is larger than that under 490 kHz ultrasound for each horn radius. Finally, particle image velocimetry method is employed to measure the velocity field in water. Both the positions of eddy and the velocity distribution are the same as the simulation results, which verifies the reliability of the present theoretical calculation model. The scenario in this work is analogous to the acoustic field and the flow field in liquid alloy, which is beneficial for the design of parameter optimization during ultrasonic processing in alloy solidification.

2017, 66 (19): 194501.
doi: 10.7498/aps.66.194501

Abstract +

It is more common for drivers to pass through multiple sections to reach destinations instead of single road section. Howerver, most of researches concentrate on improving the effect in an independent section. Based on traditional cellular automata traffic model, a multi-section model is proposed by regarding serverl road sections as a traffic system. In this model, different sections of the road might have different lengths, numbers of lanes or maximal speeds. And vehicles travel from one section to another. The main difficulty lies in dealing with the relationships among the traffic flows of different sections. Besides basic rules in NaSch model, the vehicle inflow rule, crossroad randomization brake rule and crossroad inflow rule is added in this paper to enable vehicles to flow between sections. At the beginning of section, to avoid conflicting at crossroads under open boundary condition, the concept of car pool is introduced when new vehicles enter into sections. Before arriving at the end of section, crossroad randomization brake is used to simulate the influences of crossroads. Speed decreases in probability until lower than a maximal crossroad speed. When leaving the section, vehicles go to the next section with a straight ratio. Also, new vehicles may enter according to traffic condition. Therefore, cellular automata of different sections can be connected in series.Finally, numerical simulation is demonstrated to study the influences of important parameters, including traffic inflow probability, maximal crossroad speed and crossroad randomization brake probability. Compared with traditional models, this model focuses on connecting sections. And improvements of basic models can be implanted easily, thereby increasing the accuracy of the whole model in the future.
The experimental result are as follows. 1) According to space-time graphs of different inflow probabilities, there is a new kind of traffic flow called mixed flow. Traffic congestion often starts from crossroads, and spreads to the whole section. And traffic jams in previous section might relieve traffic pressure in latter section. 2) With the increase of traffic inflow probability, crossroads tends to have a greater influence on average speed as well as average traffic density. What is more, the moderate increase of vehicle numbers could cause the road capacity to drop rapidly if it exceeds the threshold value.

2017, 66 (19): 194204.
doi: 10.7498/aps.66.194204

Abstract +

In traditional orthogonal frequency-division multiple access passive optical networks (OFDMA PON) or time-division multiplexing access (TDMA) based OFDM PONs, analog-to-digital converters (ADCs) with a high sampling rate are required to demodulate high-speed aggregated OFDM data in order to receive a small portion of the downstream data at optical network users (ONUs). Meanwhile, OFDM signal has a higher peak-to-average power ratio (PAPR) than the single carrier signal, which can result in the nonlinear effect. The resulting nonlinearity reduces the received signal performance. To enhance practicability of the present PONs, according to the sub-Nyquist sampling theory, we propose and detail a delay-division-multiplexing (DDM) scheme to enable a FDMA PON with low-sampling-rate ADCs. Based on pre-allocated relative time delays among the ONUs and discrete Fourier transform spread (DFT-S) technique, pre-processed signals sent from an optical line terminal (OLT) can be detected as different downstream signals following spectral aliasing caused by ADCs operating at a sub-Nyquist sampling rate. In the proposed scheme, as the signal distortion introduced by the propagation, aliasing and time shifted sampling is pre-compensated, the DFT and inverse discrete Fourier transform (IDFT) are unnecessary for de-mapping and picking out the signal at ONUs. Therefore, the proposed DDM scheme greatly enhances cost efficiency and enables a reduction in computational complexity. Meanwhile, DFT-S FDMA signal has low PAPR, which relieves the nonlinear effect in signal E/O conversion and transmission. As a result, the proposed scheme benefits the power budget of the OLT and power consumption of the ONUs. In experiment, we demonstrate that each ONU with an ADC operating at 1/2-1/32 of the Nyquist sampling rate is able to receive 1/2-1/32 of the downstream data, with an insignificant performance penalty. Furthermore, the details of the matrices that include channel response, aliasing and time delay are first analyzed. In addition, training symbol is very important for estimating the channel response, and how to derive and design training symbols is the first study to outline the details of this issue. The effects of fiber dispersion and the sampling instant of an ADC on signal performance are also studied. The results show that the signal performance has some degree of tolerance to sampling instant deviation and the power penalty is less than 0.5 dB to achieve a forward error correction limit of 10-3 after 25 km fiber transmission. The theoretical analysis and experimental results indicate that the proposed scheme can simplify the ONU and reduce the cost of the PON.

2017, 66 (19): 194301.
doi: 10.7498/aps.66.194301

Abstract +

The toboggan in acoustic energy will appear at the top of the slope when the sound wave radiated by a shallow water source propagates in an upslope waveguide of the continental slope area. The grazing angles of the sound rays reflected by the ocean bottom will increase in the upslope waveguide, which leads to the acoustic energy tobogganing in the shallow water at the top of the slope. In this paper, the range of acoustic energy tobogganing (RAET) at a specified depth is defined to study this phenomenon. The transmission loss (TL) is calculated by the parabolic-equation acoustic model that ie applied to the range-dependent waveguide. The RAET is defined by an average transmission loss in the abyssal water and in the shallow water corresponding to the depth. The acoustic energy toboggan is explained using the ray-based model, and the effects of source location change on it are demonstrated, including the source depth and the range away from the bottom of the slope. The sound rays from a shallow water source which transmit in the upslope waveguide can be divided into two types:one is incident to the interface vertically and will return to the water along the original path; the other is that the rays will transmit towards the sound source (the deep sea direction). However, all of them will no longer spread forward after they have transmitted to a certain distance, leading to the acoustic energy tobogganing in shallow water. The analysis results show that the RAET becomes larger with source depth increasing, and the energy toboggan phenomenon will disappear when the source is deep enough. However, the range of source away from the slope bottom has less effect on RAET. Numerical simulations are conducted in a continental upslope environment by the RAM program based on the split-step Pad algorithm for the parabolic equation. The simulation results show as follows. 1) The TL will increase rapidly after the waves have transmitted to a certain range away from the bottom of the slope when the source depth is 110 m, and the TLs is 140-160 dB propagating to the shallow water at the top of the slope. 2) The RAET will enlarge orderly when the source depths are 110 m, 550 m and 800 m respectively, and the energy toboggan phenomenon will disappear when the source depth is more than 800 m. 3) Fix the source depth at 110 m and move it along the deep sea, then the RAET will greatly varies when the distance between the source and the slope bottom changes ina range of 1-15 km. However, the RAET remain almost constant at 69.8 km when the distance between the source and the slope bottom changes in a range of 16-50 km.

2017, 66 (19): 194304.
doi: 10.7498/aps.66.194304

Abstract +

When a scatterer is located in a diffuse noise field, time domain Green's function between two different receivers can be extracted from cross-correlation of ambient noise which is recorded by the two receivers so that target detection can be implemented. However, the method based on cross-correlation strongly depends on timing synchronization of each receiver, otherwise there will be a time drift in the cross-correlation result, which can bring error in the positioning detection. Besides, two receivers that are far from each other must communicate with each other to implement cross-correlation in real-time data processing, but big data transmission is difficult in the ocean. Compared with cross-correlation, autocorrelation means that each receiver works independently and only the final autocorrelation result is to be transmitted. Actually, the scattered wave of target is always so weak that it is submerged in the autocorrelation result of the ambient noise. In this paper, we propose a method of processing the autocorrelation of the ambient noise. When the averaging noise autocorrelation of all receivers is subtracted from the autocorrelation result of the noise recorded by each receiver, the signalnoise ratio of the scattered wave will be significantly enhanced. With the help of Kirchhoff migration algorithm, detection of a scatterer can be implemented. We have conducted a scatterer passive detection experiment in Shilaoren beach, Qingdao, and accurately detected the position of a polyviny chloride pipe (about 8 m away from the nearest receiver) using only 12 min surf noise data. The experimental result shows that the processing of autocorrelation could replace cross-correlation in passive target detection when the ambient noise is time steady and the statistical characteristics of the background noise at different receivers are the same. Unlike Green's function extracted from cross-correlation of ambient noise, each receiver can work independently without considering the problems of massive data transmission and timing synchronization, which may be suitable for target detection using multi-receivers and mobile platform.

2017, 66 (19): 194701.
doi: 10.7498/aps.66.194701

Abstract +

A mathematical model is established to investigate the gravity-driven draining process of a vertical thin liquid film containing insoluble surfactants when considering the synergistic effect of surface viscosity and disjoining pressure. Lubrication theory is used to derive a coupled equation set describing the evolution of the film thickness, surfactant concentration and surface velocity. The equation set is solved numerically by the FreeFem program based on the finite element method. The film is assumed to be supported by the wire frame at both the top and bottom, thus the mass of liquid and the mass of total surfactants are conserved in the simulation. The characteristics of film evolution under the constant and variable surface viscosity are examined. Simulation results show that the surface viscosity is a crucial factor affecting the film drainage process. When neglecting the effect of surface viscosity, the film surface exhibits the mobile mode, while the film surface presents the rigid mode in the case of the surface viscosity considered. Increasing the surface viscosity, the rate of film drainage is slowed down significantly, leading to a reduction of the film thinning and enhancement of film stability, which is consistent with the results obtained by Naire et al. The disjoining pressure is a key factor in the formation of black film. When the disjoining pressure is only involved in the model, the length of the black film region is relatively short, nevertheless, if the effect of surface viscosity is only considered, a stable black film does not form. Under the synergistic effect of the disjoining pressure and surface viscosity, a very long and thin but stable black film is found in the middle segment of the film. Additionally, the thickness of black film increases and the appearance time postpones with the increase of surface viscosity. Considering the influence of concentration-dependent surface viscosity, the drainage rate is greatly affected. In the early stage, due to the smaller overall surface viscosity, the surface velocity is relatively large. With increasing surface viscosity at the bottom of film, the strength of the film surface tends to be enhanced, and then the anti-perturbation ability of the film is promoted and the film thinning is retarded. There is no significant difference in the length nor the appearance time of black film except that the thickness of black film with concentration-dependent surface viscosity is lower than that with the constant viscosity, thus the flow stability is weaker than that with the constant viscosity. In addition, the presence of the disjoining pressure slows down the thinning of blackest portion of the film and the surfactant concentration at this position. In the numerical results of the variable surface viscosity given by Braun et al. it is observed that the concentration of surfactant could almost be swept to clean in the top part of film. That is possibly because the effect of the disjoining pressure is neglected by Braun et al. It should be pointed out that the surface elasticity plays an important role in the stability of film. Therefore, it is necessary to consider the effect of surface elasticity in the future investigation.

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

2017, 66 (19): 195201.
doi: 10.7498/aps.66.195201

Abstract +

The penetration depth and the fueling efficiency of the supersonic molecular beam injection (SMBI) are affected by both the intrinsic parameters of the SMBI and the parameters of background plasma. The purpose of the present paper is to explore the possible methods of improving the fueling efficiency of SMBI by varying the beam parameters.
The penetration depths and the transport processes of SMBI with different beam densities and different beam widths are studied using the trans-neut module of the three-dimensional (3D) edge turbulence simulation code BOUT++. In our present study, the number of the injected molecules per unit time the injection speed and the injected flux are kept constant throughout the SMB fueling process, but the beam density and beam width are adjusted. The simulation is based on the real magnetic configuration of the HL-2A tokamak.
Our results indicate that the deeper injection depth can be obtained with a supersonic molecular beam (SMB) with smaller density and larger width. However, the injection depth decreases when the beam density or the beam width increases. The residence time of the beam front can be lengthened by increasing the beam density and widening the beam width. If the beam density increases or the beam width enlarges, not only the injection depth decreases, but also the residence time shortens. The front of the atom density exhibits the behaviors analogous to that of the SMB, namely, both its depth and its residence time decreases with beam density increasing and beam width decreasing. At the same time, the dissociation rate has a larger range in the spatiotemporal coordinate. The global growth of dissociation rate is inhibited by the molecular dissociation localization. However, the localization of the molecular dissociation accelerates the local growth of the dissociation rate, and the global growth of the molecular dissociation rate is promoted. When the promoting effect is dominant, under the condition of constant flux and fixed injection speed, the smaller molecular injection width will lead to the shallower molecular penetration depth.
The simulation results suggest that if we attempt to promote the fueling efficiency and to increase the injection depth of SMBI, we should utilize the SMBI with a smaller density and larger beam width. Of course, the concrete influences of the SMBI on injection depth and fueling efficiency should be studied further by varying other relevant parameters of the SMB and the backgroud plasma.

2017, 66 (19): 195202.
doi: 10.7498/aps.66.195202

Abstract +

High-power microwave (HPM) weapon, which is destructive to electronic systems, has developed rapidly due to the great progress of HPM devices and technologies. Plasma with distinctive electromagnetic characteristics is under advisement as one of potentially effective protection materials. Therefore, research on avalanche ionization effect in plasma caused by the interaction between HPM and plasma is of significance for its HPM protection performance. Based on the method of fluid approximation, the wave equation, the electron drift diffusion equation and the heavy species transport equation, explaining the propagation of microwave and the change of the charged particles inside plasma, are established to study the avalanche ionization effect under the HPM radiation. A two-dimensional physical model is built with the help of software COMSOL according to the plasma protection array designed to disturb the propagation of the HPM pulses. It can be shown that the emergence of avalanche effect is greatly affected by the incident power of microwave, and the generation time would be influenced by the initial electron density. Moreover, it can be observed that the avalanche effect appears only when the plasma array is irradiated for a period of time, which means that the performance of HPM is presented as gathering effect, and a large amount of energy is needed to change the internal particle balance in plasma. In addition, the electron density inside the plasma changes rapidly and complicatedly while the avalanche effect comes into being. Besides, the cutoff frequency of the plasma exceeds the frequency of the incident wave with the increase of electron density, which leads to that the electromagnetic wave cannot propagate in the plasma, so that the plasma can be used to protect the HPM irradiation.

2017, 66 (19): 195101.
doi: 10.7498/aps.66.195101

Abstract +

Streamer is usually present at the initial stage of atmospheric pressure air discharge, which occurs in nature as a precursor to lightning, transient luminous events in upper atmosphere and has much potential applications in industry, such as the treatment of polluted gases/liquids, assisted combustion, plasma enhanced deposition etc. Streamer is a multi-scale problem both in time and in space, which brings much difficulty to the conventional diagnostic approaches. In past decades, fluid or particle-fluid hybrid models have been frequently used for understanding the mechanisms of streamer discharges because of their high efficiencies of calculations. Accuracies of the electron transport coefficients (including drift/diffusion coefficient, ionization/attachment coefficient, electron mean energy and extra) play a key role in ensuring the correctness of the fluid or hybrid simulations. As far as we know, BOLSIG+ and MAGBOLTZ are two typical tools for obtaining the electron transport coefficients and have been widely utilized in previous models. BOLSIG+ uses two-term approximation which is not sufficient for some molecular gases, MAGBOLTZ cannot calculate the bulk transport coefficients:these data are required for some models. METHES is an additional tool for computing electron transport coefficients, however, specific platform is required which is not very user-friendly. As sorts of drawbacks exist in currently available calculating tools, in the paper, a Monte Carlo model is developed for computing the electron transport coefficients in gases, the model is flexible to choose any type of gas mixture and its accuracy has been validated by comparing with BOLSIG+ and METHES. Furthermore, the influences of N2-O2 mixture and three-body attachment process in high gas pressures on the transport coefficient are investigated. It is worth mentioning that three-body attachment process can significantly change the electron transport properties at a relatively low reduced electric field. Thus, specific attention must be paid to the transport coefficients if simulation is performed at a high pressure. In addition, differences between the bulk and flux coefficients are analyzed which are not distinguished in some previous models. Finally, we further validate the present Monte Carlo model by performing simulation of streamer discharge in atmospheric N2, which shows that the improved electron transport coefficient from our Monte Carlo model can improve the simulated plasma properties, in particular at the interior of the streamer channel. The existence of divergence at the tip of the streamer channel might be due to our local field approximation; if a density gradient term is included in the impact ionization term and local electron energy approximation of the electron transport coefficients is used, the accuracy of the fluid can be improved further.

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

2017, 66 (19): 196401.
doi: 10.7498/aps.66.196401

Abstract +

The spatial public goods game is one of the most popular models for studying the emergence and maintenance of cooperation among selfish individuals. A public goods game with costly punishment and self-questioning updating mechanism is studied in this paper. The theoretical analysis and Monte Carlo simulation are involved to analyze this model. This game model can be transformed into Ising model with an external field by theoretical analysis. When the costly punishment exists, the effective Hamiltonian includes the nearest-, the next-nearest-and the third-nearest-neighbor interactions and non-zero external field. The interactions are only determined by costly punishment. The sign of the interaction is always greater than zero, so it has the properties of ferromagnetic Ising. The external field is determined by the factor r of the public goods game, the fine F on each defector within the group, and the relevant punishment cost C. The Monte Carlo simulation results are consistent with the theoretical analysis results. In addition, the phase transitions and critical behaviors of the public goods game are also studied using the finite size scaling theory. The results show that the discontinuous phase transition has the same finite size effects as the two-dimensional Ising model, but the continuous phase transitions is inconsistent with Ising model.

2017, 66 (19): 196801.
doi: 10.7498/aps.66.196801

Abstract +

P-type hydrogenated silicon oxide (p-SiOx:H) films are prepared by radio frequency plasma enhanced chemical deposition with various CO2 flow rates. We use gas mixtures of carbon dioxide (CO2), hydrogen (H2), silane (SiH4) and diborane (B2H6) as reaction source gases. For all experiments the substrate temperature, pressure and power density are fixed at 200 oC, 200 Pa and 200 mW/cm2, respectively. The films are deposited on Corning Eagle 2000 glass substrates for optoelectronic measurements and on crystalline Si wafers for Fourier transform infrared (FTIR) measurement. The structural, optical and electronic properties of the films are systematically studied as a function of CO2 flow rate. The CO2 flow rate is varied from 0 to 1.2 cm3 min-1, with all other parameters kept constant. It is shown that with the CO2 flow rate increasing from 0 to 1.2 cm3 min-1, the Raman peak shifts from 520 cm-1 to 480 cm-1 and corresponding crystalline volume fraction decreases from 70% to 0. In addition, the FTIR spectrum shows that the oxygen content increases from 0 to 17% and the hydrogen bond configuration gradually shifts from mono-hydrogen (Si-H) to di-hydrogen (Si-H2) and (Si-H2)n complexes in the film. What is more, with the incorporation of oxygen, the optical band gap of each of all p-type SiO:H films increases from 1.8 eV to 2.13 eV, while the dark conductivity decreases from 3 S/cm (nc-Si:H phase) to 8.310-6 S/cm (a-SiOx:H phase). Furthermore, the oxygen incorporation tends to disrupt the growth of silicon nanocrystals due to the created dangling bonds that arises from an increased structural disorder. This leads to microstructural evolution of SiO:H film from a single nanocrystalline phase into first a mixed amorphous-nanocrystalline and subsequently into an amorphous phase. At a certain threshold of CO2 flow rate, a transition from nanocrystalline to amorphous growth takes place. The transition from nanocrystalline to amorphous silicon is confirmed by Raman and FTIR spectra. In the transition region or crystalline volume fraction of about 45%, Raman spectrum also reveals that the a mixture of nanocrystalline silicon and amorphous silicon oxide (a-SiOx:H) phase exists in the film. This means that nanocrystalline silicon oxide (nc-SiO:H) is a two-phase structural material consisting of a dispersion of silicon nanocrystals (nc-Si) embedded in the amorphous SiOx network. As is well known, the oxygen-rich amorphous phase can help enhance the optical band gap, while the nc-Si phase contributes to high conductivity. Finally, it is the SiO:H film deposited at phase transition that can realize a relatively high dark conductivity (about S/cm) with a wide optical band gap of 2.01 eV in the film. By using the transition p-layer as the window layer in conjunction with a suitable buffer thickness, we obtain a thin film solar cell with an open-circuit voltage of 890 mV, a short-circuit current density of 12.77 mAcm-2, fill factor of 0.73, and efficiency of 8.27% without using any back reflector.

2017, 66 (19): 196802.
doi: 10.7498/aps.66.196802

Abstract +

The excellent tribological characteristics of two-dimensional (2D) materials have received great attention, however, how to effectively predict their frictions is still lacking. Here, we propose to obtain the sliding potential energy surface by density functional theory calculations, instead of simplified potential energy function. Thus it is able to solve the frictional behaviors of 2D materials with irregular complex potential energy surfaces. Firstly, we reveal the mechanism of dual-scale stick-slip behavior between a tip and a graphene/Ru(0001) heterostructure. With a dual-wavelength potential energy surface, we observe a similar frictional behavior to those captured in atomic force microscopy experiments, in which a significant long-range stick-slip sawtooth modulation emerges with a period coinciding with the Moir superlattice structure. Secondly, we discuss the interlayer frictions of 2D materials, including graphene/graphene, fluorinated graphene/fluorinated graphene, MoS2/MoS2, graphene/MoS2 and fluorinated graphene/MoS2. With sliding potential energy surface obtained by density functional theory calculations, the interlayer friction is estimated according to the Prandtl-Tomlinson model calculation method. Compared with the friction between homostructures, the friction between heterostructures is lowered by orders of magnitude, which could be attributed to its ultralow sliding potential barrier. The stick-slip instability could be observed in homostructure, while heterostructure exihibits smooth friction loops. The 2D sliding path between the layers is recorded in the sliding process, showing its dependence on both the potential energy barrier and the spring constant. The sliding path shift increases with the increase of potential energy barrier and the decrease of spring constant in the y direction. This method is also applicable to tribological systems with dominated interfacial van der Waals interaction.

## EDITOR'S SUGGESTION

2017, 66 (19): 196201.
doi: 10.7498/aps.66.196201

Abstract +

Adseverin is a member of calcium-regulated gelsolin superfamily existing in secretory cells,which functions as an actin severing and capping protein.Adseverin is comprised of six independently folded domains (A1-A6),sharing high sequence identity (60%) with that of gelsolin (G1-G6).Calcium binding can convert both adserverin and gelsolin from a globular structure into a necklace structure and expose the actin binding sites.However,compared with gelsolin, adseverin lacks a C-terminal extension.Our previous single molecule force spectroscopy studies indicated that the Cterminal helix is critical to the force regulated calcium activation of gelsolin.It remains largely unexplored how the calcium binding to adseverin is regulated by force.
Here,using atomic force microscopy based single molecule force spectroscopy,we demonstrate that the mechanical unfolding of the sixth domain of adseverin (A6) can be significantly affected by calcium binding.In order to identify the unfolding events of A6 unambiguously,we construct a hetero-polyprotein (GB1-A6)4,in which A6 is spliced alternatively with well-characterized protein domain GB1.Therefore,in the force-extension traces,GB1 unfolding events can serve as a fingerprint to identify the unfolding signature of A6.
In the absence of calcium,the unfolding traces for (GB1-A6)4 show two distinct categories of events.The higher force events with unfolding forces of ～180 pN and contour length increments of ～ 18 nm correspond to the unfolding of GB1.The other category of events with lower unfolding forces of ～ 25 pN and contour length increments of ～35 nm are attributed to the mechanical unfolding of A6.The unfolding force for A6 is similar to that for the structural homological protein,G6.
However,in the presence of calcium ion,the unfolding force of A6 is dramatically increased to ～45 pN,indicating that the structure of A6 can be mechanically stabilized by calcium ion-binding.Moreover,we observe a clear mechanical unfolding intermediate state for the unfolding of calcium bound A6(holo A6).Upon stretching,holo A6 is first partially unfolded to an intermediate state with a contour length increment of ～7.2 nm.Then,the intermediate state is unfolded to release a contour length of ～27.8 nm.The total contour length change is the same as that for the calcium free A6 (apo A6).Because each amino acid in the unfolded structure corresponds to a contour length increment of 0.365 nm,according to the contour length change,we infer that in the unfolding intermediate state of A6,its N-terminal regions is partially unfolded.This leads to the exposure of the cryptic actin binding site on A5,which is otherwise buried in the folded structure of A6.The force regulated activation mechanism for A6 is similar to that for G6,except that they use different sequences from those in the force-sensitive region.In G6 the C-terminal helix serves as the force-responsive tail to regulate actin binding,while in A6 the N-terminal sequences are unstructured upon stretching to promote the actin binding for adseverin.
Therefore,we infer that force may be an important regulator for the actin-binding of all members in the gelsolin family proteins,including adseverin and gelsolin.Our study represents an important step towards the understanding of the function of adseverin at a molecular level.

2017, 66 (19): 196101.
doi: 10.7498/aps.66.196101

Abstract +

This investigation aims at an Nb-doped -TiAl intermetallic compound system in which part of Ti or Al atoms are substituted by Nb atoms. The structural parameters, the energy band structures, the electronic densities of states and the elastic constants of Nb-doped -TiAl intermetallic compound are calculated and studied by using the first-principles method based on the density functional theory and other physical theory. The first-principle calculations presented here are based on electronic density-functional theory framework. The ultrasoft pseudopotentials and a plane-wave basis set with a cut-off energy of 320 eV are used. The generalized gradient approximation refined by Perdew and Zunger is employed for determining the exchange-correlation energy. Brillouin zone is set to be within 333 k point mesh generated by the Monkhorst-Pack scheme. The self-consistent convergence of total energy is at 1.010-6 eV/atom. In view of geometry optimization, it is shown that doping with Nb can change the structural symmetry of the -TiAl intermetallic compound. The calculated formation energies indicate that the formation energy of the system in which Ti atom is replaced by Nb atom is smaller than that of Al atom replaced by Nb atom. Accordingly, they tend to substitute Ti atom when Nb atoms are introduced into the -TiAl system. The calculated band structures of Nb-doped -TiAl system show that they all have metallic conductivities, which implies that the brittleness of -TiAl intermetallic compound could be tailored by Nb-doping. The partial densities of states of the Nb-doped and pure -TiAl systems indicate that the intensity of covalent bond between Ti atom and Nb atom is weaker than covalent bond between Ti atom and Al atom while the Ti atoms are replaced by Nb atoms in the -TiAl system. What is more, the density of states near Fermi energy increases after Al atoms has been replaced by Nb atoms in the -TiAl system. This is an important factor for improving the ductility of -TiAl intermetallic compound. The calculated elastic constants, bulk modulus and shear modulus of Nb-doped -TiAl systems indicate that the ductility and the fracture strength of Nb-doped -TiAl system are both better than those of pure -TiAl system, especially in the system where part of Al atoms are replaced by Nb atoms. The plastic deformation capacity of Nb-doped -TiAl system is thus improved comparatively.

2017, 66 (19): 196402.
doi: 10.7498/aps.66.196402

Abstract +

Porous polymers have received much attention in recent years because of their light quality,high strength,good permeability and easy-revisable.Various fabrication methods of porous polymers have been used in which ice templating is a process which can prepare porous materials with complex structures and fine microstructures.This method has been widely used to prepare porous polymers but it still has many problems,such as poor homogeneity of pore distribution and pore connectivity.To solve these problems,it is necessary to understand the morphology of ice crystal growth in the solidification process of polymer solution.In situ observation of directional solidification is adopted in this paper to study the morphology evolution during directional solidification of polyvinyl alcohol (PVA) aqueous solution with different concentrations and molecular weights under different pulling speeds.The experimental results show that the primary dendrite spacing of PVA aqueous solution decreases with the increase of pulling speed at low concentration (1 wt%,2.5 wt%).However,increasing PVA concentration does not result in significant change in primary dendrite spacing.The primary dendrite spacing varies with pulling speed whereas the dendritic primary arm tends to shrink with increasing velocity.The effects of PVA concentration and pulling speed on morphology are partly because of diffusion instability from the classical solidification theory.When the concentration of solution is 5 wt%,there is little change of primary dendrite spacing with the velocity,which is due to the suppressed diffusion instability by high concentration of the polymer solution and large viscosity.When the concentration of solution increases to 10 wt%,ice crystal morphology is seaweed-like,where the PVA molecules are enriched and crosslinked ahead the ice crystal,leading to the continuous bifurcation of the dendrites.For the solidification morphologies of the aqueous solutions with different PVA molecular weights,the primary dendrite spacing of PVA aqueous solution decreases with the increase of pulling speed at low molecular weight (Mw=24000).Increasing PVA molecular weight does not result in significant change in primary dendrite spacing.At the low PVA molecular weight,the interface shows cell morphology.With the increase of PVA molecular weight,the large chain length leads to the stronger interaction among them and suppressing their diffusion. The corresponding constitutional undercooling is strengthened,thereby promoting the interfacial instability and dendrite formation.From the classical solidification morphology formation mechanism it may be concluded that the solidification morphology of PVA aqueous solution is determined by the competition between the two different mechanisms,i.e., interface instability induced by diffusion of PVA molecule and the local phase separation from the crosslinking of PVA polymer chains.

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

2017, 66 (19): 197501.
doi: 10.7498/aps.66.197501

Abstract +

The low-dimensional quantum spin systems have been extensively studied in the past three decades due to the novel ground states and rich magnetic behaviors,especially the quantum spin chain with diamond topology structure. Motivated by recent experimental success in Cu3(CO3)2(OH)2 compound,which is regarded as a model material of spin-1/2 diamond chain,researchers have paid a lot of attention to various variants of diamond spin chains.In this paper,we mainly examine the magnetic properties of an antisymmetric spin-1/2 Ising-Heisenberg diamond chain with the secondneighbor interaction between nodal spins.By using exact diagonalization and transfer-matrix methods,the ground-state phase diagram,magnetization behavior and macroscopic thermodynamics are exactly solved for the particular case that all magnetic bonds yield antiferromagnetic couplings,which usually shows the most interesting magnetic features closely related to a striking interplay between geometric frustration and quantum fluctuations.To clearly illustrate the effect of second-neighbor interaction item,we consider a highly frustrated situation that all Ising-Heisenberg bonds and Heisenberg bonds possess the same interaction strength.The calculation results indicate that the second-neighbor interaction item will enrich ground states and magnetization plateaus.A classical ferrimagnetic phase FRI1 corresponding to a novel two-thirds of intermediate plateau with translationally broken symmetry is introduced,manifesting itself as up-up-up-down-up-up spin configuration at a ground-state.In addition,there are other four distinct ground states which can be identified from the phase diagram,i.e.,one saturated paramagnetic phase SP,one classical ferrimagnetic phase FRI2,one quantum ferrimagnetic phase QFI and the unique quantum antiferromagnetic phase QAF.The classical phase FRI2 and quantum phase QFI both generate one-third of magnetization plateau.It is worth mentioning that all the values of these magnetization plateaus satisfy the Oshikawa-Yamanaka-Affleck condition.Besides,the results also have shown a rich variety of temperature dependence of total magnetization and specific heat.The magnetization displays the remarkable thermal-induced changes as the external field is sufficiently close to critical value where two or more than two different ground states coexist.At the critical field relevant to a coexistence of two different states,the total magnetization displays a monotonic decrease trend.The thermal dependence of zero-field specific heat displays relative complex variations for different second-neighbor interactions between nodal spins.At first,the specific heat presents only a single rounded Schottky-type maximum.Using the second-neighbor interaction,another sharp peak arises at low-temperature and is superimposed on this round maximum,and the specific heat exhibits a double-peak structure. On further strengthening,the low-temperature one keeps its height shifting towards high temperature,while the hightemperature round peak suffers great enhancement and moves in an opposite direction.Finally,the low temperature peak entirely merges with the Schottky-type peak at a certain value of second-neighbor interaction,and above this value, the specific curve recovers its single peak structure.The observed double-peak specific heat curves mainly originate from thermal excitations between the ground-state spin configuration QAF and the ones close enough in energy to the ground state.

2017, 66 (19): 197302.
doi: 10.7498/aps.66.197302

Abstract +

Crystal structures of GaAs nanowires prepared by employing molecular beam epitaxy technique are often dominated by the wurtzite (WZ) phase.Recently,Galicka et al.found that the WZ GaAs nanowires grown along the[0001]direction in smaller size are energetically more favorable than other nanowires with the zinc blende phase grown along a specific direction (2008 J.Phys.:Condens.Matter 20 454226).The native nanowire usually has abundant unsaturated surface dangling bonds (SDBs) inducing significant surface states,leading to electrons accumulating at the nanowire surface. Thus the electrical property of the nanowire is very sensitive to the surface condition.However,surface passivation can effectively remove the surface states from the SDBs,and optimize the device performance.In this paper,using the first-principle calculations in combination with density function theory,we investigate the effect of surface passivation on the electronic structure of the GaAs nanowires grown along the[0001]direction.Various passivation species (hydrogen (H),fluorine (F),chlorine (Cl) and bromine (Br)) with different coverage ratios are considered.The GaAs nanowires hydrogenated with different locations and coverage ratios display different electronic properties.It is found that the GaAs native nanowire with a smaller diameter shows a semiconductor characteristic with indirect band gap,which originates from the fact that at smaller diameter,the surface stress becomes more remarkable,and then leads to surface atomic reconstruction.After passivation,the indirect band gap is translated into the direct band gap.For the GaAs nanowire with an As SDB hydrogenated,one deep donor level is located in the gap,and its band structure shows an n-type characteristic.For the GaAs nanowire with a Ga SDB hydrogenated,one shallow acceptor level is located in the gap,and its band structure shows a p-type characteristic.For the GaAs nanowire with a Ga-As dimer hydrogenated, its band structure shows an intrinsic semiconductor characteristic.For the GaAs nanowire with all of the Ga SDBs hydrogenated,the band structure shows a metallic characteristic.The band gap of the GaAs nanowire gradually increases as the hydrogen passivation ratio increases.For 50% hydrogen passivation,the band gap for the symmetrical passivation is slightly bigger than that for the half-side passivation.For the F-,Cl-and Br-passivation,the band gap decreases compared with for H-passivation.This is due to the fact that the ability of passivating atoms to compensate for surface atoms is weak,thereby reducing the band gap.The mechanism for the surface passivation is the suppression of surface states by the ability of the passivating atoms to compensate for surface atoms.These results show that the electronic properties of GaAs nanowires can be modulated by surface passivation,which is helpful for using GaAs nanowires as components and interconnections of nanoscale devices.

2017, 66 (19): 197301.
doi: 10.7498/aps.66.197301

Abstract +

Gallium nitride (GaN)-based high electron mobility transistor (HEMT) power devices have demonstrated great potential applications due to high current density, high switching speed, and low ON-resistance in comparison to the established silicon (Si)-based semiconductor devices. These superior characteristics make GaN HEMT a promising candidate for next-generation power converters. Many of the early GaN HEMTs are devices with Schottky gate, which suffer a high gate leakage and a small gate swing. By inserting an insulator under gate metal, the MIS-HEMT is highly preferred over the Schottky-gate HEMT for high-voltage power switche, owing to the suppressed gate leakage and enlarged gate swing. However, the insertion of the gate dielectric creates an additional dielectric/(Al) GaN interface that presents some great challenges to AlGaN/GaN MIS-HEMT, such as the threshold voltage (Vth) hysteresis, current collapse and the reliability of the devices. It has been reported that the poor-quality native oxide (GaOx) is detrimental to the dielectric/(Al) GaN interface quality that accounted for the Vth instability issue in the GaN based device. Meanwhile, it has been proved that in-situ plasma pretreatment is capable of removing the surface native oxide. On the other hand, low power chemical vapor deposition (LPCVD)-Si3N4 with free of plasma-induced damage, high film quality, and high thermal stability, shows great potential applications and advantages as a choice for the GaN MIS-HEMTs gate dielectric and the passivation layer. In this work, an in-situ pre-deposition plasma nitridation process is adopted to remove the native oxide and reduce surface dangling bonds prior to LPCVD-Si3N4 deposition. The LPCVD-Si3N4/GaN/AlGaN/GaN MIS-HEMT with a high-quality LPCVD-Si3N4/GaN interface is demonstrated. The fabricated MIS-HEMT exhibits a very-low Vth hysteresis of 186 mV at VG-sweep=(-30 V, +24 V), a high breakdown voltage of 881 V, with the substrate grounded. The hysteresis of our device at a higher positive end of gate sweep voltage (VG +20 V) is the best to our knowledge. Switched off after an off-state VDS stress of 400 V, the device has a dynamic on-resistance Ron only 36% larger than the static Ron.

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

## EDITOR'S SUGGESTION

2017, 66 (19): 198501.
doi: 10.7498/aps.66.198501

Abstract +

The high-speed deep space communication is one of the key technologies for deep space exploration. Laser communication system equipped with sensitivity of single photon will improve existing deep space communication speed. However, laser communication at single photon level needs to consider not only the effect of transmission environment, but also the performance of used single photon detector and the photon number distribution. As a new single photon detector, superconducting nanowire single photon detector (SNSPD) outperforms the traditional semiconducting SPDs at near infrared wavelengths, and has high detection efficiency, low dark count rate, low timing jitter, high counting rate, etc. The SNSPD can be used for detecting single photons efficiently, rapidly and accurately. In this paper, we introduce the system detection efficiency and dark count rate of SNSPD based on the photoelectric detecting model without considering the effect of atmospheric turbulence, establish the mathematical model of bit error, and put forward the formula of system bit error rate. What should be emphasized is that the bit error rate is an important parameter for measuring the performance of laser communication system. Error is partly from background thermal radiation and circuit electromagnetic interference; in addition, error appears when photons reach the surface of device without being absorbed to successfully produce resistance area or photons are absorbed but there occurs no response. As a result, the calculation of bit error rate includes the whole process of photoelectric conversion. In order to analyze how to affect the size of system bit error rate, first we simulate two factors of the formula, i.e., light intensity and laser pulse repetition frequency. The results show that the light intensity has the greatest influence on error bit rate. With the light intensity increasing from 0.01 to 1000 photon/pulse, the error bit rate significantly decreases from 10-1 to 10-7 level. The influence of laser pulse repetition frequency is restricted by the light intensity, which declines with the increase of pulse repetition frequency. Then we measure the error bit rate experimentally, which validates the simulation model. However, when increasing light intensity or speed, experimental bit error rate is about 10-4 times higher than simulation result. The reason may be that the insufficiency of actual communication modulation extinction ratio of optical signal to the background noise through optical fiber increases the dark count rate. The above model and experimental results could be the foundation of high-speed deep space laser communication such as moon-earth and Mars-earth based on SNSPD.

2017, 66 (19): 198502.
doi: 10.7498/aps.66.198502

Abstract +

Silicon-based photonics has aroused an increasing interest in the recent year, mainly for optical telecommunications or optical interconnects in microelectronic circuits. The waveguide photodetector is one of the building blocks needed for the implementation of fast silicon photonics integrated circuits. The main considerations for designing such a device are the bandwidth, the power consumption and the responsivity. Germanium is now considered as an ideal candidate for fully integrated receivers based on silicon-on-insulator (SOI) substrates and complementary metal oxide semiconductor (CMOS)-like process because of its large optical absorption coefficient at the wavelength for optical communication. Therefore, the study of high speed and high responsivity Ge waveguide photodetectors is necessary. In this paper, high concentration phosphor doped SOI substrate is achieved by using solid-state source diffusion at first. Secondly, the high quality epitaxial germanium (Ge) is grown on phosphor doped SOI substrate by using low temperature Ge buffer layer technique based on the UHV/CVD system. The surface profile, crystal quality and strain of epitaxial Ge film are characterized by using atomic force microscopy, X-ray diffraction (XRD), and Raman scattering spectrum. The results show that the Ge film has a smooth surface of 1.12 nm roughness and about 0.2% tensile strain, which is verified by XRD characterization result. Thirdly, ptype Ge region is formed by BF2+ implantation, and rapid thermal annealing to repair the implantation damages and activate impurity. Finally, the highperformance Ge PIN waveguide photodetectors with different sizes are fabricated by standard COMS technology. Moreover, the device performances, in terms of dark current versus voltage characteristics, photocurrent responsivity and 3 dB bandwidth, are well studied. The results show that the detector with a size of 4 m20 m demonstrates a dark current density of 75 mA/cm2 at -1 V and a photocurrent responsivity of 0.58 A/W for 1.55 m optical wavelength. In addition, an optical band width of 5.3 GHz at -2 V for 1.55 m is also demonstrated, which is far below theortical value of about 40 GHz. This can mainly be attributed to two aspects. On the one hand, Ge PIN structure contains low temperature Ge buffer layer, which has highdensity dislocation because of large lattice mismatch between Si and Ge. Those dislocations or defects can trap and release the photo-generated carrier, which increases the transit time. On the other hand, the contact characteristics of Al with n+-Si and p+-Ge are not very good, leading to a large contact resistance and RC delay. Through improving the above two aspects, the performance of Ge PIN waveguide photodetector will be further enhanced.

2017, 66 (19): 198201.
doi: 10.7498/aps.66.198201

Abstract +

Inverted polymer solar cells with P3HT:PCBM as active layer are fabricated based on poly(dopamine)/ZnO (PDA/ZnO) as composite cathode buffer layer. Effects of PDA/ZnO composite cathode buffer layer with the different self-polymerization times on the device performance are investigated. According to the results, the short circuit current and photoelectric conversion efficiency of polymer solar cells first increase then decrease with the increase of the self-polymerization time of PDA. For 10-min PDA self-polymerization, the photovoltaic performance of the device achieves the optimal values:open circuit voltage 0.66 V, short circuit curent density 9.70 mA/cm2, fill factor 68.06%, and power conversion efficiency 4.35% under irratiation of light with a strength of 100 mW/cm2. We conclude that the improvement of device performance is due to the PDA/ZnO composite cathode buffer layer reduced the contact resistance between the ZnO and ITO, at the same time, the presence of a large number of nitrogen groups in PDA is advantageous for the electronic collection of the inverted polymer solar cells. Meanwhile, polymer solar cell with PDA/ZnO as composite cathode buffer layer also exhibits excelent stability. In addition, PDA has a strong adhesive force that makes the ZnO interface layer on its surface not easy to fall off. This provides a new way of fabricating the flexible polymer solar cell devices.

2017, 66 (19): 198503.
doi: 10.7498/aps.66.198503

Abstract +

Molecular spintronics has attracted much attention because of many novel functionalities at the single molecule level over the past decades.Recently,much research has focused on organic molecules containing transition metals in the field of molecular spintronics,which possesses desired spin-dependent transport properties for spintronic device applications. In this paper,based on non-equilibrium Green's function formalism combined with the first-principles density functional theory,the spin-dependent transport properties of an organic Co-Salophen molecule sandwiched between two zigzag graphene nanoribbon (ZGNR) electrodes are investigated.By applying an external magnetic field,the spin directions of the left and right ZGNR electrodes may be switched to two different configurations:the parallel (P) and antiparallel (AP) spin configurations.It is found that for the P spin configuration,the spin-up current is significantly larger than the spin-down one which is nearly zero in a bias range from -1.0 V to 1.0 V,exhibiting a nearly perfect spin filtering effect (up to 100%).Moreover,the spin-up current shows negative differential resistance behavior at 0.3 V.For the AP spin configuration,the spin-down current is much larger than the spin-up one at the positive bias.On the contrary,the spinup current is much larger than the spin-down one at the positive bias.Therefore,the device exhibits bipolar spin filtering effect.It is also found that the spin-up current at the negative bias is much larger than that at the corresponding positive bias,while the spin-down current at the negative bias is much smaller than that at the corresponding positive bias,which shows the outstanding spin rectifying effect.Besides,a significant giant magnetoresistance effect is also obtained in the device when the spin directions of the left and right ZGNR electrodes switch between P and AP spin configurations. The spin transport properties of the device under P and AP spin configurations are attributed to the different orbital symmetries of spin subbands (* and ) of the electrodes and the spatial distribution of molecular orbitals within the bias window.By analyzing the spin-polarization transmission spectrum,the local density of states,the band structures and symmetries of the ZGNR electrodes and the projected self-consistent Hamiltonian states of molecular orbitals,the internal mechanism for multiple functional characteristics of the device is explained in detail.Our results indicate the Co-Salophen molecule can be a promising candidate for future applications in molecular spintronics device,and also provide a theoretical reference for designing the next-generation molecular nano-devices.