## Vol. 65, No. 9 (2016)

##### 2016-05-05

###### GENERAL

2016, 65 (9): 090501.
doi: 10.7498/aps.65.090501

Abstract +

The main purpose of this article is to explore the bursting behaviors as well as the mechanism when multiple equilibrium states evolve into the bursting attractors. Taking the controlled Lorenz model with periodic excitation for example, the coupling effect of different scales in frequency domain corresponding to the case that an order gap exists between the exciting frequency and the natural frequency of the system with multiple equilibrium states is investigated. Unlike the autonomous slow-fast coupling system, neither obvious slow nor fast subsystems can be observed in a periodically excited system. Since the exciting frequency is far less than the natural frequency of the system, the whole exciting term can be considered as a slow-varying parameter, leading to the generalized autonomous system. With the variation of the slowly-varying parameter, the bifurcation forms as well as the behaviors for different equilibrium states in the generalized autonomous system are explored. It is pointed out that for certain conditions, Hopf bifurcation and fold bifurcations related to different equilibrium points can be observed. According to the conditions related to different bifurcations, the bursting oscillations in two typical cases are presented. In order to explore the mechanism of bursting oscillation, transformed phase portraits are introduced in which the whole exciting term is treated as a generalized state variable so that the relationship between the original state variables and the slow-varying parameter can be clearly described. By employing the transformed phase portraits, the bifurcation mechanisms of different bursting attractors are presented. For the conditions where only fold bifurcation exists between two equilibrium states in the generalized autonomous system, two un-symmetric bursting attractors can be observed. With the variation of parameters, when the repetitive spiking oscillations pass across the attracting basin of another equilibrium states, the two bursting attractors interact with each other to form an enlarged symmetric bursting attractor. For the conditions where both the Hopf and fold bifurcations evolve into the bursting attractors, multiple quiescent states as well as repetitive spiking states exist in the bursting oscillations, which may lead to complicated behaviors. It is found that the coexistence of multiple equilibrium states as well as the related bifurcation forms not only leads to multiple forms of quiescent states and the spiking states, but also results in different switching forms between different quiescent states and the spiking states.

2016, 65 (9): 090502.
doi: 10.7498/aps.65.090502

Abstract +

Multifractal detrended fluctuation analysis is an effective tool for dealing with the non-uniformity and singularity of nonstationary time series. For the serious issues of the trend extraction and the inefficient computation in the traditional polynomial fitting based multifractal detrended fluctuation analysis, based on the dual-tree complex wavelet transform, a novel multifractal analysis is proposed. To begin with, as the dual-tree complex wavelet transform has the anti-aliasing and nearly shift-invariance, it is first utilized to decompose the signal through the pyramid algorithm, and the scale-dependent trends and the fluctuations are extracted from the wavelet coefficients. Then, using the wavelet coefficients, the length of the non-overlapping segment on a corresponding time scale is computed through the Hilbert transform, and each of the extracted fluctuations is divided into a series of non-overlapping segments whose sizes are identical. Next, on each scale, the detrended fluctuation function for each segment is calculated, and the overall fluctuation function can be obtained by averaging all segments with different orders. Finally, the generalized Hurst index and scaling exponent spectrum are determined from the logarithmic relations between the overall detrended fluctuation function and the time scale and the standard partition function, respectively, and then the multifractal singularity spectrum is calculated with the help of Legendre transform. We assess the performance of the dual-tree-complex wavelet transform based multifractal detrended fluctuation analysis (MFDFA) procedure through the classic multiplicative cascading process and the fractional Brownian motions, which have the theoretical fractal measures. For the multiplicative cascading process, compared with the traditional polynomial fitting based MFDFA methods, the proposed multifractal approach defines the trends and the length of non-overlapping segments adaptively and obtains a more precise result, while for the traditional MFDFA method, for the negative orders, no matter the generalized Hurst index, scaling exponents spectrum, or the multifractal singularity spectrum, the acquired results each have a significant deviation from the theoretical one. For the time series with different sizes, the proposed method can also give a stable result. Compared with the other adaptive method such as maximal overlap discrete wavelet transform based MFDFA and the discrete wavelet transfrom based MFDFA, the proposed approach obtains a very accurate result and has a fast calculation speed. For another time series of fractional Brownian motions with different Hurst indexes of 0.4, 0.5 and 0.6, which represent the anticorrelated, uncorrelated, correlated process, respectively, the results of the proposed method are consistent with those analytical results, while the results of the polynomial fitting based MFDFA methods are most greatly affected by the order of the fitting polynomial. The method in this article provides a valuable reference for how to use the dual-tree complex wavelet transform to realize the multifractal detrended fluctuation analysis, and we can benefit from the signal self-adaptive trend extraction and the high computation efficiency.

2016, 65 (9): 090701.
doi: 10.7498/aps.65.090701

Abstract +

As air pollution is becoming more and more serious in recent years, gas-sensing devices have attracted intensive attention. In particular, NO2 is one of the most toxic gases in the atmosphere, which tends to produce acid rain and photochemical smog. Thus, there is a strong demand of cheap, reliable and sensitive gas sensors targeting NO2. Gas sensors fabricated on silicon substrates with room-temperature operation are very promising in power saving, integrated circuit processing and portable detectors. More important, the silicon nanowires (SiNWs)-based devices are compatible with very large scale integration processes and complementary metal oxide semiconductor technologies. In the present work, the novel nanocomposite structure of (SiNWs)/vanadium oxide (V2O5) nanorods for NO2 detection is successfully synthesized. The SiNWs are fabricated by a combination of nanosphere lithography and metal-assisted chemical etching. Vanadium films are deposited on SiNWs by DC magnetron sputtering, and then V2O5 nanorods are synthesized with subsequent thermal annealing process for full oxidation in air. The morphology and crystal structure of product obtained are characterized by field-emission scanning electron microscopy and X-ray diffraction. The characterization results indicate that V2O5 nanorods are uniformly distributed on the surfaces of SiNWs. The increased specific surface area of SiNWs/V2O5 nanocomposite provides more adsorption sites and diffusion conduits for gas molecules. Therefore, the novel structure of the nanocomposite is conducive to gas-sensing. In addition, the sputtering time has an obvious influence on the morphology of vanadium oxide. With the increase of the sputtering time, the specific surface area and the number of p-n heterojunctions formed in the nanocomposite are both less than those of nanocomposite with appropriate sputtering time. The gas-sensing properties are examined by measuring the resistance change towards 0.5-4 ppm NO2 gas at room temperature by the static volumetric method. Results show that the nanocomposite with shorter deposition time has better gas-sensing properties to low-concentration NO2 gas than those of bare SiNWs and nanocomposite with longer deposition time. On the contrary, the responses of the nanocomposite to other high-concentration reducing gases are very low, indicating good selectivity. The enhancement in gas sensing properties may be attributed to the change in width of the space charge region, which is similar to the behavior of p-n junction under forward bias, in the high-density p-n heterojunction structure formed between SiNWs and V2O5 nanorods. In conclusion, these results demonstrate that the SiNWs/V2O5 nanocomposite has great potential for future NO2 gas detection applications with low consumption and good performance.

###### NUCLEAR PHYSICS

2016, 65 (9): 092101.
doi: 10.7498/aps.65.092101

Abstract +

-beta-decay half-lives are not only important parameters for studying the structures and decay properties of the exotic nucleus far from stability, but also basic parameters for understanding the astrophysical phenomenon. Astrophysicists need exact data of -decay half-lives as input to build nucleosynthesis models for understanding the elements abundances of our universe and solar system. For nuclei far from stability, experimental synthesis and further measurements on their half-lives are rather difficult due to the rarity and radioactivity of target material for synthesizing these nuclei. In theoretical respect, although there are many models such as finite-range droplet model plus quasi-particle random-phase approximation (QRPA), microscopic density functional theory plus QRPA, Hatree-Fock-Bogoliubov theory plus QRPA, and shell model etc., it is still a challenge to calculate -decay half-lives in a reliable way for nuclei far from the -stable line, partly because of the intrinsic complexity of nuclear multi-body problem. In empirical respect, Sargent made an empirical study of -decay half-lives in 1933 and discovered a law which is consistent with the Fermi -decay theory proposed one year later. From then on, there have been a few parametric models based on some of real physical behaviors, which describe complex quantum many-body systems, such as the Kratz-Herrmann formula and the gross theory. Recently, Zhang et al. discovered an exponential law describing -decay half-lives and the nucleon number (Z,N) of parent nuclei far from the stable line. A formula is proposed to calculate the -decay half-lives of nuclei far from stability, which can describe experimental data reasonably well. However, the differences between allowed transitions and forbidden transitions are not fully considered in this formula. Zhang et al. used a set of parameters to describe both allowed transitions and forbidden transitions. In this paper, we consider the different -decay half-lives of allowed transitions and forbidden transitions, and propose an updated parameterization of this formula. A set of parameters is obtained through fitting experimental data of different kinds of transitions with a least-square method. With these new parameters, the theoretical calculation results are in good agreement with the experimental values. The calculation accuracy is improved compared with previous version. By comparison with the complicated and time-consuming microscopic calculation, the improved exponential formula can give the results of -decay half-lives for the allowed transitions and the forbidden transitions in an effective and reliable way. According to the updated formula, we predict half-lives of --decay half-lives of some unknown nuclei far from the -stable line. These predictions are very useful references for the experimental study of --decay of nuclei far from stability and for astrophysical applications.

2016, 65 (9): 092501.
doi: 10.7498/aps.65.092501

Abstract +

In this paper, we introduce the on-the-fly (OTF) Doppler method which is used to calculate the temperature-dependent cross section. After substituting Adler-Adler multilevel representation into Doppler formulation, the theoretical formulation of temperature dependent cross section is obtained. This theoretical formulation can be approximated by a Taylor series expansion and asymptotic series expansion, which is the base of OTF method. The OTF method can be used to calculate the cross section of any nuclide at any temperature in a range of 300-3000 K based on cross section library of 300 K. For the OTF method, firstly, a series of temperature dependent cross section libraries is produced by NJOY. Secondly, a uniform energy grid is evaluated by the temperature dependent cross section libraries. Thirdly, a polynomial is used to fit the temperature dependent cross section on each energy grid. The coefficient of the polynomial is obtained by single value decomposition algorithm. Finally, the coefficients of the polynomial in all energy grids and the energy grids themselves are written in a text file. To test the cross section polynomial produced by OTF method, we compare the total and absorption cross sections of 238U and 235U calculated by the polynomial with those produced by NJOY. The errors of these cross sections obtained by these two programs are presented in the paper.
The text file produced by OTF method can be read by the Monte Carlo code JMCT, which is a coupled neutron/photon transport code developed by IAPCM. After providing the temperature and energy of the particle, the temperature dependent cross sections in two adjacent energy grids are calculated by the polynomial respectively. The cross section of target energy is obtained by linear interpolation. Two benchmarks including a pin-cell model and an assembly model are used to verify the applications of OTF method in JMCT. The results are presented in the paper.

2016, 65 (9): 092801.
doi: 10.7498/aps.65.092801

Abstract +

The traditional fixed-point iteration method is typically used for neutronics/thermal-hydralics coupling problems in most nuclear safety analysis codes. But the fixed-point iteration method has a tendency to fail to be used in computing the coupling problems due to slow convergence rate in some cases and even no convergence, and thus resulting in a limited efficiency, especially for the tight-coupling and fast-transient problems. In addition, for the reactor thermal-hydraulic calculation, the traditional finite difference or volume method (FDM or FVM) is used. However, both FDM and FVM require fine mesh size to achieve the desired precision and thus also result in a limited efficiency for the large scale problems. In this paper, to ensure the accuracy, efficiency and convergence for large-scale and complicated coupling problems, the new methods-NEM_JFNK are successfully developed to simultaneously solve the neutronics/thermal-hydralics coupling problems by combining the advantage of the efficient coarse nodal expansion method (NEM) and Jacobian-Free Newton-Krylov method (JFNK).
The NEM has been widely used in the reactor physics analysis due to its high efficiency and accuracy in the reactor physics analysis, and it has proved to be superior to FDM and FVM. To improve the efficiency and accuracy for the large scale problems, the NEM is first extended to thermal hydraulic problems from the reactor physics calculation. Then all the governing equations of the neutronics/thermal-hydralics coupling problems can be discretized by the NEM and all the variables can be solved on the coarse meshes so that the size of coupling problems is greatly reduced. To ensure the high accuracy for the coupling problems on the coarse meshes, the high-order coefficients in NEM are successfully transferred between the coupling terms by our research. After that, to ensure the convergence of complicated coupling problems, JFNK based on the NEMs needs to be developed. However, the researches of JFNK based on the NEM in reactor analysis are less and the existing JFNK methods are mostly based on FVM or FDM or the finite element method. In this paper, the NEM discretization equations are successfully integrated into the framework of JFNK through the special treatment and the NEM_JFNK with linear-based preconditioner named LP_NEM_JFNK is also successfully developed. In addition, to take advantage of the existing code and avoid the construction of residual formulations, the non-residual construction NEM_JFNK named NRC_NEM_JFNK is presented and the black-box coupling method is achieved by NRC_NEM_JFNK so that the existing codes only need the simple modification to achieve the combination of the NEM and JFNK. Numerical results show LP_NEM_JFNK and NRC_NEM_JFNK outperform traditional fixed-point iteration method in the sense of convergence rate and efficiency. Further studies are needed to extend the NEM_JFNK method to the multi-dimensional neutronic/thermal hydraulic coupling problems in the high temperature gas-cooled reactor.

###### ATOMIC AND MOLECULAR PHYSICS

2016, 65 (9): 093201.
doi: 10.7498/aps.65.093201

Abstract +

The production and research of ultracold heteronuclear molecules have aroused the great interest recently. On the one hand, these molecules are extremely popular in experiments for exploring the collision dynamic behaviors in threshold, photoassociative spectrum and strong dipole-dipole interactions. On the other hand, ultracold polar molecules populated at deeply bound levels in the singlet ground state are the right candidates to investigate quantum memories for quantum simulation, and to study strongly interacting quantum degenerate gases. The precise knowledge of cold collision processes between two different types of alkali atoms is necessary for understanding and utilizing ultracold heteronuclear molecules, sympathetic cooling, and thus formation of two species BEC.
The goal of the present investigation is to study the collisions between ultracold sodium atoms and cesium atoms. We systematically demonstrate simultaneously trapping ultracold sodium and cesium atoms in a dual-species magneto-optical trap (MOT). The sodium atom trap loss rate coefficient Na-Cs is measured as a function of Na trapping laser intensity. At low intensities, the trap loss is dominated by ground-state hyperfine-changing collisions, while at high intensities, collisions involving excited atoms are more important. A strong interspecies collision-induced loss for Na atoms in the MOT is observed. In order to obtain the trap loss coefficient Na-Cs, we proceed in two steps. First, the Cs repumping laser is blocked to avoid the formation of ultraold Cs atoms. The loading process of Na atoms is recorded when the Cs trapping laser is on. Second, the loading curves of the Na atoms are obtained as Cs atoms are present with the repumping laser beams. The total losses PNa and PNa' are acquired by fitting the two loading curves of trapped Na atoms. Thus, the trap loss coefficient Na-Cs can be derived from the difference between total losses PNa and PNa' divided by the density of the Cs atoms.
The coefficient Na-Cs decreases in a range of 5-10mW/cm2, which originates from the hyperfine-state changing (HFC) collision. A Doppler model is used to calculate the Na atom trap depth, in that the atom trap depth and exoergic energy determine the behavior of the collisional trap loss rate coefficient. The three corresponding calculated critical intensities of Na trapping laser are 7.98, 14.82, 16.2 mW/cm2 respectively in the Na-Cs HFC collision process. The first calculated critical intensity value agrees well with the experimental result. Our work provides a valuable insight into HFC collision between Na and Cs atoms and also paves the way for the production of ultracold NaCs molecules using Photoassociation (PA) technology. Furthermore, the experimental results lay a great basis for the obtainments of high sensitive heteronuclear NaCs molecular PA spectrum and the creation of deeply bound ground state molecules.

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

2016, 65 (9): 094101.
doi: 10.7498/aps.65.094101

Abstract +

A single-side and one-dimensional left handed-material on the basis of periodic Ⅱ structure is designed in this paper. The Ⅱ structure of left-handed material is very simple and has some advantages, such as large bandwidth, small size and low loss. The results of simulation with the software HFSS show that for this structure in a frequency range from 8.79 GHz to 15.75 GHz, the real part and imaginary part of refraction index are less than 0 and approximately 0 respectively, and the real part of wave impedance is greater than 0, the largest loss per unit is 0.27 dB, and the relative bandwidth is 55.78%, showing that this Ⅱ periodic structure has the negative character. Based on the simulation results, this periodic Ⅱ structure is fabricated and measured with the vector network analyzer and wave guide method. The measurements are in agreement with the simulations. All these results prove that this left-handed material has some better characters than traditional meta-materials.

2016, 65 (9): 094201.
doi: 10.7498/aps.65.094201

Abstract +

The property of optoelectronic detector indicates that the response between incident light intensity and output digital number is non-linear. For imaging polarimeter, if light intensity is directly substituted by digital number when restoring polarization information from polarization images, the restored polarization information will apparently deviate from the true scene polarization information. This deviation makes the quantitative application of polarization information meaningless. To solve this problem, in this paper we propose a new algorithm for restoring the polarizaiton information with considering the property of polarimeter of the detector. Firstly, theorical correction equation of polarization information restoration are proposed for division-of-time polarimeter and for simultaneous polarimeter respectively. And then, specific implementation and polarization imaging test experiment are carried out. For divison-of-time polarimeter, we firstly test the property of the detector, and then use the tested parameter and the proposed restoration equation to restore polarizaiton information from the data of polarzation imaging test experiment. The degree of linear polarization (DoLP) restored with digital number directly changes from 0.932 to 0.753 when changes from 1.0 to 1.5. The DoLP restored with correction equation proposed in this paper varies from 0.932 to 0.926, which approaches to the ture scene DoLP value 1.0. For simultaneous polarimter, the instrument matrixes of the polarimeter are calibrated first under different setting values, and then the calibrated instrument matrixes are used to restore poliarzation information from the data of polarization imaging test experiment. The DoLP restored with digital number directly changes from 1.3763 to 1.1582 when changes from 1.0 to 1.5, which exceeds the possible DoLP range from 0 to 1.0. The DoLP restored with correction equation proposed in this paper varies from 0.8428 to 0.8683, which approaches to the ture scene DoLP value 1.0. Experimental result shows that the property of polarimeter has an apparent effect on the restored polarization information, and that the polarizaiton information restoration error increases with setting deviation from 1.0. With the restoration algorithm proposed in this paper, the restored polarization information can steadily approach to the scene polarization information with acceptable slants small. The poroposed polarization restoration algorithm with considering property establishes a theoretical foundation for the future study of polarimeter and its quantitative application.

2016, 65 (9): 094202.
doi: 10.7498/aps.65.094202

Abstract +

In this paper, we consider a single BaF molecule driven by an external field. When the symmetry is broken, the states of the BaF molecule demonstrate the permanent dipole moments. An external laser field to excite BaF molecule transition from its ground state to its excited state, and a radio frequency field couple with the permanent dipole moment of the BaF. The first order and second order cumulants of the emission photons and the waiting time distribution are studied via the recently developed generating function approach, which is very convenient to study the counting statistics and the corresponding probability distributions. The results demonstrate that the radio frequency field could help the BaF molecule to absorb photons from the driving field. The second and third order waiting time distributions oscillate with the evolution time, which reflects the states oscillating with the external radio frequency field.

2016, 65 (9): 094203.
doi: 10.7498/aps.65.094203

Abstract +

Continuous-wave single-frequency 589 nm yellow laser can be used in laser cooling of sodium atoms. Besides, the interaction between 589 nm laser and sodium atoms can be studied by resonance fluorescence, which provides an important basis for the sodium guide star in the adaptive optics. In this paper, single frequency 589 nm yellow light is generated by sum frequency of single-block non-planar ring cavity 1064 nm and 1319 nm laser in periodically poled KTiOPO4 crystal. The geometric parameters of single-block non-planar Nd:YAG crystal and magnetic field intensity are optimally designed by simulation calculation through using Jones matrix. The output powers 1080 mW and 580 mW are obtained for continuous-wave single-frequency 1064 nm and 1319 nm laser in the experiment, respectively The two fundamental beams are expanded to be the same as perfectly as possible in size and are focused into a spot with a size of about 60 m by an achromatic lens. The sum-frequency generation takes place in a 1 mm2 mm20 mm phase-matched type-I periodically poled KTiOPO4 crystal with a matching temperature of 55℃ and polarization period of 12.35 m The crystal is anti-reflection coated for all three wavelengths (1064 nm, 1319 nm and 589 nm). A 14.8 mW output of 589 nm laser is obtained with beam quality factor M2=1.14 and the corresponding sum-frequency efficiency is 0.9%. The influence of periodically poled KTiOPO4 temperature on the sum-frequency efficiency is studied and the temperature acceptance bandwidth is measured to be 1.5 degrees The wavelength of 589 nm yellow light can be tuned to the sodium atom D2a absorption line by changing the temperature of 1064 nm Nd:YAG crystal and 0.164 pm of tuning accuracy is reached. The whole laser system is stable and reliable, so it provides a practical and effective technical means to obtain the continuous-wave single-frequency 589 nm laser, for it is relatively simple and easy to implement.

2016, 65 (9): 094204.
doi: 10.7498/aps.65.094204

Abstract +

In this paper, we systematically study the (1+1)-dimensional spatial optical solitons in nematic liquid crystals with negative dielectric anisotropy. Firstly, with the perturbation method, we obtain a (1+1)-dimensional soliton solution in the second approximation.Numerical simulations confirm the analytical soliton solution in the strongly nonlocal case, the critical power of a strongly nonlocal solition is directly proportional to wm2/w3, where wm is a characteristic length of the material response, and w is the soliton width. Secondly, the soliton solutions in nematic liquid crystal with negative dielectric anisotropy are obtained by numerical computation. It is found that the bright solitons exist only when the degree of nonlocality is above a critical value. The analytical solutions in the second approximation accord with the numerical ones very well even under the general degree of nonlocality. Finally, to investigate the stability, we conduct the linear stability analysis, and find that all the solitons are stable, which is also confirmed by the numerical simulations.

Multiple scattering and attenuation for electromagnetic wave propagation in sand and dust atmosphere

2016, 65 (9): 094205.
doi: 10.7498/aps.65.094205

Abstract +

For an electronic system operation under the conditions of all-weather in arid and semiarid areas, the studies of the attenuation and multiple scattering are necessary for electromagnetic wave propagations in sand and dust atmosphere. Based on Mie theory, a method of calculating the attenuations for electromagnetic wave propagation through sand and dust atmosphere is presented in this paper, which relate to the particle size distributions and visibilities for sand and dust atmosphere. The attenuations at 37 GHz are given for various visibilities, and are compared with the results calculated from other formulas and the experimental data from the literature. The attenuations are closer to the experimental results. In order to investigate into electromagnetic wave propagations in lower visibility sand and dust atmosphere, the multiple scattering in sand and dust storms are necessarily analyzed. At 37 and 93 GHz, the extinction cross sections, albedos and asymmetry factors are calculated by Mie theory for various size sand and dust particles. By the Monte Carlo (MC) simulation method, the attenuations for including the multiple scattering effects are calculated under the conditions of dry and 5% water content in sand and dust particles, respectively, and are compared with the results from Mie theory. The results are shown that the difference between the attenuation obtained by Mie and that by MC is small at 37 GHz. The influence of the multiple scattering on attenuation is small and may be ignored at 37 GHz. At 93 GHz, the difference between the attenuation obtained by Mie and that by MC is clear, and the attenuation obtained by using Mc simulation is smaller than that based on Mie theory. The effect of the multiple scattering on attenuation is evident at 93 GHz. The lower the visibility, the more notable the effect on attenuation is. For different sand and dust storms, the particle refractive indexes and the particle size distributions are different. For the sand and dust storms in Tengger desert and the blowing sand and dust atmosphere in north China, the attenuations at 93 GHz are analysed. In Tengger desert, the attenuation and the multiple scattering are larger than in blowing sand and dust atmosphere. The results show that the more the large size particles in sand and dust storms, the stronger the effect multiple scattering on attenuation is. Hence, for stronger sand and dust storms, the attenuation and the effect of multiple scattering become important. With the increase of water content in particle, the imaginary part of refractive index increases, the attenuation greatly increases, and the effect of the multiple scattering on attenuation is weakly varied. The analyses show that the attenuations for electromagnetic wave propagation in arid sand and dust atmosphere are smaller than in moisture sand and dust atmosphere under the condition of the same visibility.

2016, 65 (9): 094206.
doi: 10.7498/aps.65.094206

Abstract +

Sub-wavelength grating is a critical element in micro and nano-photonics. So its fabrication and application have attracted a great deal of research attention. While the existing lithography technologies of sub-wavelength grating fabrication have some insufficient points, such as high cost, low output, technical complexity, or difficult to change the period of the sub-wavelength grating. In this paper, an adjustable period and large area sub-wavelength grating with low cost and maskless is proposed and theoretically realized. The sub-wavelength grating is inscribed by the interference between two TE0 waveguide modes, where the TE0 waveguide mode is existent in an asymmetric metal-cladding dielectric waveguide structure excited by the prism coupling method. The dispersion curve of TE0 waveguide mode, the relationship between the period of the sub-wavelength grating and the exciting light source, the refractive index of the prism and the photoresist, especially the thickness of the photoresist are theoretically analyzed in detail. The distribution of the interference optical field of TE0 waveguide mode in the multilayer structure including metal film, photoresist and air layer is numerically simulated using the finite element method. The shorter the exciting light wavelength with the identical photoresist condition, the smaller the period of sub-wavelength grating inscribed by TE0 waveguide modes interference lithography is. For further studying the influences of refractive index and thickness of photoresist and the refractive index of the prism on the period of sub-wavelength grating, the exciting light with 442 nm wavelength and the Ag matel film are used. The period of sub-wavelength grating is smaller with thicker photoresist film, when the refractive indexes of photoresist and prism are the same. The larger refractive index of photoresist is beneficial to inscribing the sub-wavelength grating with smaller period when the refractive index of prism and the thickness of photoresist are identical. The prism with higher refractive index can provide wave vector-matching condition with lager propagation constant, and can inscribe sub-wavelength grating with smaller period. Compared with surface plasmons interference lithography which needs the thicker photoresist film due to the finite penetration depth of SPs, TE0 waveguide modes interference can realize adjustableperiod sub-wavelength grating writing for thicker photoresist condition by changing exciting light source, the refractive index of prism, the refractive index of photoresist and especially the thickness of photoresist. The realization of adjustable period sub-wavelength grating inscribed by TE0 waveguide modes interference lithography will provide important theoretical support for reducing the fabrication cost of sub-wavelength gratings and broadening the application scope of sub-wavelength grating.

2016, 65 (9): 094207.
doi: 10.7498/aps.65.094207

Abstract +

Laser-beam illumination uniformity is a key issue in inertial confinement fusion facilities. In order to fulfill the requirement of improving illumination uniformity, a radial smoothing (RS) scheme is proposed. For smoothing the focal-spot pattern on a short time scale compared with the hydrodynamic response time of the target, the optical Kerr effect with extremely response time is taken into consideration. The basic principle of RS based on optical Kerr effect is that by using the interaction between optical Kerr medium and periodic Gaussian pulses to modulate a periodic spherical phase, to modulate periodic sphericel phase added at the wavefront of laser transmission wave, change the focal-spot size of the laser beam in far field, and further induce the fast radial redistribution of the speckles inside the focal spot in far field, and further induce the fast radial redistribution of the speckles inside the focal spot in far field. This fast radial redistribution of the speckles smoothes the intensity modulation of the focal spot on the target and eventually achieves the beam smoothing in the radial direction. The application of RS in the beamline is detailed. The optical Kerr medium is inserted in the front-end of the bemline, before the laser beam is injected into the main amplifier. The periodic Gaussian pulse for pumping the optical Kerr medium is obtained by the pulse stacking system based on fibers. The pulse width of stacked Gaussian pulse and the time delay between Gaussian pulses are set to be on a picosecond time scale or subpicosecond time scale. The induced refractive index of the optical Kerr medium by the pump laser fits spherical distribution with periodic variation, and results in the radial distribution of the speckles in focal plane. By establishing the theoretical model of the radial beam smoothing scheme implemented with continuous phase plate (CPP), the focusing characteristics of laser beam with RS and CPP are discussed in detail. The influences of the selection of optical Kerr medium and the characteristics of the radial redistribution on the radial smoothing effect are simulated and analyzed. Results indicate that the RS based on optical Kerr effect could efficiently achieve the periodic radial redistribution of the speckles on focal plane, and therefore improves the illumination uniformity in the radial direction while eliminating the stripe pattern presented in far field by one-dimensional smoothing spectral dispersion (SSD). The smoothing performance of RS is different from that of the conventional SSD due to its radial smoothing direction. Moreover, the combined application of RS with continuous phase plate could achieve a better smoothing level with a shorter time. The utilization of radial smoothing scheme in high power laser system may significantly improve the laser-beam irradiation with little influence on the performance of the beamline.

2016, 65 (9): 094208.
doi: 10.7498/aps.65.094208

Abstract +

In visible light communication, the wavelength division mutiplexing (WDM) technology can improve system data rate by increasing the number of channels. However, because the emission spectrum of the light emitting diode (LED) has a certain width, a phenomenon of spectral overlapping will occur when the number of channels increases and channel-spacing decreases, which results in channel crosstalk although optical filters are adopted. The channel crosstalk will restrict the capacity of WDM-VLC (visible light communication) system, which has great research significance. In this paper, the channel crosstalk based on LED spectra overapping is disscussed. The LED emission spectrum is modeled by combining the physical mechanism of LED emission with real shape of LED spectrum. According to the literature, the LED shape can be fitted greatly by Gauss function, and the full-width at half-maximum E is in a range from about 4.3kBTj to 6.8kBTj when the peak wavelengths of InGaN and AlInGaN LEDs are both less than 560 nm, E values range from 2.1kBTj to 3.3kBTj when the peak wavelength of InAlGaP LED is larger than 560 nm. In order to reduce the overall system complexity we use the following values: when the peak wavelength is less than 560 nm, E = 5.5kBTj; when the peak wavelength is larger than 560 nm, E = 3.0kBTj. Then, according to the overlapping spectra and VLC channel with considering optical filter transmittance and detector spectral response, the channel crosstalk formula is derived. Some quantities are given before simulation such as the semi-angle at half illuminance of an LED is 60; all LEDs are so closely arranged together to mix light in free space that spacing between LEDs can be ignored with respect to the propagating distance; the strongest signal situation is considered. The simulation result shows that although at the same channel spacing, different channels have different crosstalks because spectra are different. And the crosstalk from one adjacent channel will not exceed -13.6 dB when channel-spacing is larger than and equal to 28 nm, which means that when OOK modulation is used and the BER achieve 10-6, the channel-spacing should not be less than 28 nm. Finally, an experiment of channel crosstalk with using two-channel WDM VLC system and LEDs with different wavelengths is conducted and the correctness of the crosstalk analysis is verified. The colors of red (635 nm), red-orange (620 nm) and amber (596 nm) LEDs are used and two of them are used each time. Two sine signals with different frequencies are launched by AWGs (Agilent 33250A) and through amplifiers and Bias-Tees, drive two LEDs. The signal analyzer (Agilent N9020A) is used to observe the signal power. The experimental results of channle crosstalk are close to theoretical results. The analysis of channel crosstalk in muliti-channel WDM-VLC system will give some guidance in increasing the number of channels for optical communication in the future.

2016, 65 (9): 094209.
doi: 10.7498/aps.65.094209

Abstract +

Until now, the air temperature sensors inside thermometer screens and radiation shields are affected by solar radiation, which causes the measuring result to become greater than the actual temperature. The temperature rise can reach 0.8 K or even higher. In this paper, a temperature sensor array design is established for obtaining high precision measurement results. The temperature sensor array consists of an array of radiation shields which features a tube-shape, a platinum resistance sensor array, an aluminum plate with a silver mirror surface and a temperature measurement module that includes a high accuracy thermometer circuit. There is always at least one radiation shield that supplies relatively good ventilation under any airflow direction. A computational fluid dynamic method is implemented to analyze and calculate the temperature rise induced by radiation under various environmental conditions. A correction equation of the temperature rise is obtained by surface fitting using a genetic algorithm. The measurement accuracy can be further improved by this correction equation. In order to verify the performance of the sensor array, a forced ventilation temperature measurement platform is constructed, which consists of a platinum resistance sensor, an L-shaped radiation shield and an air pump. The airflow rate inside the radiation shield can be up to 20~m/s, and the L-shaped radiation shield can horizontally rotate under the control of a software to minimize the error caused by the heated radiation shield. The temperature sensor array, a temperature sensor with traditional radiation shield, and the forced ventilation temperature measurement platform are characterized in the same environment. To experimentally verify the computational fluid dynamic method and the genetic algorithm, a number of contrast tests are performed. The average temperature rise of sensors equipped with the traditional radiation shields is 0.409 K. In contrast, the temperature rise of the sensor array is as low as 0.027K. This temperature sensor array allows the error caused by solar radiation to be reduced by a percentage of approximately 93%. The temperature rise of temperature sensor array, caused by the angular variation of airflow direction is on the order of several mK. When the solar radiation intensity and the airflow rate are 1000W/m2 and 0.1m/s, respectively, the temperature rise is 0.097 K. The temperature rise is 0.05K, when the airflow rate is greater than 0.4 m/s. The temperature rise can be reduced to 0.01 K, when the airflow rate is greater than 2 m/s. The average offset and root mean square error between the correction equation and experimental results are 0.0174 K and 0.0215 K, respectively, which demonstrates the accuracy of the computational fluid dynamic method and genetic algorithm proposed in this research. The temperature measurement accuracy has the potential to be further improved by utilizing the computational fluid dynamics method and the genetic algorithm.

2016, 65 (9): 094301.
doi: 10.7498/aps.65.094301

Abstract +

Self-collimation, a peculiar effect that allows acoustic signals to propagate in sonic crystals (SCs) along a definite direction with almost no diffraction, possesses a promising prospect in integrated acoustics as it provides an effective way to transmit acoustic signals between on-chip functionalities. There exists, however, the intrinsic inability of self-collimation to efficiently bend and split acoustic signals. Most of existing schemes for bending and splitting of self-collimated acoustic beams are based on SC of square lattice, thus their bending and splitting angles are restricted to 90. In this paper, the finite element method is used to investigate self-collimation of acoustic beams in an SC of hexagonal lattice. It is shown that 60 and 120 bending of self-collimated acoustic waves can be simultaneously realized by simply truncating the two-dimensional hexagonal SC. Bended imaging for a point source with a subwavelength resolution of 0.38 0 can also be realized by truncating the SC structure. In addition, a scheme for 60 and 120 splitting of self-collimated acoustic waves is also proposed by introducing line-defects into the hexagonal SC. It is demonstrated that an incoming self-collimated beam can be split into a 60 (or 120 bended one and a transmitted one, with the power ratio adjusted by the value of defect size. We believe that this hexagonal-SC-based bending and splitting mechanism will offer more flexibilities to the beam control in the design of acoustic devices and will be useful in integrated acoustic applications.

2016, 65 (9): 094302.
doi: 10.7498/aps.65.094302

Abstract +

The receiver at larger depth can receive the direct-arrival signal from a shallow source in a certain range in deep water. During a deep-water experiment conducted in 2014, a vector sensor located at a depth of 3146 m received the direct-arrival signals from the transducer towed at about 140 m depth by the source ship. In this paper, the propagation properties of the sound field in the direct-arrival zone in deep water are studied based on the ray theory and subsequently a source-range-estimation method is proposed. In the direct-arrival zone, the arrival angle is one of the most important properties of sound field, and the sound field is mainly composed of the contributions of a direct ray and a surface-reflected ray. The theoretical analysis and simulation results show that the amplitudes of horizontal particle velocity and vertical particle velocity are related to the mean arrival angle of the direct ray and the surface-reflected ray, and the larger the arrival angle, the greater the vertical particle velocity is, but the weaker the horizontal particle velocity is. Furthermore, the energy difference between horizontal particle velocity and vertical particle velocity can be approximately expressed by a monotonic function of the arrival angle, which varies fast with the horizontal distance between source and receiver. This property is applied to the estimation of source range. The analysis of the experimental data shows that the estimated source ranges are consistent with the GPS ranges within the range of 8 km, and the mean relative error of source range estimation is within 10%.

2016, 65 (9): 094501.
doi: 10.7498/aps.65.094501

Abstract +

With the development of space technology, flexible appendages such as lightweight manipulators and satellite antennas, often appear in spacecrafts. Usually, the large overall motion of the flexible appendage will bring about large deformation problem. And there is a strong nonlinear coupling between the large overall motion and deformation of the flexible appendage, which brings about a large challenge to the precise control of the spacecraft. Dynamics of a rotating flexible planar beam with large deformation is investigated in this paper. A new nonlinear dynamic model of a flexible beam with large deformation is established based on an absolute node coordinate formulation (ANCF). The longitudinal and bending deformations of the flexible beam are both considered in the model. The longitudinal strain energy and bending strain energy of the beam can be calculated by using Green-Lagrangian strain tensor and the exact expression of the flexible beam curvature, respectively. A new concise expression of the bending deformation energy can be obtained by using the Lagrange identical equation. The new elastic force model is derived from the new expression of the deformation energy. The dynamic equations of the present model can precisely deal with the large deformation problem of flexible beams. Then, simulation results from three dynamic models, including the ANCF model, the high order coupling model (HOC model), and the BEAM188 model in ANSYS, are compared to prove the validity of the ANCF model proposed in this paper. And we can also find the deficiency of the HOC model from the simulation. Further study shows that the new generalized elastic force model can be simplified properly. Two simplified models are presented in this paper. The applicabilities of the simplified models are pointed out from the viewpoints of computational efficiency and accuracy. A dimensionless parameter denoted as is introduced to describe the extent to which a flexible beam pendulum undergoing free falling motion is deformed. The deformation of the flexible beam increases as increases. Considering the calculating efficiency of the dynamic model, when is small, simplified model I is chosen preferentially; when is big, simplified model Ⅱ is adopted preferentially.

2016, 65 (9): 094502.
doi: 10.7498/aps.65.094502

Abstract +

With increasingly strict requirements for control speed and system performance, the unavoidable time delay becomes a serious problem. Fractional-order feedback is constantly adopted in control engineering due to its advantages, such as robustness, strong de-noising ability and better control performance. In this paper, the dynamical characteristics of an autonomous Duffing oscillator under fractional-order feedback coupling with time delay are investigated. At first, the first-order approximate analytical solution is obtained by the averaging method. The equivalent stiffness and equivalent damping coefficients are defined by the feedback coefficient, fractional order and time delay. It is found that the fractional-order feedback coupling with time delay has the functions of both delayed velocity feedback and delayed displacement feedback simultaneously. Then, the comparison between the analytical solution and the numerical one verifies the correctness and satisfactory precision of the approximately analytical solution under three parameter conditions respectively. The effects of the feedback coefficient, fractional order and nonlinear stiffness coefficient on the complex dynamical behaviors are analyzed, including the locations of bifurcation points, the stabilities of the periodic solutions, the existence ranges of the periodic solutions, the stability of zero solution and the stability switch times. It is found that the increase of fractional order could make the delay-amplitude curves of periodic solutions shift rightwards, but the stabilities of the periodic solutions and the stability switch times of zero solution cannot be changed. The decrease of the feedback coefficient makes the amplitudes and ranges of the periodic solutions become larger, and induces the stability switch times of zero solution to decrease, but the stabilities of the periodic solutions keep unchanged. The sign of the nonlinear stiffness coefficient determines the stabilities and the bending directions of delay-amplitude curves of periodic solutions, but the bifurcation points, the stability of zero solution and the stability switch times are not changed. It could be concluded that the primary system parameters have important influences on the dynamical behavior of Duffing oscillator, and the results are very helpful to design, analyze or control this kind of system. The analysis procedure and conclusions could provide a reference for the study on the similar fractional-order dynamic systems with time delays.

2016, 65 (9): 094503.
doi: 10.7498/aps.65.094503

Abstract +

Complex network is the abstract topology of a large number of nodes and edges in reality. How to reveal the influences of internal network topology on network connectivity and vulnerability characteristics is a hotspot of current research. In this paper, we analyze the influence of assortativity according to Newman's definition of assortativity in a given degree distribution. To fully understand the influence of assortativity we should change the assortativity to see how the topology of network changes. But we find the existing greedy algorithm cannot improve assortativity effectively. First we put forward a deterministic algorithm based on degree distribution and an uncertain algorithm based on probability distribution to increase assortativity. The deterministic algorithm can create a certain network which has a large assortativity without changing node degree. The uncertain algorithm can increase the assortativity continuously by changing the connection of edges. And the uncertain algorithm creates different graphs each time, so the result of the algorithm is uncertain. Then we test our algorithms on three networks (ER network, BA network, Email network) and compare with greedy algorithm, and the experimental results show that the uncertain algorithm performs better than greedy algorithm in three networks which have a large span of assortativity. And our deterministic algorithm performs well in a real world network. We find that we can increase assortativity coefficient up to 1 in ER network. This is because nodes in the ER network are peer to peer. We can also show that that the assortativity cannot increase up to 1 in some networks because nodes in these networks are not in the same status. Because we obtain a large span of assortativity, we can fully understand the change of network topology. On this basis, we analyze the changes of clustering coefficient when using the uncertain algorithm based on a probability distribution to increase the assortativity. We find that there is a certain correlation between assortativity and clustering. And we study the micro influence of uncertain algorithm on network, by which the reason of the change of clustering coefficient is explained. We calculate the changes of giant branches and small branches. The changes of the number of nodes in giant branches and the number of small branches show that the scale of giant branches becomes smaller, which means that the connection between nodes in giant branches becomes closer. The increase of the number of small branches means that the network as a whole becomes more fragile. So we can show how the uncertain algorithm changes the topology of the network without changing the degree of nodes in the network. Then we can use this algorithm to change the network to obtain a larger span of assortativity for further study.

2016, 65 (9): 094701.
doi: 10.7498/aps.65.094701

Abstract +

Unsteady distribution of spray is experimentally studied when a round liquid jet is injected into a supersonic crossflow vertically. An oscillation distribution model for the liquid column and spray is established. Tyndall scattering caused by the sol medium is put forward to eliminate the interference effect of monochromatic laser passing through the supersonic gas flow field. The scattering causes the disordering of laser propagation direction and phase, thus makes the planar light source uniform and eliminate the interference effect of laser at the same time. Then a uniform light source is formed and can be set as the uniform background with a pulse width of 7 ns. The camera, with dimension of CCD pixel space of 40002672 pixel, is located directly in front of planar light source, and the shooting area is between both. The frozen liquid jet/spray images with high spatiotemporal resolution are captured using the pulsed laser background imaging (PLBI) method in supersonic crossflows. And the drag phenomenon caused by the too-long exposure time in the ordinary and traditional high-speed imaging process is avoided. Based on the maximizing inter-class variance method (Otsu) and Canny method, the out boundary of liquid jet/spray are extracted from an instantaneous image. A dimensionless parameter named intermittency factor (the logogram is r) is defined and used to quantitatively analyze the oscillation distribution characteristics of jet/spray. The intermittency factor of the whole spray field could be calculated by sample probability statistic method. An empirical jet/spray oscillation distribution model, in supersonic crossflows, is summarized based on parameter studies. Various conditions are studied, including stagnation pressure range of gas (642 kPa to 1010 kPa), practical pressure range (0.36 MPa to 4.61 MPa), nozzle diameters (0.48 mm/1.0 mm/1.25 mm/1.52 mm), distances down from nozzle (10 mm to 125 mm), and jet-gas momentum flux ratio range (0.11 to 7.49). The empirical model is used to predict the oscillation distribution of water jet penetrated in a Ma2.1 supersonic crossflow. It is indicated that the predictive result matches well with the experimental result. It could be concluded that the PLBI method presented in this paper reasonably utilizes the high energy and short pulse characteristics of the laser to successfully complete the frozen image of liquid jet/spray under the condition of supersonic crossflow. The dimensionless parameter r defined in the study can be used to quantitatively analyze the oscillation distribution characteristics of jet/spray well. This study has important significance for understanding the diffusion characteristics of liquid jet in supersonic crossflows.

## EDITOR'S SUGGESTION

2016, 65 (9): 094702.
doi: 10.7498/aps.65.094702

Abstract +

Study of isentropic sound speed of two-phase or multiphase flow has theoretical significance and wide application background. As is well known, the speed of sound in fluid containing particles in suspension differs from that in the pure fluid. In the particular case of bubbly liquids (gas liquid two-phase flow), the researches find that the differences can be drastic. Up to now, the isentropic speed of sound in the flow field with a small volume fraction of bubbles (less than 1%), has been investigated fully both experimentally and theoretically. In this paper, we consider another situation, as the case with solid particles in gas, which is the so-called gas particle two-phase flow. Although many results have been obtained in gas liquid two-phase flow, there is still a lot of basic work to do due to the large differences in the flow structure and flow pattern between gas particle two-phase flow and gas liquid two-phase flow. Treating the gas particle suspension as the relaxed equilibrium, thermodynamic arguments are used to obtain the isentropic speed of sound. Unlike the existing work, we are dedicated to developing the computational model under dense condition. The space volume occupied by particle phase and the interaction between particles are overall considered, then a new formula of isentropic sound speed is derived. The new formula includes formulae of the pure gas flow and the already existing dilute gas particle two-phase flow as a special case. On the one hand, the correctness of our formula is verified. On the other hand, the new formula is more general. The variations of sound speed with different mass fractions of particle phase are analyzed. The theoretical calculation results show that the overall physical law of sound speed change is that with the increase of the particle mass fraction, the sound speed first decreases and then increases. The velocity of sound propagation in gas particle two-phase flow is far smaller than in pure gas in a wide range, so it is easy to reach the supersonic condition. When the particle volume fraction is below 10%, the result is consistent with Prandtl theoretical analysis. In this range, the influences of the particle phase pressure modeling parameters can be neglected. When the particle volume fraction is more than 10%, the particle phase pressure modeling parameters produce influences. Furthermore the corresponding physical principles and the mechanisms are discussed and revealed. The new formula and physical understandings obtained in this paper will provide a theoretical support for the researches of dense gas particle two-phase compressible flow and related engineering applications.

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

2016, 65 (9): 095201.
doi: 10.7498/aps.65.095201

Abstract +

In this paper, intensity distribution of 69.8 nm soft X-ray laser pumped by capillary discharge is studied theoretically and experimentally. In experiment, a main current of 11.5 kA is chosen to generate the plasma in a 35 cm long capillary filling with Ar at pressures in a range of 14-16 Pa. The intensity distribution is characterized by two different components. There is a big peak in the center of the intensity distribution profile, and its full width at half maximum divergence is about 0.4 mrad. There are also two small off-axis peaks on the either side of the center peak, and the peak-to-peak divergence is about 1.5 mrad. In theory, three different distributions of electron density are used to study the propagation of Ne-like Ar 69.8 nm laser in a cylindrical plasma column. The spatial intensity distribution for the 69.8 nm laser beam is calculated with the geometrical optics approximation. The results show that the center peak, which has such a small divergence, could be attributed to the slight depression of electron density on the axis.

2016, 65 (9): 095202.
doi: 10.7498/aps.65.095202

Abstract +

In order to improve the two-dimensional imaging diagnostic accuracy of inertial confinement fusion (ICF) experiment, a calibration method of the dynamic spatial resolution of X-ray framing image-converter (XFIC) is proposed. When an object with straight edge function is projected onto the photocathode of XFIC as an input, edge spread function can be obtained by recording an image of the XFIC output. The first derivative of the edge spread function produces the line spread function (LSF). Then the modulation transfer function (MTF) of the system can be worked out by Fourier transform of the LSF. Therefore, the spatial resolution can be deduced. According to this theory, the spatial resolution of XFIC can be obtained. Based on SG-Ⅱ laser facility, the calibrating X-ray source is generated by 8 bundled lasers bombarding the target. High-Z knife-edge is irradiated by the X-ray and imaged on the photocathode of XFIC, and then a dynamic image is obtained as the system works in the gating mode. By handling the dynamic image, the LSF of XFIC is given by analyzing the edge image. Then the MTF of the camera can be indicated by the Fourier transform of the LSF. When the MTF is 0.1, the corresponding spatial resolution of the imaging system is 20 lp/mm. According to the dynamic spatial resolution theory of framing image-converter, the extreme spatial resolution is 22.8 lp/mm. The calibration result agrees with the theoretical results. By contrast, the static spatial resolution of the same X-ray framing image-converter calibrated by the traditional method is presented here. The calibrated static spatial resolution is 22 lp/mm, a little higher than the dynamic one. During the two-dimensional imaging diagnosis, the X-ray framing image-converter works in the dynamic gating mode, thus the calibrated dynamic spatial resolution can be more truthful to reflect its imaging diagnosis ability.

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

2016, 65 (9): 096101.
doi: 10.7498/aps.65.096101

Abstract +

It is highly desirable to undercool titanium based alloy melts and modulate their dendritic solidification process due to the relevant applications in aerospace engineering. But the serious chemical reactivities of this category of alloys result in potent heterogeneous nucleation and suppress remarkable undercoolings in the course of normal material processing. This paper shows that such a challenge can be solved by containerless processing approach. Liquid Ti-25 wt.%Al alloy is highly undercooled and rapidly solidified under containerless state by both electromagnetic levitation and drop tube techniques. Its metastable undecoolability, crystal nucleation mechanism and dendrite growth process are examined experimentally and analyzed theoretically. Those heterogeneous nuclei with wetting angles above 60 are found to be quite difficult to eliminate even during levitation processing, thus reducing the undercoolability of this alloy. The maximum undercooling of bulk alloy melt reaches 210 K (0.11 TL). The thermodynamic driving force to initiate the nucleation of -Ti phase increases almost linearly with the enhancement of undercooling. The phase dendrite displays a growth velocity up to 11.2 m/s, indicating that the rapid solidification is realized at the relatively slow cooling rate of levitated alloy melt. With the increase of undercooling, phase dendrite experiences a kinetic transition from solute diffusion controlled to thermal diffusion controlled growth. Once undercooling exceeds 100 K, the nonequilibrium solute trapping effect brings about the practically desirable segregationless solidification. Nevertheless, the single condition of substantial undercooling is insufficient to suppress the solid state transformation of phase. It is decomposed into 2-Ti3Al phase plus a small amount of -TiAl compound after containerless solidification at levitated state. A more efficient approach to controlling and modulating the solidification microstructures is to utilize the coupled effects of high undercooling and rapid quenching, which proves to be feasible through the rapid solidification of alloy droplets inside drop tube. For those alloy droplets with diameters ranging from 77 to 1048 m, their cooling rates attain a maximum of 1.05105 K/s, and the predicted maximum undercooling is 227-778 K. In this case, phase dendrites are well refined and kept in a metastable state until ambient temperature. The heat transfer calculations indicate that the thermal radiation is the dominant cooling mechanism for the large alloy droplets above 690 m, whereas thermal convection becomes the major cooling mechanism for the small alloy droplets below 690 m. The microgravity condition during free falling does not show apparent effect on the microstructural formation of these alloy droplets.

2016, 65 (9): 096102.
doi: 10.7498/aps.65.096102

Abstract +

The granular system has complicated force chain network and multiple relaxation mechanisms. The different relaxation mechanisms have largely effects on others. The force chains divide the whole system into many soft zones which dominate the main dissipation process. The system evolves into lower potential energy state gradually and forms directional arrangement under an external load. During the evolution, the complex relaxation behaviors such as transport and migrant processes, make it difficult to distinguish different dissipated mechanisms. Each single physical mechanism stripping from multiple mechanisms should be studied in depth. While among all the mechanisms, the structure evolution plays a crucial role and needs to be paid more attention to.
From the view of potential energy, the detailed energy transformation is illustrated. The granular system is often at a metastable state. When the external disturbance is large enough, the system would step over the energy barrier to a new state. The height of energy barrier is related to the packing structure and grain property. In energy landscape, there exist many energy valleys which correspond to different metastable states. The grain rearrangement and structure reorganization are two main evolution processes at a quasi-static state. The former brings about major potential energy change because of friction and forms certain contact relations. While the latter evolves on the basis of the skeleton formed by grain rearrangement and reaches lower energy state. The conversion among different energy valleys can be used to explain stress relaxation process.
In a complex granular system, the choosing of appropriate internal state variables becomes important, which can reflect specific relaxation process and internal characteristics. The energy fluctuation in the system has a huge influence on dissipation process and macroscopic response and is an effective internal variable to have an insight into the structure evolution. Then granular temperature rooted from gas kinetics is introduced to model the macroscopic behaviors. For loose and rapid granular flow, the kinetic granular temperature itself is the root to affect the flow process. While in a dense granular system, the granular temperature at a quasi-static state is referred to as elastic energy fluctuation. The structure can be kept stable when granular temperature exists on account of the mutual confinement among particles. And the granular temperature at a stable state is just a representation of internal structure of granular assembly. When the granular temperature stimulated by the external disturbance exceeds the stable value, the irreversible process happens and the difference between the excited state and stationary state is the driving force for evolution.
The stress relaxations under different surface properties and confining pressures are simulated using non-equilibrium theory with new change for granular temperature. It can be found that the granular temperature difference triggers elastic relaxation and force chains reorganization. And the larger the temperature difference, the further away from the steady state the system is and the larger the stress change is. The more smooth the surface and the smaller the confining pressure, the lower resistance is generated, so that the initial granular temperature difference is larger and the stress change is larger during stress relaxation. The granular temperature decreases as time goes by because of its own relaxation. When the difference is equal to zero, the process of stress relaxation finishes and the system evolves into a global minimum of potential energy.

2016, 65 (9): 096103.
doi: 10.7498/aps.65.096103

Abstract +

Ferromagnetic shape memory alloys (FSMAs) have received much attention as high performance sensor and actuator materials, since a large magnetic-field-induced strain by the rearrangement of twin variants in the martensitic phase was reported. Up to now, several FSMAs including Ni-Mn-Ga, Ni-Fe-Ga, Co-Ni-Ga, Ni-Mn-Al systems have been studied. Vast amount of knowledge accumulated at the properties of Ni-Mn-Ga Heusler alloys in the past decade can foresee the possibility of employing these alloys in device applications. However, the actuation output stress level of the Ni-Mn-Ga alloy is only less than 5 MPa, which represents a shortcoming of this alloy system. Recently, an unusual type of FSMAs Ni-Co-Mn-In Heusler alloy has been experimentally investigated. It shows magnetic-field-induced reverse martensitic transition (MFIRT), making it more attractive for practical application as magnetically driven actuator because it possesses a magnetostress level on the order of tens of MPa. An almost perfect shape memory effect associated with this phase transition is induced by a magnetic field and is called the metamagnetic shape memory effect. NiMnIn is the basic ternary alloy system of the NiMnInCo alloy, and possesses the same metamagnetic shape memory effect. Moreover, large magnetoresistance, large entropy change that generates giant reverse magnetocaloric effects (MCEs), giant Hall effect have been discovered in Ni-Mn-In alloys.
Composition adjustment must be carried out around stoichiometric Ni2MnIn in order to obtain the appropriate martensitic transformation temperature and Curie temperature. Therefore, a variety of point defects would be generated in this process. In this paper, the defect formation energy and magnetic properties of the off-stoichiometric Ni-X-In (X= Mn, Fe and Co) alloys are systematically investigated by the first-principle calculations within the framework of the density functional theory through using the Vienna ab initio software package. The In and Ni antisites at the site of the X sublattice (InX and NiX) have the relatively low formation energies. For most cases of the site occupation, the excess atoms of the rich component directly occupy the site (s) of the deficient one (s), except for In-rich Ni-deficient composition. In the latter case, the defect pair (InX+XNi) is energetically more favorable. The formation energy of Ni vacancy is the lowest and that of In vacancy is the highest in the vacancy-type defects. It is confirmed that the In constituent is dominant for the stability of the parent phase.
The value of the Ni magnetic moment sensitively depends on the distance between Ni and X atoms. The smaller the distance, the larger the Ni magnetic moment will be. For the anti-site type point defect, when the extra X atom occupies a Ni site, most of the free electrons gather around the extra X atom; while the extra X occupies an In position, the charges are regularly distributed between Ni and extra-X atoms. Moreover, with the increase of the X atomic number, the number of the valence electrons increases, and the bonding strength between the extra X and its neighboring Ni is also enhanced. The results are particularly useful in guiding composition design and developing new type of magnetic shape memory alloy.

2016, 65 (9): 096104.
doi: 10.7498/aps.65.096104

Abstract +

In this paper, a series of hot carriers tests of irradiated 130 nm partially depleted silicon-on-insulator NMOSFETs is carried out in order to explore the HCI influence on the ionizing radiation damage. Some devices are irradiated by up to 3000 Gy before testing the hot carriers, while other devices experience hot carriers test only. All the devices we used in the experiments are fabricated by using a 130 nm partially depleted (PD) SOI technology. The devices each have a 6nm-thick gate oxide, 100 nm-thick silicon film, and 145 nm-thick buried oxide, with using shallow trench isolation (STI) for isolation scheme. The irradiation experiments are carried by 60Co- ray at the Xinjiang Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, with a dose rate of 0.8~Gy(Si)/s. During irradiation all the samples are biased at 3.3V, i.e., VGS=3.3V and other pins are grounded, and when the devices are irradiated respectively by total doses of 500, 1000, 2000 and 3000Gy(Si), we test the characteristic curves again. Then 168-hour room temperature anneal experiments are carried out for the irradiated devices, using the same biases under irradiation. The HCI stress condition is chosen by searching for the maximum substrate current. The cumulative stress time is 5000s, and the time intervals are 10, 100, 500, 1000 and 5000s respectively. After each stress interval, the device parameters are measured until stress time termination appears. Through the comparison of characteristic between pre-irradiated and unirradiated devices, we find that the total dose damage results in the enhanced effect of hot carriers: the substrate current value which characterizes the hot carrier effect (for SOI device are the body to the ground current) increases with the increase of total dose, as the pre-irradiated and unirradiated device do under the same conditions of hot carrier stress, the degradations of key electrical parameters are more obvious for the pre-irradiated one. In order to analyze the physical mechanism of the experimental phenomena, the wide channel device is tested too, we also analyze the phenomenon of the decrease of the substrate current of the wide channel device. From the contrasts of pre-irradiated and unirradiated devices, and narrow and wide channel device test results, we can obtain the following conclusions: SOI devices (especially the narrow channel device) with additional ionization irradiation field induced by ionizing radiation enhance the rate of injecting electrons into the silicon dioxide, and produce oxide trap charge and interface states, which leads to the fact that the channel carrier scattering becomes stronger, transfer characteristic curve of the device, output characteristic curve, transconductance curves and the related parameters of VT, GMmax, IDSAT degradation degree increase. So, when designing 130nm PD SOI NMOSFETs which are applied to the space environment, one should make a compromise between radiation resistance and HCI reliability.

2016, 65 (9): 096401.
doi: 10.7498/aps.65.096401

Abstract +

Interface characterizes describes how the atoms/molecules attach themselves to the solid/liquid interface from the liquid when the crystallization takes place, which plays a key role in revealing the kinetic mechanism during the crystal growth. For common non-facet/non-facet metallic systems, the kinetic undercooling is usually small and it becomes only significant when the growth velocity is high. However, high growth velocity can be usually realized under large undercooling condition. In this case, the interface temperature cannot be measured, thus the kinetic undercooling cannot be determined quantitatively either. Compared with the atom and small molecule materials, the polymer has its distinctive characteristic of different long chains, which are entangled together in a liquid state. Thus the crystallization of the polymer system usually proceeds in the two-dimensional manner, which provides an ideal way to obtain large kinetic undercooling under the small growth velocity condition. The directional crystallization technique has been widely adopted to study the scaling law of undercooling and growth velocity due to its accurate controlling of growth velocity and temperature gradient. Therefore, it offers an appropriate way to make a quantitative investigation. In this paper, the in-situ observations of the solidification of polyethylene glycol 6000 at different pulling velocities are performed and the interface temperature is examined as well by using the directional crystallization technique. The effect of the pulling rate on the growth kinetics is examined. The results reveal that the interface temperature decreases and the undercooling increases gradually with the pulling velocity increasing. A change in the growth regime is observed at T=13.5 K, where regime Ⅱ-regime Ⅲ transition occurs according to Hoffman's kinetic theory of polymer crystallization. The comparison of undercooling between the present work and DSC isothermal crystallization is made, and it shows that the data obtained in the directional growth and the isothermal growth follow the same trends but the undercooling in isothermal growth is larger than in directional growth under the same growth velocity. This indicates that the undercooling in the latter case is over-estimated since it contains the thermal undercooling. Undercooling is the driving force for crystallization, which usually includes solute undercooling, curvature undercooling, thermal undercooling, and kinetic undercooling. Because of the flat interface and the pure material, there is no solute undercooling nor curvature cooling in the present case. The thermal undercooling is also zero in the unidirectional crystallization process. Thus the total undercooling in the present work is the kinetic undercooling. The maximum kinetic undercooling reaches 20 K, indicating that the interface kinetic controlling growth takes place due to the two-dimensional nucleation in polymer.

2016, 65 (9): 096402.
doi: 10.7498/aps.65.096402

Abstract +

Since the discovery of the first metallic glass (MG) with the composition of Au75Si25 in 1960, vast efforts have been devoted to understanding the mechanisms of glass formation in metals, because this class of glassy alloy usually possesses unique properties that may have the potential application as engineering material. As is well known, structure determines properties of material. Therefore, understanding the glass formation of MG from the structural perspective is helpful for guiding researchers in developing more MGs. So far, icosahedral clusters are regarded as the preferred clusters contributing to the formation of amorphous structure due to its five-fold symmetrical feature and high atomic packing. However, it has been found that an ideal icosahedron usually does not have a high concentration in many MG compositions. Thus, we wonder whether icosahedral clusters are popular in microstructures of amorphous alloys. In this work, a feasible scheme for identifying the icosahedron-like clusters in MGs is developed to address this issue. It is found that icosahedron-like clusters are popular structural units in amorphous structure indeed, contributing to the glass formation in alloy. A projection method of reflecting the styles of shell-atom connections in Voronoi-tessellation indexed clusters is developed in detail, so that all clusters can be further geometrically indexed as different projected types of polyhedra. It is revealed that there are three kinds of clusters (0, 2, 8, 1, 0, 2, 8, 2 I-type, and 0, 1, 10, 2) which have the most similar geometrical features to that of the so-called ideal icosahedron, 0, 0, 12, 0. Therefore, besides the ideal icosahedron, these three types of clusters can be regarded as the icosahedron-like clusters. The ideal icoshahedron (0, 0, 12, 0) has a coordination number (i.e., the number of shell atoms) of 12, while these three icosahedron-like clusters have coordination numbers ranging from 11 to 13, so that structural balance between the geometrical atomic stacking and the chemical interactions among various elements in MGs (especially multicomponent MGs) is more easy to achieve. Furthermore, structural models of three selected ZrCu compositions are studied, which are obtained by systematic experimental measurements combined with reverse Monte Carlo simulation. It is found that both the icosahedron-like cluster and the ideal icosahedron have the similar values of some structural parameters, in terms of high atomic packing efficiency, high cluster regularity, fruitful five-fold symmetrical feature, etc. In addition, it is revealed that these ideal icosahedra and icosahedron-like clusters can contain almost all the atoms in these structural models, enhancing the space filling efficiency. In conclusion, these identified icosahedron-like clusters should be the popular building blocks, contributing to the glass formation in alloy. This work provides an insight into the glass formation in alloy from the cluster-level structural angle and will shed light on developing more MGs.

2016, 65 (9): 096501.
doi: 10.7498/aps.65.096501

Abstract +

Diamond-Like Carbon (DLC) is thought to be a potential material in solving heat dissipation problems in light emitting diode module packages. It is of vital importance in evaluating the thermal conductivity of DLC film deposited on a silicon substrate.
In this paper, the molecular dynamics method is used to simulate the formation of a DLC film by the deposition of carbon atoms on a isilicon substrate. Tersoff potential is adopted to reproduce the structures and densities of silicon, carbon, and SiC. A silicon substrate consisting of 544 atoms is located at the bottom of the simulation domain. The substrate is kept at a temperature of 600 K through a Noose-Hover thermostat. Carbon atoms are injected into the substrate individually every 0.5 ps at an energy of 1 eV. After a 7.5 ns deposition process, a 4 nm amorphous film containing 15000 carbon atoms is formed. Injected carbon atoms and substrate silicon atoms are intermixed at the bottom layer of the deposited film while the rest of the film contains only carbon atoms. The density of the film decreases slightly with the increase of the height of the deposited film and the average density is 2.8 g/cm3. Analysis of the coordination number shows that the sp3 fraction of carbon atoms in the film also decreases with the increase of the height of the deposited film, with a maximum value of 22%. It might be caused by the continuous impacts of the subsequently injected carbon atoms on the previously formed DLC film.
The thermal conductivities of the DLC film in the planar and normal directions are calculated by the Green-Kubo method. The thermal conductivity of pure diamond film is also calculated for comparison. The results show that the planar thermal conductivity of the deposited DLC film is approximately half of that of the pure diamond film with the same size. It is higher than the normal thermal conductivity of the deposited film. The thermal conductivities of the DLC film in both planar and normal directions increase with the increase of film density and sp3 fraction in the DLC film. The results indicate that the local tetrahedral structure of sp3 carbon atoms contributes to the improvement of thermal conductivity in the DLC film.

2016, 65 (9): 096801.
doi: 10.7498/aps.65.096801

Abstract +

Germanene, one of the most important two-dimensional materials after graphene and silicone have been discovered, is attracting wide attentions due to its many excellent physical properties. Since a suitable band gap is needed for the electronics and optoelectronics, the lack of a band gap has essentially restricted the practical applications of germanene in macroelectronics. In this article, density functional theory calculations with van de Waals corrections is utilized to study the geometric and electronic properties of germanene (Ge), germanane (GeH) and germanene/germanane (Ge/GeH) bilayer. The band gaps for Ge and GeH are zero and 1.16 eV, respectively. For the Ge/GeH bilayer, a considerable binding energy of 273 meV/unit cell is obtained between Ge and GeH layers. This value is smaller than that of Ge bilayer (402 meV/unit cell), but larger than that of GeH bilayer (211 meV/unit cell), indicating a considerable GeH/ bonding. This means that Ge and GeH layers could be combined steadily by the interlayer weak interactions. Meanwhile, a band gap of 85 meV is opened, which is contributed to the breaking of the equivalence of the two sublattices in the Ge sheet, yielding a nonzero band gap at the K point. Charge density difference indicates that the electrons on the s orbital of H transfer to the Ge_p orbital, enhancing the interlayer interactions. It should be noted here that the van de Waals corrections are pretty important for the geometric and electronic properties of the Ge/GeH bilayer. Without the van de Waals corrections, the binding energy of the Ge/GeH bilayer is reduced from 273 meV/unit cell to only 187 meV/unit cell, severely underestimated the strength of the weak forces between Ge and GeH layers, resulting in a much smaller band gap of 50 meV. Interestingly, no band gap is obtained for the sandwich structure GeH/Ge/GeH, in which the equivalence of two sublattices in germanene is kept. Finally, all the results are confirmed by the high accurate hybrid functional calculations. At the Heyd-Scuseria-Ernzerhof level, the band gap of Ge/GeH bilayer is 117 meV, slightly larger than 85 meV at the Perder-Burke-Ernzerhof level. Our work would promote utilizing germanene in microelectronics and call for more efforts in using weak interactions for band structure engineering.

2016, 65 (9): 096802.
doi: 10.7498/aps.65.096802

Abstract +

It is observed that the addition of Nb or Ge to Zr alloy can improve its corrosion resistance. Because of the extreme importance of the mechanism of oxidation to corrosion properties of Zr alloy, we systematically investigate the O adsorption properties on pure Zr surface and Zr surface with Nb or Ge using first-principle calculations based on density functional theory. Firstly, we present the absorption energies to reveal the influences of Nb and Ge on the O absorption capacity of Zr surfaces, resepctively. According to the calculated absorption energies, we find that Nb and Ge reduce the oxygen absorption capacities of most of surfaces, and the only exception is that Nb enhances the oxygen absorption capacity of Zr(1120) surface. Moreover, the absorption energy of O at favorable site on Zr(0001) surface is much lower than on Zr(1010) or (1120) surface. Therefore, the initial oxidation of polycrystalline Zr should occur at Zr(0001) surface and the absorption capacity of this surface takes a predominant role in the initial oxidation of polycrystalline surface. Secondly, the segregation of Nb or Ge in Zr alloy is anisotropic. We find that the segregation of Ge to surface is exothermic, while the segregation of Nb to surface is endothermic. Nb and Ge migrate to Zr(0001) surface more easily than to Zr(1120) and Zr(1010) surfaces. Therefore, the influence of Nb or Ge on absorption property of Zr(0001) is much larger than that of Zr(1010) or (1120) surface. Based on the adsorption and segregation properties of Nb and Ge on Zr surfaces, both Nb and Ge can reduce the oxygen absorption capacity of Zr surface and inhibit the initial oxidation of Zr alloy surface, which can be used to understand the experimental observation that Nb and Ge can improve the corrosion resistance of Zr alloy. Finally, the electronic structure analysis shows that the influences of Nb and Ge on oxygen adsorption capacity of Zr surface are exerted by changing the d-band distribution. According to Hammer-Norskov d-band center theory, the absorption energy of absorpate on transition metal surface is mainly determined by d-band center. The addition of Nb or Ge to the Zr surface changes the location of d-band of the surface, which results in the variation of absorption energy of O atom on the Zr surface. For absorption at top site on each surface, it is found that the absorption energy of O only depends on the d-band center of the surface atom below the O atoms. However, for absorption at favorable sites on each surface, the absorption energy of O atom is influenced not only by the d-band center of surface atom, but also by atomic structural properties of the surface.

## EDITOR'S SUGGESTION

2016, 65 (9): 096803.
doi: 10.7498/aps.65.096803

Abstract +

In this paper, we study the effect of anisotropic surface tension on the interface morphological stability of deep cellular crystal during directional solidification. We assume that the process of solidification is viewed as a two-dimensional problem, the anisotropic surface tension is a four-fold symmetry function, the solute diffusion in the solid phase is negligible, the thermodynamic properties are the same for both solid and liquid phases, and there is no convection in the system. On the basis of the basic state solution for the deep cellular crystal in directional solidification, by the matched asymptotic expansion method and the multiple variable expansion method, we obtain the asymptotic solution, and then the quantization condition of interfacial morphology for deep cellular crystal is obtained.
The results show that by comparison with the directional solidification system of surface tension isotropy, the interface morphological stability of surface tension anisotropy also possesses two types of global instability mechanisms: the global oscillatory instability (GTW-mode), whose neutral modes yield strong oscillatory dendritic structures, and the low-frequency instability (IF-mode), whose neutral modes yield weakly oscillatory cellular structures. Both of the two global instability mechanisms have the symmetrical mode (S-mode) and the anti-symmetrical mode (A-mode), and the growth rate of the S-mode with the same index n is greater than that of the A-mode. In this sense we say that the S-mode is more dangerous than the A-mode. All the neutral curves of the GTW-S-modes and LF-S-modes divide the parameter plane into two subdomains: the stable domain and the unstable domain. In the paper we show the neural curves of the GTW-S-modes and LS-S-modes for various n, respectively. It is seen that among all the GTW-S-modes (n=0, 1, 2), the GTW-S-mode with n=0 is the most dangerous oscillatory mode, while among all the LF-S-modes (n=0, 1, 2), the LF-S-mode with n=0 is the most dangerous weakly oscillatory mode. We also show the neural curves of the GTW-S-mode (n=0) and LS-S-mode (n=0) for various anisotropic surface tension parameters, respectively. It is seen that as the anisotropic surface tension increases, the unstable domain of global oscillatory instability decreases, and the unstable domain of the low-frequency instability increases.

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

2016, 65 (9): 097101.
doi: 10.7498/aps.65.097101

Abstract +

As is well known, the structure inversion asymmetry (SIA) and Rashba spin splitting of semiconductor heterostructure can be modulated by either electric field or engineering asymmetric heterostructure. In this paper, we calculate the Rashba coefficient and Rashba spin splitting for the first subband of Al0.6Ga0.4N/GaN/Al0.3Ga0.7N/Al0.6Ga0.4N QW each as a function of thickness (ws) of the inserted Al0.3Ga0.7N layer (right well) and external electric field. The thickness of GaN layer (left well) is 40-ws . With ws increasing, the Rashba coefficient and Rashba spin splitting for the first subband increase first, because the polarized electric field in the well region increases and the electrons shift towards the left heterointerfaces, and then decrease when ws20 since the electric field in the well region decreases, and the confined energy increases as effective well thickness decreases. But when ws30 , the Rashba spin splitting decreases more rapidly, since kF decreases rapidly. Contributions to the Rashba coefficient from the well is largest, lesser is the contribution from the interface, which varies slowly with ws, and the contribution from the barrier is relatively small. Then we assume ws=20 , and find that the external electric field can modulate the Rashba coefficient and Rashba spin splitting greatly because the contribution to the Rashba coefficient from the well changes rapidly with the external electric field, and the external electric field brings about additional potential and affects the spatial distribution of electrons, confined energy and Fermi level. When the direction of the external electric field is the same as (contrary to) the polarization electric field, the Rashba coefficient and Rashba spin splitting increase (decrease) with external electric field increasing. With the external electric field changing from -1.5108 V m-1 to 1.5108 V m-1, the Rashba coefficient approximately varies linearly, and the Rashba spin splitting first increases rapidly, then approximately increases linearly, and finally increases slowly. Because the value of kF increases rapidly first, then increases slowly. Results show that the Rashba coefficient and the Rashba spin splitting in the Al0.6Ga0.4N/GaN/Al0.3Ga0.7N/Al0.6Ga0.4N QW can be modulated by changing the relative thickness of GaN and Al0.3Ga0.7N layers and the external electric field, thereby giving guidance for designing the spintronic devices.

2016, 65 (9): 097301.
doi: 10.7498/aps.65.097301

Abstract +

The polycrystalline anatase TiO2 nanowires with a diameter of about 300 nm are successfully prepared by the sol-gel method together with electrospinning method under a heat treatment at 500℃. The effect of illumination on electronic transport property and magnetoresistance (MR) effect are studied via voltage-current (V-I) curves measured at room temperature in the cases of the dark and the ultraviolet irradiation. The results show that the V-I plots are straight lines without passing through zero point and the resistance of the nanowire is as high as 7.51011 in the dark. The resistance decreases gradually with the magnetic field increasing and after reaching a minimum 4.71011 at B=0.7 T it turns to increase rapidly, but is still smaller than the resistance without magnetic field, indicating a negative MR effect. With the increase of the magnetic field, the negative MR effect increases and then decreases, and the negative MR achieves a maximum value of -37.5% under B=0.7 T. Interestingly, the resistance of nanowires in the ultraviolet irradiation is reduced by about 10 times compared with that in the dark without applying a magnetic field. As the magnetic field increases, the resistance increases monotonically, presenting a positive MR effect. The MR increases rapidly with the increase of magnetic field, and reaches the maximum positive MR effect 620% under B=1.0 T. At room temperature only a few carriers are generated by the thermal excitation in the TiO2 nanowires, which leads to a large resistance in the dark situation. In the ultraviolet irradiation case, the carrier concentration of the nanowires increases because of the generation of a large number of electron-hole pairs, resulting in huge decrease of resistance compared with in the dark. We attribute the change of the MR to the competition betwen two MR mechanisms: negative MR effect due to the localization of d electron and positive MR effect due to spin splitting of the conduction band. In the dark, due to the low carrier concentration, the negative MR mechanism caused by the localization of d electron is dominant under the magnetic field. However, in the ultraviolet irradiation, because carrier concentration increases hugely due to the irradiation, the positive MR mechanism caused by spin splitting of the conduction band is dominant. The fact that the V-I curves does not pass through zero point implies that the contact between TiO2 nanowire and Pt metal is Schottky contact due to the difference in work function. In the dark, the initial voltage first increases with the increase of magnetic field, and then remains steady. In the ultraviolet irradiation the initial voltage is smaller than in the dark and increases monotonically with the magnetic field increasing. In this paper, the physical mechanism of the electrical transport property and MR effect of TiO2 nanowire are discussed, which may provide a meaningful exploration for developing the new electronic device based on the oxide nanowires.

2016, 65 (9): 097302.
doi: 10.7498/aps.65.097302

Abstract +

Sn-doped ZnO and pure ZnO thin films are deposited on glass substrates with prepared electrode by the chemical vapor deposition method. The gas sensing performances of Sn-doped ZnO and pure ZnO thin films are investigated by our home-made system at room temperature, and the gas sensing test results reveal that Sn-doped ZnO thin film exhibits high gas response to ethanol and acetone, while no response is detected for pure ZnO to ethanol or acetone at room temperature. Sn-doped ZnO thin film also has high selectivity that the response to ethanol is higher than that to acetone in the same measurement conditions, and the response of Sn-doped ZnO thin film sample to ethanol is almost the third largest when the concentration is 320 ppm. The typical scanning electron microscopy images reveal that these two samples are tetrapod-shaped ZnO whiskers with diameters in a range of about 150-400 nm. X-ray diffraction results indicate that all the samples are of wurtzite structure. Neither trace of Sn, nor that of Sn alloy nor that of Sn oxide is detected in the Sn-doped ZnO film, while its diffraction peak shifts towards the left compared with that of pure ZnO sample, which suggests that Sn atoms exist in the form of interstitial atoms in the ZnO crystal. The energy dispersive spectrum shows that the Sn-doped ZnO thin film is composed of Zn and O elements, and no Sn signal is defected. Photoluminescence spectra reveal that both Sn-doped ZnO and pure ZnO films have ultraviolet light emission peaks and green emission peaks, while the intensities of the defect emissions are significantly enhanced by doping of Sn. In addition, no gas response to ethanol is detected after Sn-doped ZnO thin film has been annealed in the air, which indicates that the room temperature gas sensitivity of the Sn-doped ZnO thin film may be related to its high defect concentration. The working mechanism of Sn-doped ZnO thin film is explained by a free electron random scattering model. As is well known, ZnO semiconductor gas-sensor is of surface-controlled type. In this work, upon exposure to ethanol vapor, the physical absorbed ethanol molecules acting as scattering centers can reduce the mean free path of the electrons in the surface of the film, changing the mean free time n, which would increase the resistance of Sn-doped ZnO thin film at room temperature. This work provides a simple method of fabricating the highly sensitive ethanol gas sensor operating at room temperature, which has great potential applications in gas sensor field.

## EDITOR'S SUGGESTION

2016, 65 (9): 097401.
doi: 10.7498/aps.65.097401

Abstract +

In YBa2Cu3O7-x (YBCO) film there exists thickness effect: the critical current density of YBCO film drops precipitously as the coating thickness increases, especially in the case that the thickness of YBCO film exceeds 1 m. In this paper, we introduce very thin layers of CeO2 into YBCO layers and successfully fabricate the structure of YBCO/YBCO/CeO2/YBCO superconducting thick film. Firstly, YBCO films with two layers are fabricated on a LaAlO3 substrate by a multiple coatings process using a trifluoroacetate metal organic deposition method. Secondly, CeO2 thin films are deposited on YBCO films by RF-sputtering. Finally, we prepare the third YBCO film on CeO2interlayer. No cracks are observed in scanning electron microscopy images of these films; further, the majority of the grains in the films are well-textured and c-axis oriented. The full-width-half-maximum of the out-of-plane texture is measured to be 1.395 for the multilayer YBCO film at a thickness of 2 m Using this multilayer technology, we achieve Jc values of up to 1.36 MA/cm2 (77 K, self-field) in films as thick as 2 m, for an extrapolated critical current of 272 A/cm. We attribute the enhanced performance of the thick YBCO film to the CeO2 interlayer which playsan important role in transmission texture and stress relaxation.

2016, 65 (9): 097801.
doi: 10.7498/aps.65.097801

Abstract +

The effective wavelength scaling theory for optical antennas indicates that an optical antenna does not respond to the wavelength of incident electromagnetic wave, but to a shorter effective wavelength which depends on the plasma wavelength and optical dielectric permittivity of the antenna material, and also on the geometric structure of the antenna.
In this paper, based on the effective wavelength scaling theory for optical antennas and on the assumption that metallic carbon nanotube (CNT) can be described by a free electron gas according to the Drude model, the general relationship between effective wavelength and dielectric properties of the antenna material for a metallic carbon nanotube optical antenna is derived. According to this relationship, the investigation into the effective wavelength that a metallic CNT optical antenna responds to can be transferred to easier theoretical calculation for the dielectric properties of CNT, instead of exploring its plasma wavelength. Following first-principle calculations for dielectric properties of CNT with 4 diameter, the effective wavelength versus incident wavelength for each of two types of metallic 4 CNT antennas is investigated. In addition, the resonance characteristics of metallic 4 CNT dipole antennas are analyzed.
It is shown that the effective wavelength approximately follows a linear relationship with wavelength of the incident light for the 4 metallic CNT antenna, which is consistent with the wavelength scaling theory. In addition, CNT optical antenna has good wavelength scaling performance compared with nano-antennas made of conventional metals like silver and gold; hence metallic CNTs as optical antennas are beneficial for constructing more compact devices. Moreover, according to the simulation results of resonance characteristics of metallic 4 CNT dipole antennas, there are several 4 metallic CNT dipole antennas with small difference in length meeting the resonance conditions for incident electromagnetic wave with a certain frequency, while there are one or more corresponding resonant modes in the optical and near-infrared spectral range concerned for a 4 metallic CNT dipole antenna with fixed length. Therefore, it is easier to meet the resonance conditions for CNT optical antenna than for conventional metal optical antenna, which also arises from the superior wavelength scaling ability of CNT. These advantages of CNT can help to miniaturize the optical antenna and improve the efficiency of energy conversion of the incident radiation in the optical and near-infrared spectral range.
Reliability of the assumption and the theoretical process in this paper are validated by comparing the simulation results with existing investigations. Therefore, the theoretical investigations in this paper may provide a new approach to studying metallic CNT optical antennas. The simulation results also demonstrate the potential applications of CNT optical antenna, including solar energy harvesting and conversion.

2016, 65 (9): 097901.
doi: 10.7498/aps.65.097901

Abstract +

In recent years, the carbon nanotube (CNT) emitters used for ion sources or gas sensors have been investigated, and the progress of several approaches such as field ionization and field desorption sources has been reported. However, a major concern for these applications is possible loss of CNTs caused by field evaporation, which can shorten the lifetimes of CNT-based emitters used for high electric field ion sources. So in CNT-based field emitter technology, emitter lifetime and degradation will be key parameters to be controlled. However, up to now only very few investigations in this direction have been conducted. The reason for this might lie in the fact that one often considers that the threshold value of field evaporation for a kind of material ( 40 V/nm) is much higher than the field of ionization or desorption ( 10 V/nm) according to the metal material characteristics (such as the threshold values of field evaporation for tungsten and molybdenum are 54 V/nm and 45 V/nm, respectively). In this work, the carbon nanotube thin-film (the density of CNTs is about 2.5108/cm2) is fabricated by screen-printing method, and the field evaporation behavior of CNT thin-film is studied experimentally in an ultrahigh vacuum system typically operating at a pressure of lower than 10-9 Torr after a 4-hour bake-out at ~200℃. Unlike the vertically aligned CNT array having higher electric field around the edge of the array because of the shielding effect, the printed CNT thin-film has more uniform distribution of electric field and is very easy to relize the mass production. The results show that the prepared CNT thin-film has quite obvious field evaporation behavior (some contaminants have deposited on the surface of grid after field evaporation, and energy-dispersive X-ray spectroscopy elemental mapping result of the grid indicates that the contaminants consist mainly of carbon elements), with turn-on field in a range of 10.0-12.6 V/nm, ion current could reach up to hundreds of pA. Meanwhile, the results with scanning electron microscope analysis and field electron emission measurement indicate that the CNT distribution turns into more non-uniform distribution after field evaporation; even some CNTs are directly dragged away from the substrate by the strong field. The field evaporation of CNT thin-film also leads to field electron emission onset voltage increasing from 240 V to 300 V, field enhancement factor decreasing from 8300 to 4200, and threshold field of field evaporation rising from 10.0 V/nm to 12.6 V/nm. However, the repeatability of sample treated by the field evaporation brings about an improvement to a certain extent. It could be understood in this way: upon applying a positive voltage, the most protruding parts, which have the strongest emissive capability, are evaporated first, which leads to the declined field enhancement factor; the parts of CNTs which have relatively weak emissive capability are not evaporated. So the uniformity of electric field is improved through reducing the difference in field enhancement factor rather than surface morphology between carbon nanotubes. The field evaporation of CNT thin-film is also a process which improves the uniformity of electric field. Therefore, the stability and repeatability of the field electron emission for carbon nanotube thin-film are improved naturally.

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

2016, 65 (9): 098101.
doi: 10.7498/aps.65.098101

Abstract +

In this paper we analyze the reason of the etching trenches in chemical vapor deposition (CVD) graphene domain and study the influence factor in the distribution and morphology of wrinkles.
Graphene is synthesized on Cu substrate. The Cu substrate is annealed at 1050℃ for 60 min with 1000 sccm Ar and 200 sccm H2. After annealing, 500 sccm Ar, 20 sccm H2, and 1 sccm dilute CH4 (mixed with Ar) are introduced into the CVD system for graphene growth. Hydrogen etchings of graphene are conducted with flows of 500 sccm Ar and 200 sccm H2 at atmospheric pressure, and etching are performed at 950 and 1050℃.
The striated and reticular etching trenches are observed after etching via optical microscope and scanning electron microscope. Every graphene domain is divided into island structures by these etching trenches. However, the edge of graphene domain is not etched and the size of domain is not changed. Electron backscatter diffraction (EBSD) is conducted to analyze the different morphologies of etching trenches. According to the EBSD analysis, the etching trench is closely associated with the Cu crystal orientation. Different Cu planes result in differences in mode, shape, and density of the etching trench.
We conduct a verification experiment to judge whether the etching trenches are caused by the gaps between graphene and Cu substrate or by the hydrogenation of wrinkles.
The graphene domains grown on Cu substrate with the same growth condition are etched immediately after growth without cooling process. We select graphene which grows across the Cu grain boundary, via optical microscope. A small number of regular hexagons are observed in graphene surface and the region of Cu boundary, but no etching trench is found. As the graphene growing across Cu boundary is the suspending graphene and there is no etching trench, we consider that the gap between graphene and Cu species is not a significant factor of forming etching trench. For comparison, the etching trenches are observed in the graphene domains with cooling process. Thus, the trench formation is bound up with the cooling process after growth, which can lead to the wrinkle formation on the graphene surface, giving rise to a large thermal expansion coefficient difference between the graphene and Cu species. As a major type of structural imperfection, wrinkles can show that enhanced reactivity is due to hydrogenation because of high local curvature. So we consider that the trench formation is caused by the hydrogenation of wrinkles.
Then the as-grown graphene domains are transferred to SiO2 substrate and atomic force microscope (AFM) is employed to measure the surface appearance of graphene. The AMF image shows lots of wrinkles in the graphene surface. The morphology and density of wrinkles are similar to those of the etching trenches extremely. Thus, the AFM testing result provides another evidence to prove that the etching trenches are related to the hydrogenation of wrinkles.
From the above we can draw some conclusions. Numerous trenches are observed in the graphene domains after etching, and the trench patterns are closely associated with the Cu crystal orientation. A different Cu crystal orientation leads to variations in mode, shape, and density of the etching trench. We prove that the etching trenches are caused by the hydrogenation on wrinkles formed in the cooling down process instead of the gap between Cu and graphene. This hydrogen etching technology is a convenient way to detect the distribution and morphology of wrinkles. Furthermore, it provides a reference for improving the quality of CVD graphene.

2016, 65 (9): 098102.
doi: 10.7498/aps.65.098102

Abstract +

Solid oxide fuel cell (SOFC) is considered to be a highly efficient device to convert chemical fuels directly into electrical power. Because of multilayer composite arrangement of cells, mismatch of the thermal expansion coefficients, chemical/thermal gradient, or phase change of the materials will result in residual stresses, which are reflected in the pronounced bending of unconstrained cells and cause a reliable problem. Considerable efforts have been devoted to the analysis of residual stresses in an elastic multilayer system, and one of the efforts that are to improve not only electrochemical performance for high energy conversion efficiency but also long term stability, is to process a continuously gradient anode functional layer (CG-AFL) between dense electrolyte and porous anode. Hence to understand the stress distribution and deformation of the multilayer with a CG-AFL is needed for the cell design. As the chemical reduction takes place at the interface between NiO-YSZ and the previously reduced porous Ni-YSZ, a reduced layer, together with the unreduced layer and the electrolyte will cause the residual stresses to be re-distributed. In this paper, taking the CG-AFL into account, the curvature and residual stresses of half-cell during reduction are analyzed. The results show that the curvature of half-cell with a CG-AFL increases as the reduction process. And the curvature would also increase as the thickness of the CG-AFL increases, and decrease with the increase of the index of power function that expresses young's modulus and thermal expansion coefficient of gradient layer. The residual stresses among the layers are correspondingly influenced by the thickness of the gradient layer, the index of power function and reduction extent. When taking power function as a linear function, the gradient layer obviously reduces the residual stress in the electrolyte. However, the increase of the index in power function will cause the increase of electrolyte residual stress. These mentioned analyses reveal that the CG-AFL cannot offer a solution that simultaneously improves the residual stress and curvature in a half-cell in terms of thickness and profile exponent of CG-AFL, i.e., the mitigation of residual stress will give rise to the increase of curvature, and vice versa. On the other hand, for part-reduced half-cell, the maximum tensile stress is found at anode/gradient layer interface in anode layer, which may facilitate structural failure since tensile residual stress is so high that it reaches the fracture strength of anode material. Consequently, it is important to ensure that the anode is fully reduced in practice. In conclusion, the existing gradient layer is helpful for enhancing the cell reliability via suitable design.

2016, 65 (9): 098501.
doi: 10.7498/aps.65.098501

Abstract +

The need for low-power alternatives to digital electronic circuits has aroused the increasing interest in spintronic devices for their potentials to overcome the power and performance limitations of (CMOS). In particular, all spin logic (ASL) technology, which stores information using the magnetization direction of the nano-magnet and communicates using spin current, is generally thought to be a good post-CMOS candidate for possessing capabilities such as nonvolatiliy, high density, low energy dissipation. In this paper, based on nano-magnetic dynamics described by Landau-Lifshitz-Gilbert-Slonczewski (LLGS) equation and transport physics of spin injection and spin diffusion, a coupled spin-transport/magneto-dynamics model for ASL is established. Under different channel lengths and applied voltages, the switching characteristics of ASL device comprised of Co and Permalloy (Py) nano-magnets are analyzed by using the coupled spin-transport/magneto-dynamics model. The results indicate that the switch delay, energy dissipation and thermal noise effect of PyASL are lower than those of CoASL. The main reason is that the saturation magnetization of Py is less than that of Co. Under the same applied voltage, the maximal channel length of PyASL is longer than that of CoASL when ASL device can switch accurately. Moreover, the two ASL devices' switching delay can be reduced by reducing channel length or increasing applied voltage, and the energy dissipation can be reduced by reducing channel length or applied voltage, whereas there are no optimized applied voltages to minimize the energy-delay product. In addition, the influences of thermal noise on switching delay and energy dissipation can be improved by lowering channel length, but increasing applied voltage can only improve the influence of thermal noise on switching delay. The above-mentioned conclusions will supply essential guidelines for optimizing the ASL devices' materials and configuration.

2016, 65 (9): 098901.
doi: 10.7498/aps.65.098901

Abstract +

Large network average path length will cause large network delay which brings difficulty in supporting the time sensitive services and applications. Large hop distance between source node and destination node in traditional network leads to significant network delay. By adding long-ranged links, path length from source node to destiny node will be reduced and original network can be transformed into a scale-free network with a small network average path length. The network delay is optimized by minimizing hop distance, in which information can transfer more efficiently and rapidly. Adding links can lower network delay effectively, but on the other hand, it will increase its cost. Common network construction methods focus on separating networks that are very different from each other and mostly unaware of each other, such as fixed and mobile networks planning. But in many real networks, networks are dependent on each other; therefore ignoring these network interactions cannot become more efficient. Cost and effectiveness play a key role in real network construction and layering network is an effective way to analyze coupling network especially in heterogeneous network. In this paper, the model of a toward cost-effective scale-free coupling network construction method is proposed. It combines the advantages of layered network and cost-effective network. A layered coupling network model is established in which network is divided into several networks based on link property. Links in the same layer have the same property and the upper layer capability is higher than lower layer capability. The nodes in the upper network are selected from the lower layer network coupling with the corresponding nodes with the same spatial location. Based on the network optimization and evolving network researches, the increases of node degree and local network radius are supposed to be continuous, moreover cost-effective indicator is introduced which characterizes the costs and effectiveness of adding links. Based on continuum, links are added to upper layer network with a certain probability by two continuous processes and thus network evolves into a scale-free network. The two continuous processes include node degree increasing process and local network radius increasing process. In the previous processes, cost-effective indicator is introduced and only the links satisfied cost-effectiveness are added. Cost-effective indicator characterizes the cost and effectiveness of network construction. Cost is proportional to Euclidean distance and effectiveness includes revenue of network average path length decreasing and link property benefit. In the coupling network, traffic prefers to transmit in the upper layer network for reducing network latency, and consequently leading to traffic congestion in upper layer. In the simulation, network topology evolution and dynamic traffic performance are evaluated. The simulation result shows that this method can effectively reduce the network latency within cost-effective requirement and initial network characteristics are maintained. The results also show that the network average path length declines slowly when network average path length is small because lower average path length needs higher cost when average path length is small. To investigate the traffic behaviors in the coupled layered networks, the traffic dynamic transition model is taken and dynamic traffic performance is given in this evolved scale-free network. Moreover, the cooperation between the two layers can be used to optimize network traffic performance by adjusting the link capacity to satisfy the requirements for the network congestion.