Vol. 65, No. 16 (2016)
2016-08-20
GENERAL
2016, 65 (16): 160201.
doi: 10.7498/aps.65.160201
Abstract +
In order to study the essential structural characteristics of monolayer MoS2 nanoribbons in natural state, and also the effects of the aspect ratio and the ambient temperature on the relaxation properties of the nanoribbons, the relaxation properties of monolayer MoS2 nanoribbons with different aspect ratios are simulated by molecular dynamics (MD) method based on REBO potential functions at different thermodynamic temperatures from 0.01 K to 1500 K. The energy curves and surface morphologies of monolayer MoS2 nanoribbon are obtained, and the dynamic equilibrium process of the monolayer MoS2 nanoribbons is also discussed in all the simulation process. The simulation results show that the monolayer MoS2 nanoribbons do not generate a fluctuation at the ideal temperature (0.01 K) for the reason that the kinetic energy of the monolayer MoS2 nanoribbons is almost zero and the vibration amplitude is small. However, a certain degree of fluctuations occurs at the edges and inside of the monolayer MoS2 nanoribbons at the room temperature or high temperature. The fluctuation height and the fluctuation degree also increase with increasing the ambient temperature and the aspect ratio of the MoS2 nanoribbon, even a high aspect ratio monolayer MoS2 nanoribbon exhibits a surface curved fluctuation, which is perpendicular to the surface of the MoS2 nanoribbons under high temperature condition. Finally, the influences of chirality on relaxation property under different temperature conditions are studied in this paper further, the results show that unlike the armchair structure, the zigzag monolayer MoS2 nanoribbons not only present a surface fluctuation, but also exhibit an obvious bending phenomenon along the width direction simultaneously. Like the armchair nanoribbons, the surface fluctuation height and the surface fluctuation degree of the zigzag nanoribbons also increase with increasing both the ambient temperature and the aspect ratio of the MoS2 nanoribbons. It is also observed that the armchair and zigzag monolayer MoS2 nanoribbons with a similar aspect ratio have a similar surface fluctuation degree at the same ambient temperature. Unlike the armchair nanoribbons, the bending phenomenon along the width direction of the zigzag nanoribbons is more significant, and the bending width and the bending degree increase with increasing the ambient temperature and the aspect ratio of the MoS2 nanoribbons. Although the bending degree of the zigzag nanoribbons becomes larger with the increase of temperature, the increasing rate of the bending degree will become smaller and smaller until the bending degree reaches a maximum value.
2016, 65 (16): 160202.
doi: 10.7498/aps.65.160202
Abstract +
With the development of factional calculus theory and applications in different fields in recent years, the rational approximation problem of fractional calculus operator has become a hot spot of research. In the early 1950s and 1960s, Carlson and Halijak proposed regular Newton iterating method to implement rational approximation of the one-nth calculus operator. Carlson regular Newton iterating method has a great sense of innovation for the rational approximation of fractional calculus operator, however, it has been used only for certain calculus operators. The aim of this paper is to achieve rational approximation of arbitrary order fractional calculus operator. The realization is achieved via the generalization of Carlson regular Newton iterating method. To construct a rational function sequence which is convergent to irrational fractional calculus operator function, the rational approximation problem of fractional calculus operator is transformed into the algebra iterating solution of arithmetic root of binomial equation. To speed up the convergence, the pre-distortion function is introduced. And the Newton iterating formula is used to solve arithmetic root. Then the approximated rational impedance function of arbitrary order fractional calculus operator is obtained. For nine different operational orders with n changing from 2 to 5, the impedance functions are calculated respectively through choosing eight different initial impedances for a certain operational order. Considering fractional order operation characteristics of the impedance function and the physical realization of network synthesis, the impedance function should satisfy these basic properties simultaneously: computational rationality, positive reality principle and operational validity. In other words, there exists only rational computation of operational variable s in the expression of impedance function. All the zeros and poles of impedance function are located on the negative real axis of s complex plane or the left-half plane of s complex plane in conjugate pairs. The frequency-domain characteristics of impedance function approximate to those of ideal fractional calculus operator over a certain frequency range. Given suitable initial impedance and for an arbitrary operational order, it is proved that the impedance function could meet all properties above by studying the zero-pole distribution and analyzing frequency-domain characteristics of the impedance function. Therefore, the impedance function could take on operational performance of the ideal fractional calculus operator and achieve the physical realization. It is of great effectiveness in the generalization of this kind of method in both theory and experiment. The results educed in this paper are the basis for further theoretic research and engineering application in constructing the arbitrary order fractional circuits and systems.
2016, 65 (16): 160301.
doi: 10.7498/aps.65.160301
Abstract +
Quantum secret sharing is an important way to achieve secure communications, which has critical applications in the field of information security for its physical properties. According to the perspective of the practical applications, improving the confidentiality and integrity of secret sharing schemes is a good method to increase the security and reliability of communications. In this paper, we propose a quantum secret sharing scheme based on generator matrix segmentation and the structural features of quantum graph states. The security of the secure secret sharing scheme is guaranteed by the pattern of transferring information by stabilizers, scalability of the information and new recovery strategy provided by the entanglement of the related graph states. It puts forward an effective solution to the problem of matrix cycle period, where some numbers without the primitive element cannot construct the generation matrix.
First of all, the physical properties of quantum bits (qubits), such as uncertainty principle, no-cloning theorem and indistinguishability, not only optimize the classical schemes but also ensure the absolute safety of communication. Secondly, the application of matrix segmentation makes secret information has better scalability. It improves the coding diversity and the difficulty in deciphering. Thirdly, the favorable entanglement properties and mature experiment preparation techniques of graph states provide an approach to the practical applications. The superiority of the yielded graph states is described in graphical fashion with an elegant stabilizer. Fourthly, the shuffling operation can ensure the independence of the message among participants. Therefore, Eve can not obtain any useful information by measuring randomly. Two group-recovery protocols are proposed to show the secret recovering processing through rebuilding sub-secrets among legal cooperative participants.
In the scheme design, the dealer extracts the classical secret information according to the corresponding principle between the classical and quantum information, and divides the classical secret through generated matrix which is produced with the primitive elements in finite domain satisfying the linear independence for any k column vectors. Then the dealer encodes information into graph states and distributes particles to the legal participants with unitary operations. Subsequently, the credible center obtains sub-secrets by the theory of graph states and the group recovery protocol. He can achieve the initial classical secret via the inverse algorithm of matrix segmentation. After getting the classical secret, he recovers quantum secret according to the relationship between classical information and quantum information.
Theoretical analysis shows that this scheme can provide better security and scalability of the information. It is appropriate to realize the secret sharing in the quantum network communication to protect secrets from eavesdropping. Also, it can provide an approach to designing diverse and scalable quantum secure communication schemes based on quantum graph states, the algorithm of matrix segmentation, and group-recovery protocol.
EDITOR'S SUGGESTION
2016, 65 (16): 160501.
doi: 10.7498/aps.65.160501
Abstract +
Quantum walk (QW), the quantum mechanical counterpart of classical random walk, has recently been studied in various fields. The evolution of the discrete time quantum walk can be described as follows: the walker changes its spin state by the coin operator C, then takes one step left or right according to its spin state. For homogeneous quantum walk, the coin operator is independent of time and the standard deviation of the position grows linearly in time. It is quadratically faster than that in the classical random walk. In this work, we numerically study the dynamical behaviors of spreading in a one-dimensional discrete time quasiperiodic quantum walk (DTQQW). The DTQQW is that the coin operator is dependent on time and takes two different coins C() and C() arranged in generalized Fibonacci (GF) sequences. The GF sequences are constructed from A by the recursion relation: AAmBn, BA, for m, n are positive integers. They can be classified into two classes according to the wandering exponent . For 0, they belong to the first class, and for 0, they belong to the second class. For one dimensional system, the behaviors of two classes of GF systems are different either for the electronic spectrum of an electron in quasiperiodic potentials or for the quantum phase transitions of the quasiperiodic spin chains. In this paper, we discuss the cases of two different C operators (C();C()) arranged in GF sequences and find that the spreading behaviors are superdiffusion (the standard deviation of the position ~t; 0:5 1) for the two classes of GF DTQQW. For the second class of GF DTQQW, the exponent values are larger than those of the first class of GF DTQQW in the case of two identical C operators. By exploring the probability distribution in the real space, we find that for the first class of GF DTQQW, the probability distributions are almost the same for different initial states and are similar to the classical Gaussian distribution. For the probability distributions of the second class of GF DTQQW, there are two peaks at the two edges and the height of the two peaks can be different for different initial states. They are similar to the ballistic distribution of the homogeneous quantum walk. Therefore, we conclude that for the first class of GF DTQQW, the spreading behaviors are close to those of the classical random walk ( = 0:5) while for the second class of GF DTQQW, they are close to those of the homogeneous quantum walk ( = 1). This result is quite different from the characteristics of the quantum phase transitions in two classes of GF quasiperiodic quantum spin chains.
2016, 65 (16): 160502.
doi: 10.7498/aps.65.160502
Abstract +
As a basic problem, anomalous diffusions in various fields of physics and related science have been studied for several decades. One of the topic problems of anomalous diffusion is Lévy flight, which is employed as the statistical model to solve the problems in various fields. Therefore, studying the dynamical mechanism of Lévy flight, especially in the existence of external potential, is of importance for relative theoretical and experimental research. In this paper, within the framework of dynamical continuous time random walk method, the Lévy flight diffusive behaviors and dynamical mechanisms driven by nonlinear friction are studied in the force-free potential and periodic potential. The nonlinear friction instead of Stokes friction is considered in each step of Lévy random walker through the dynamical continuous time random walk method. In the force-free potential, the nonlinear friction term can be considered to be inharmonic potential in the velocity space which can restrain the velocity of random walker, so the anomalous superdiffusion of Lévy flight turns into a behavior in the normal case because of the strong dissipative effect of the nonlinear friction. Due to the introduction of the nonlinear friction, the velocity steady probability density distribution behaves as transitions between bimodal shape and unimodal shape, which is detrmined by the Lévy index μ and the friction indexes γ0 and γ2. The bimodality is most pronounced at μ =1, with μ increasing the bimodality becomes weaker, and vanishes at μ =2 which is the Gaussian case. Besides, there is a critical value γ0c=0.793701, which also determines the bimodal behaviors. For γ0=0 the bimodality is most pronounced, as γ0 increases it smooths out and turns into a unimodal one for γ0 > γ0c. In the existence of periodic potential, the Lévy random walker can be captured by the periodical potential due to the introduction of nonlinear friction, which behaves as the mean square displacement x2(t)> of the random walker and can reach a steady state quite quickly after a short lag time. However, the restraint is not equivalent to truncation procedures. Since the velocity of random walker obeys Lévy distribution, there is still extremely large jump length for random walker with extremely small probability. When the extremely large jump length is long enough and the barrier height U0 is not comparably high, the random walker can cross the barrier height of the periodic potential and jump out of the periodic potential, which behaves as the mean square displacement x2(t)> and a leap from a steady state to another one appears. However, the restraint on the random walker from the nonlinear friction always exists, so the random walker is captured again by the periodic potential, which means that the mean square displacement comes into a steady state again.
2016, 65 (16): 160701.
doi: 10.7498/aps.65.160701
Abstract +
Applications in industrial and scientific areas need high-quality multispectral images. However, because of various disadvantages, e.g., adverse environment of sensing, limited spectral bands, etc., multispectral images suffer low contrasts, low signal-noise-ratios, etc. As inputs of applications such as tracking, recognition and so on, multispectral images with low quality may cause those applications to fail. In this paper, aiming to meet practical requirements, we propose an algorithm of efficiently improving miultispectral images. The framework of the proposed method is as follows. According to Retinex theory, a multispectral image can be modeled as multiplicative combination of an illumination component and a reflection component. Typically, an illumination component is of low frequency and determines the dynamic range of pixel intensity, while a reflection component is of high frequency and determines the property of an image. Once we can successfully enhance both an illumination component and a reflection component, which is described later, respectively, we achieve the enhancement of a multispectral image by multiplying two enhanced components. First, we estimate the illumination component based on the principal structures extracted from the multispectral image on a multiple scale, i.e. on a low-level scale, middle-level scale and high-level scale, respectively. The mean value of the three principal structures is used as the estimation of an illumination component. Then the global structure contained in the illumination component, can be obtained by analyzing the corresponding information about its histogram, and is involved to enhance the global contrast and the edge details of the principal structure. Second, with the previously computed illumination component, we can easily derive the reflection component from the multispectral image in pixel-wise division operation. There are adequate image details as well as noise in the reflection component. We suppress the noise and keep the image details by using a non local mean filter. And then we enhance the image details by means of local variances. Finally, multiplying the enhanced illumination component by the filtered reflection component, we enhance the multispectral image. In order to verify the efficiency of our algorithm, experiments are conducted over multispectral image sets including X-ray images, ultraviolet images, well illuminated visible light images, poorly illuminated visible light image, and infrared images. The experimental results show that the proposed algorithm can efficiently remove halo artifacts, well suppress noise and obviously improve local details as well as global contrast. Compared with the state-of-the-art algorithms, the proposed method significantly enhances multispectral images both in vision and in objective analysis by means of information entropy and average gradient.
ATOMIC AND MOLECULAR PHYSICS
2016, 65 (16): 163101.
doi: 10.7498/aps.65.163101
Abstract +
Accurate spectroscopic constants and rovibrational level information are of great importance in molecular physics and astrophysical field. In this work, a new computational scheme is presented to further improve the accuracy of spectroscopic constants, and the two lowest electronic states of sulfur monoxide molecule are investigated as a typical case study. High-level ab initio calculations are carried out to compute the potential energy curves (PECs) of the lowest bound states, the ground triplet states X3Σg-, and the first excited singlet states a1Δg of SO molecule. The explicitly correlated multi-reference configuration interaction method (MRCI-F12) and cc-pCVQZ-F12 basis set are adopted in the electronic structure computations. The Davidson correction is taken into account to eliminate the size-consistency error. The core-valence (CV) electron correlations of the n=2 shell of S atom and n=1 shell of O atom are estimated by the MRCI-F12 method, whereas only the 1 s orbital of S atom is excluded in the CV calculations. Moreover, we introduce the scalar relativistic effect into our study by utilizing the second-order Douglas-Kroll and Hess (DKH) one-electron integrals by the MRCI method through combining with the uncontracted cc-pCVQZ-F12 basis set. On the basis of the PECs of the SO dimer, the spectroscopic constants (Te, Re, ωe, ωeχe, Be, αe and De) of the two electronic states are determined by solving the one-dimensional nuclear rovibrational Schrödinger equations. Our spectroscopic constants are found to be in excellent agreement with previous experimental and theoretical values. Furthermore, the detailed information about the vibrational energy levels and rotation spectroscopic constants (Bν, Dν) of the two states is also presented with a deviation of 0.1% order of magnitude from the available experimental results. Our present computational work is valuable for future experimental studies on the rovibrational energy levels for the SO molecule and also indicates that the MRCI-F12 approach is cheap and accurate and expected to have wide applications in the PECs of other small molecular systems.
2016, 65 (16): 163102.
doi: 10.7498/aps.65.163102
Abstract +
In this paper, the QCISD(T), CCSD(T), B3PW91 and B3LYP methods and the basis sets of aug-cc-pVTZ, 6-311G are used to calculate the structure of the ground state of free radical BeH molecule. The equilibrium distance and the energy of the molecule are optimized. The calculated results are compared with the experimental data, and the B3LYP method with the basis sets 6-311G is found to be able to provide the results that are the closest to the experimental values. So, in this paper the density function B3LYP method and the basis sets 6-311G are chosen and used to optimize the geometric structures of the ground state of free radical molecule of BeH in electric fields ranging from -0:02 to 0:02 a.u. The effects of external electric field on bond distance, system energy, charge distribution, energy levels, dipole moment, HOMO-LUMO gap, and infrared spectrum are studied．The results show that the molecular bond distance, the total atomic charge, the dipole moment, and the IR intensity decrease gradually with the increase of the external electric field along the molecular axis H→Be. At the same time, the total energy, the HOMO-LUMO gap, and the frequencies increase. The total energy increases sharply while the reverse electric field Be→H increases.
ELECTROMAGNETISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
2016, 65 (16): 164101.
doi: 10.7498/aps.65.164101
Abstract +
The unique electromagnetic properties of left-handed materials have received much attention due to their applications in microwave devices, radar antenna and electromagnetic stealth and so on. However, the double side etching of dielectric substrate for most traditional left-handed materials will lead to the complexity of manufacture, narrow left-handed bandwidth and higher loss. In order to overcome the shortages of traditional left-handed materials, a new broadband and low-loss left-handed material with multi-opening cross shape structure is presented in this paper. The structure is fabricated on a single side of a dielectric substrate by integrating an electric resonator with a magnetic resonator to form a left-handed unit. Through theoretical analysis, software simulation and effective electromagnetic parameters extraction, the results show that the structure has a double negative characteristics (ε μ <0) in the 12.7-21.1 GHz range, which is basically in the Ku band. The absolute bandwidth of left-handed character is up to 8.4 GHz and the loss of unit cell is less than 0.3 dB. The structure realizes a wider left-handed bandwidth with smaller cell size and lower loss than the conventional left-handed materials, and it provides an important reference for the application and design of the broadband and low-loss microwave left-handed materials.
EDITOR'S SUGGESTION
2016, 65 (16): 164201.
doi: 10.7498/aps.65.164201
Abstract +
Artificial spin-orbit coupling in neutral cold atom have been experimentally implemented in alkali-metal atoms. Nowadays people begin to explore its possible applications. One of the most interesting applications is the atomic mirror, which is a key element in atom optics. And spin-orbit coupling provides the atomic beam with the possibility that the atomic spin can flip during its propagation, thus can be used to prepare the quantum-state-selective atomic mirror. In 2008, Juzeliūnas, et al. [Juzeliūnas G, et al. 2008 Phys. Rev. Lett. 100 200405] studied a spin-orbit-coupled matter wave packet of cold atom gas impinging on an infinite step potential created by the optical light field. Results showed that there is not only ordinary specular reflection, but also non-specular one. The reflected atoms split into two beams and double reflection takes place. Based on the previous study, here we consider a matter wave packet of spin-orbit-coupled cold atom gas impinging on a finite step potential created by the optical light field. Due to the effect of the spin-orbit coupling, in addition to the propagating state, the eigenstates of cold atoms include evanescent state and oscillating evanescent state. Under suitable conditions double reflection will take place. If there are just evanescent waves in the step potential, total internal reflection will take place. In other words, when there is propagating wave in the step potential, partial reflection will take place. By taking into account both the total internal reflection and partial reflection, we study not only the polarization rate but also the reflectivity each as a function of incident energy, incident angle and spin-orbit coupling strength. The properties different from those of previous studies are found. In the case of total internal reflection, we find that the polarization rate of the reflected atoms is sensitive to incident angle instead of the spin-orbit coupling strength and incident energy. While in the case of partial reflection, all these factors strongly affect the polarization rate and reflectivity. We carefully study these properties and find that, on one hand, high efficiency atomic mirror can be acquired in the case of total internal reflection, and on the other hand, we can acquire the different polarization rates by adjusting the incident angle, the spin-orbit coupling strength and incident energy in the case of partial reflection.
2016, 65 (16): 164202.
doi: 10.7498/aps.65.164202
Abstract +
Environmental forces can produce undesired vibrations in mechanical structures that can limit the precision of mechanical equipment and cause mechanical failure. Piezoelectric-shunt damping is an attractive technique for controlling the vibrating structures, which is reliable, economical and light-weight. Phononic crystal is an internal component whose elastic constant, density and sound velocity change periodically. When the elastic wave passes through a phononic crystal, special dispersion curve is formed due to the interaction of periodic arrangement materials. In order to study the electromagnetic oscillation band gap of the piezoelectric phononic crystal with LCR shunt network at torsional and flexural vibration, we propose a new phononic piezoelectric beam, which is composed of aluminum and epoxy resin. When the piezoelectric patch is strained, the electrical energy is dissipated as current flows through an external LCR shunting circuit. By combining the piezoelectric effect with the mechanical vibration of the smart material, the equivalent additional stress of piezoelectric patches is deduced. Moreover, coupling the energy band theory of phononic crystal with the effect of electromagnetic oscillation, we calculate the band gap characteristics of torsional and flexural vibration of intelligent material. Using the transfer matrix method and Bloch theorem for periodic boundary conditions, the band gap of the phononic beam can be calculated. With the increase of resistance, the amplitude attenuation of the band gap decreases. However, it can expand the frequency range. The inherent frequency of the electromagnetic oscillation is 1/[2π√L(C + CP)]. The sum of capacitance and inherent capacitance is the total capacitance of the shunting circuit. Therefore, the frequency of the electromagnetic oscillation decreases with the increases of the capacitance and inductance. The amplitude attenuation of the band gap increases with the increase of the inductance and decreases greatly with the rise of the capacitance. Three main differences between the LCR shunt networks and traditional circuits are found. First, the band gaps of the phononic piezoelectric smart material are composed of Bragg band gaps and local resonant band gaps. The former one is due to the mismatch between aluminum and epoxy resin, which makes the elastic waves have no corresponding vibration modes at certain frequencies. The latter one is from the effect of electromagnetic oscillation in LCR shunt networks, which consume the energy by resistor. Second, by tuning the resistance, capacitance and inductance, we can change the singularity position and stress magnitude of equivalent additional force curve. The amplitude attenuations of locally resonant band gaps and electromechanical coupling coefficient will be changed. Third, both locations and widths of the band gaps can be tuned by simply varying the value of negative capacitance of the shunting networks without needing to modify the configuration of the structure. Therefore, it provides a new idea for controlling the vibration and reducing the noise of the phononic piezoelectric smart material.
2016, 65 (16): 164203.
doi: 10.7498/aps.65.164203
Abstract +
High-power, high-efficiency 808 nm laser diode arrays for pumping solid-state lasers have been widely used in industrial, scientific, medical and biological applications. The tendency of development of 808 nm laser diode pumping with high power, high efficiency and long lifetime is well-known. Diode-pumped solid-state system with high-efficiency laser diode array has many advantages such as compact volume, lower weight and energy saving. Currently, commercial 808 nm diode laser arrays with lower power conversion efficiency of about 50%-55%, due to the optical absorption losses for GaAs-based epitaxial materials, have been reported.
In order to reduce series resistance and thermal resistance, heavily doped p-type waveguide and cladding layers are employed. However, the absorption loss on the free carriers in heavily doped p-type layers is dominant, leading to a lower power conversion efficiency. In order to achieve a high efficiency, the following requirements must be considered: improving the internal quantum efficiency by reducing the carrier leakage and increasing the electron injection efficiency; minimizing the voltage drop by optimizing the operating voltage; reducing the series resistance and thermal resistance of device; minimizing the internal loss including free-carrier absorption loss and scattering loss by designing optimized waveguide and cladding structure.
In this paper, optimizing the epitaxial structure and fabricating technologies are demonstrated to achieve the high efficiency and high power. The asymmetric broad waveguide epitaxial structure with lower absorption loss in p-type waveguide and cladding layer is designed in order to achieve the above goals. The high-efficiency epitaxial structure is optimized including the thickness, doping and composition for each layer structure. The strained quantum well diode laser with lower transparency current and higher differential is of benefit to achieving the high power. A novel asymmetric broad waveguide structure is designed by optimizing the waveguide thickness and component of p-waveguide so as to reduce carrier absorption loss, the optical absorption loss in this epitaxial structure is achieved to be as low as 0.63 cm-1.
The wafer is grown by metalorganic chemical vapor deposition on an n-GaAs substrate. The optimized growth conditions and substrates orientation are extensively studied to improve the crystal quality and reduce the internal loss and defects. The wafer is processed using standard procedures. For the fabricated 1-cm laser diode array mounted on P-side down on copper microchannel cooled heatsink, the device shows an output power of 150 W under an operating current of 135 A with an emitting wavelength of 809 nm, an operating voltage of 1.76 V, a slope efficiency of as high as 1.25 W/A, and maximum power conversion efficiency of as high as 65.5%, which is the highest level of 808 nm diode laser array with an output power of 150 W.
2016, 65 (16): 164204.
doi: 10.7498/aps.65.164204
Abstract +
Due to the potential applications in optical storage, optical logic gates and all-optical signal shaping, the polarization switching (PS) and bistability (PB) of vertical-cavity surface-emitting lasers (VCSELs) under external disturbance have attracted much attention. In this work, based on the spin-flip model, the characteristics of PS and PB in a variable-polarization optical injection 1550 nm VCSEL (VPOI-1550 nm-VCSEL) are investigated numerically. In this scheme, the output from a master tunable laser passes through a rotating polarizer, an optical isolator (ISO), and a neutral density filter, then is injected into a 1550 nm-VCSEL. The polarization angle and power of the injection light are controlled by the RP and ISO, respectively. The results show that for a free-running 1550 nm-VCSEL, the parallel polarization-mode (Y polarization-mode) is lasing while the orthogonal polarization-mode (X polarization-mode) is suppressed in the 1550 nm-VCSEL. After introducing a variable-polarization optical injection, for a given frequency detuning Δν (defined as the frequency difference between the injection light and the X polarization mode), type I PS occurs during continuously increasing the injection power and type II PS occurs in the inverse process if the polarization angle θp of the injection light (defined as the angle difference between the polarization direction of injection light and Y polarization mode of the 1550 nm-VCSEL) is large enough. Moreover, the injection power required for generating type I PS is different from that for generating type II PS, namely PB is observed. When Δν is fixed, with the increase of θp, the injection power for the occurrences of the two types PS and the width of PB decrease. For a larger value of |Δν|, the injection power for the occurrence of type I PS is higher meanwhile the width of the PB is larger than that for a relatively small value of |Δν|. On the other hand, for a given injection power, type I PS, type II PS, and corresponding PB can also be observed in the 1550 nm-VCSEL through continuously increasing and reducing θp within the range from 0° to 90° under an appropriate Δν. For a relatively small |Δν|, the value of θp required for the occurrence of type I is similar to that for type II PS, which results in the very narrow width of the PB. Contrastively, for a relatively large |Δν|, the values of θp required for the occurrences of the two types PS and the width of PB severely fluctuate with the variation of Δν. Therefore, for the fixed parameters of the 1550 nm-VCSEL, through adjusting the power and polarization angle of the injection light, the performances of the PS and PB can be optimized. It is expected that this work can provide an effective guidance for optimizing the VCSEL-based bistable devices.
2016, 65 (16): 164301.
doi: 10.7498/aps.65.164301
Abstract +
The physical mechanism and signal characteristics of acoustic scattering are the vital basis for target recognition. But underwater target acoustic scattering components are aliasing in time-frequency (TF) domain, for which the target elastic acoustic scattering characteristics are difficult to detect. Additionally, the existing blind source separation methods are effective only on condition that the number of array elements is equal to or greater than the number of the source signals. To address these problems, a novel TF domain blind source extraction method of separating target acoustic scattering components is proposed in this paper. The method only uses the TF energy characteristic differences among the target acoustic scattering components, and special limitations on target echo structures are unnecessary. Image morphology filter is used to remove the cross-term interference from time-frequency distribution (TFD) of the received array signals. Then, the single source which shows maximum energy concentration at the corresponding auto-term TF points is extracted through three operations: i) selecting the single source auto-term TF points from the auto-term ones; ii) constructing the spatial TFD matrix according to the selected single source auto-term TF points; iii) obtaining the single source by decomposing the eigenvalue of their spatial TFD matrix. Finally, the extracted single signal is excluded by the tightening process from the received array signals, and each single signal is separated successively by repeating the above steps. In addition, a signal processing model which can describe the physical characteristics of the target echoes is established based on the separated signal components. Simulations illustrate that the image morphological filter can remove the cross-term interference and improve the TF resolution of the Wigner-Ville distribution. Anechoic pool experimental results show that the TF domain blind source extraction algorithm can well separate each target acoustic scattering component, it can also achieve a higher output signal-to-noise ratio. Furthermore, the separated elastic acoustic scattering components are in good agreement with the results computed by the surface wave generating theory, so the method can provide the robust and reliable feature for underwater target recognition.
2016, 65 (16): 164302.
doi: 10.7498/aps.65.164302
Abstract +
In the direct-arrival zone in deep water, the sound ray arrival angle is one of the most important properties of the sound field. However, it is complicated to estimate the arrival angle only by using the information about the sound pressure. Vector sensors have significant advantages in direction-of-arrival estimation, and the acoustic energy flux detection is one of the most important estimation methods. In this paper, the properties of complex acoustic intensity in the direct-arrival zone in deep water are analyzed, and the arrival angles of sound rays are estimated with the complex acoustic intensity extracted from the experimental data. Firstly, the expressions of horizontal particle velocity, vertical particle velocity and complex sound intensity are provided based on the ray theory. It is shown that the amplitudes of the horizontal and vertical particle velocities and the components of the complex sound intensity are closely related to the sound ray arrival angle. The larger the sound ray arrival angle, the greater the vertical particle velocity and the vertical component of the complex sound intensity are, but the weaker the horizontal particle velocity and the horizontal component of the complex sound intensity are. Secondly, for the direct-arrival zone of the sound field generated by a shallow source in deep water, the properties of the complex sound intensity with deep receiver are analyzed based on the sound ray arrival structure. The theoretical and simulation results show that the arrival angles of the sound rays can be estimated with the complex sound intensities of pulses received by a deep receiver. The mean arrival angles of the direct ray and the surface-reflected ray can be estimated with the complex sound intensities of the pulses of the direct-arrival wave and the surface-reflected wave. The mean arrival angles of the bottom-reflected ray and the surface-reflected-bottom-reflected ray can be estimated with the complex sound intensities of the pulses of the bottom-reflected wave and the surface-reflected-bottom-reflected wave. The angle obtained with the complex sound intensity of the total field comprised of all sound rays is approximately equal to the mean arrival angles of the direct ray and the surfaced-reflected ray. Thirdly, the validity of the arrival angle estimation with the complex sound intensity is verified by the experimental data. During a deep water experiment conducted in 2014, a vector sensor was placed at a depth of 3146 m to receive the experimental signals. Within the range of 17 km, the vector sensor received the direct ray from the sound source towed at about 140 m. By using the pulses of the direct-arrival wave and the surface-reflected wave received by the vector sensor, the mean arrival angles of the direct rays and the surface-reflected rays are estimated. It is shown that the estimated arrival angles are consistent with the theoretical results.
2016, 65 (16): 164303.
doi: 10.7498/aps.65.164303
Abstract +
The purpose of this paper is to optimize the performance of a quantum thermoacoustic micro-cycle. Thermoacoustic devices, such as thermoacoustic engines, thermoacoustic refrigerators, and thermoacoustic heat pumps are a new class of mechanical equipments without moving part and pollution. The thermoacoustic technology associated with these devices will hasten significant revolution in power engineering and mechanical devices. The work substance of a thermoacoustic device is composed of a number of parcels of fluid. Each parcel consists of a lot of molecules or atoms. The thermodynamic cycle is realized by the heat exchange between the parcel and the solid wall of the channel. The thermodynamic cycle of the parcel of fluid is called the thermoacoustic micro-cycle. The thermodynamic behavior of a thermoacoustic system may be described by studying that of the thermoacoustic micro-cycle. It is necessary to study the model and performance of the thermoacoustic micro-cycle in order to promote the development of thermoacoustic technology. The quantum mechanics, which was one of the great achievements in the 20 th century, can reveal the secret of the micro particle world. Quantum thermodynamics is an inter-discipline that combines quantum dynamics and thermodynamics. It provides a useful tool for analyzing the quantum cycles and devices. In this paper, the method of the quantum thermodynamics is employed to analyze the performance of a quantum thermoacoustic micro-cycle. The thermoacoustic parcel is modeled as a gas composed of many micro particles, which abide by the quantum mechanics. These particles are referred to as thermal phonons. Thermal phonons are bosons. The evolution of each thermal phonon must satisfy the Schrö dinger equation in quantum mechanics. The quantum mechanics model of the thermoacoustic micro-cycle, which is called the quantum thermoacoustic micro-cycle, is established in this paper. The quantum thermoacoustic micro-cycle consists of two constant force processes and two quantum adiabatic processes. The quantum thermodynamical behavior and evolution of the thermal phonon in a one-dimensional harmonic trap are investigated based on the Schrö dinger equation and the two-eigenstates system. The energy eigenvalue of the thermal phonon are employed. The analytical expressions of the optimal dimensionless power output P*, the thermal efficiency η and the critical temperature gradient (dT/dx)ex for the quantum thermoacoustic micro-cycle are derived by considering Gibbs probability distribution. The optimal relationship between dimensionless power output P* and thermal efficiency η is obtained. The analysis shows that both the power output and the thermal efficiency decrease with the increase of width of the harmonic trap L1. One can find that the characteristic curve of P*-η is parabolic-shaped. There exist a maximum dimensionless power output P* and the corresponding frequency η. It is noteworthy that there is a critical temperature gradient for the quantum thermoacoustic micro-cycle. The critical temperature gradient is important because it is the boundary between the heat engine and the heat pump. The optimal design and these operating conditions for the quantum thermoacoustic micro-cycle are determined in this paper. The results provide a new method for studying the thermoacoustics by means of the quantum thermodynamics, thereby broadening the application range of the quantum thermodynamic.
2016, 65 (16): 164304.
doi: 10.7498/aps.65.164304
Abstract +
Environmental uncertainty is one of the limiting factors in the matched-field localization. Within a Bayesian framework, environmental focalization has been widely used to solve the augmented localization problem, in which the environmental parameters, source ranges and depths are considered to be the unknown variables. However, the position of the moving source varies with time, which limits the observation interval and the number of acoustic signals. Therefore, it has to estimate lots of unknown parameters with the limited observation information. When the source moves fast or the environment has great uncertainty, the environmental focalization gets worse. Taking the parameter estimation of Kalman filter in the non-stationary process as a reference, the acoustic signals from a series of observations are treated in a simultaneous inversion. This provides the most informative solution, since data from multiple source locations are brought to bear simultaneously on the environmental unknowns, which in turn constrain the source locations better. In this article, the time-unvarying parameters are introduced to describe the source position. The source positions are inverted indirectly by the time-unvarying parameters, which reduces the estimated parameter dimension effectively. At the same time, the current estimated results are treated as the priori information of the next inversion, which establishes the new prior distribution and cost function. It could compensate for some individual abnormal data effectively and realize continuous localization of the moving source. The noise signals radiated from a surface ship target are processed and analyzed. The Bayesian tracking algorithm greatly increases the observation interval and the number of acoustic signals, and enhances the localization accuracy in an uncertain water environment. Tracking results of the ship noise indicate that simultaneous inversion of multiple acoustic observations with constant velocity track model and the Thikhonov regularization provides a better solution than sequential independent inversions. It is indicated that the Bayesian tracking method learns the uncertain environment as more observations become available. It is discovered that the maximum a posteriori solution and the two-dimensional solution have similar results according to the global positioning system value. The reason is that the source locations are treated implicitly by the source speed, which is similar to the marginal probability distribution by reducing the multidimensional posterior probability density to the representative two-dimensional probability distributions.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
EDITOR'S SUGGESTION
2016, 65 (16): 165201.
doi: 10.7498/aps.65.165201
Abstract +
In inertial confined fusion (ICF) experiments, the temporal evolution of X-ray spectrum can provide important diagnostic information such as electron temperature and density on laser-plasma interaction. Accurate diagnostic requires a wide range of X-ray spectrum from several hundred eV to kilo eV to be measured with high temporal resolution. For traditional single grating spectrometer coupled with streak cameras, the limited recording length of streak cameras severely restricts measured X-ray spectral range in one laser shot. Here we design a shifted dual transmission grating (SDTG) spectrometer for laser-produced plasma X-ray diagnostics in ICF experiments which can provide wide-range X-ray spectrum measurement from 100 eV to 5 keV with high temporal and spectral resolution. This SDTG spectrometer comprises two X-ray gratings: one with high line density and the other with low line density. The high line density grating is used to measure X-ray spectrum from 1000 eV to 5000 eV and the low line density grating measures X-ray spectrum from 100 eV to 1000 eV respectively. These two kinds of X-ray gratings are arranged in a plane with their centers shifted by a certain distance. A shifted double slit component is designed according to the spatial positions of the two gratings and set in front of the photocathode in the streak camera to ensure that two sets of X-ray spectra by two shifted gratings are projected on the photocathode without overlapping. This novel SDTG-based X-ray spectrometer can take the most of recording panel space, offering a path to realize a high resolution and broad spectral ranges in diagnosing soft X-rays. In this paper, the design method and the technical data of the SDTG-based X-ray spectrometer are given. The SDTG-based X-ray spectrometer is integrated, debugged and used to measure X-ray pulse at SG-III prototype facility located in Laser Fusion Research Center of Chinese Academy of Engineering Physics. The time integral results are captured by the SDTG spectrometer in the ICF fluid RT experiments and time-resolved spectra are recorded in indirect drive implosion experiment. Experimental results show the SDTG-based X-ray spectrometer can capture X-ray spectrum ranging from 0.1 keV to 5 keV, with a spectral resolution of 0.04 nm and a temporal resolution of better than 30 ps. By fully utilizing limited recording length, the SDTG-based X-ray spectrometer can realize a wide range temporal X-ray spectrum measurement with enough spectral resolution and temporal resolution. This SDTG spectrometer is a good temporal X-ray diagnostic tool for ICF experiments and other high energy density physics experiments.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
2016, 65 (16): 166101.
doi: 10.7498/aps.65.166101
Abstract +
The initial growing dendrite is influenced significantly by the complicated solidification conditions in continuously oscillating mold. The uneven growth of dendrite causes some defects seen commonly such as internal crack, subsurface porosity, subsurface inclusion and other defects of continuous casting billet. The induced initial defects in mold can be expanded and propagated in the following process such as secondary cooling, straightening, rolling and other subsequent handling procedure and then evolve into serious defects that can restrict the development and the quality refinement of final steel products. The mechanical stress caused by mold oscillation and the melt flowing is a crucial factor that leads to the uneven microstructure growth of initial solidifying shell in continuous casting mold.
In this work, we simulate the growth and the morphology evolution of primary dendrites in mold area by using the cellular automaton (CA) method in combination with the actual conditions for continuously cast low carbon billet (Fe-0.6 wt.%C). Further, the mechanical state of initial dendrite is analyzed by regarding primary dendrite as a cantilever beam and its mechanical stress is calculated by combining thermo-physical properties and flow rate of steel based on the principle of materials mechanics to shed light on the formation of initial defects formation in mold area of continuous casting process. The results show that the solute concentration of initial dendrite tip gradually increases with undercooling from 2 to 10 K, and the maximum concentration rises by 0.07% when the increment of undercooling is 2 K. The length of dendrite arm increases significantly with undercooling from 2 to 6 K. However, the length of dendrite arm remains steady in a stable growth rate of 0.08 mms-1 when the undercooling is enhanced from 6 to 10 K. The increase of undercooling reduces the bending stress at dendrite root when the flow rate of molten steel is improved from 0.13 to 0.33 ms-1, while the mechanical stress continuously increases with the growth of primary dendrite at a constant undercooling. The bending stress of dendrite root has a high possibility to exceed its critical fracture strength under the condition of undercooling below 6 K or dendrite grow up more than 1 s. The primary dendrite is likely to be fractured and form initial defects of billet.
2016, 65 (16): 166201.
doi: 10.7498/aps.65.166201
Abstract +
The Hugoniot elastic limit (HEL) of ceramics is explained as the limit of elastic response and the onset of failure under dynamic uniaxial strain loading, which is an important parameter for understanding the dynamic properties of ceramic materials. Previous experimental impact studies showed an interesting phenomenon that the HEL decreases with the increase of sample thickness, which is termed the elastic precursor decay. This phenomenon has not been explained by a suitable mechanism to date.
Recently it has become apparent that mechanical response of polycrystalline ceramics is governed by mechanism operating at a grain level. So the objective of the present work is to develop a mechanism that can illustrate this phenomenon on a mesoscale. In this paper, the plate impact experiments of alumina with varying thickness values are conducted by using one-stage light gas gun. The histories of the rear free surface velocity of the samples are recorded by a Velocity Interferometer System for Any Reflector (VISAR). The HELs of alumina samples with different thickness values are obtained from turning point of elastic phase to inelastic phase in the temporal curves of free surface velocity. It is confirmed that the HEL of alumina decreases with increasing the sample thickness obviously, namely elastic precursor decay phenomenon. It is considered that this phenomenon is related to the failure mechanism of shocked alumina at a grain level. Thus, the mesoscopic model of alumina, including alumina grain phase and glass binder phase, is developed according to the microstructure properties of tested sample observed experimentally. Mesoscale simulations are presented to study the mesoscale failure properties of alumina at various impact velocities. The results show that inelastic responses, such as microcracking, grain plasticity, are observed in microstructure of alumina even when the peak-shock stress is less than the magnitude of HEL. As is well known, the evolution process of cracking or plasticity is the energy dissipation process essentially, which will reduce the amplitude of elastic wave. Furthermore, the properties of elastic precursor wave propagation in microstructure of alumina are also captured in the present simulations. Since the acoustic impedance of glass binder phase is much lower than that of alumina grain phase, complex reflection and transmission of elastic wave will occur at grain boundaries. Due to a large number of randomly oriented crystals, the wave front, well defined at the continuum, is dispersed to lateral or reverse directions at these length-scales, which can also decay the elastic precursor amplitude in the initial propagating direction. Therefore, the results suggest that energy dissipation caused by the failure process should occur below HEL and energy dispersion due to reflection and transmission of elastic wave at grain boundaries should play a dominant role in the phenomenon of elastic precursor decay.
2016, 65 (16): 166801.
doi: 10.7498/aps.65.166801
Abstract +
In the injecting process of liquid jet, the disturbance wave on jet interface will grow continually, leading to the spatial development and atomization of liquid jet. Studying the spatial evolution of liquid jet will help to deepen the understanding of the mechanism of jet breakup and atomization. In this paper, based on the linear and nonlinear stability theories, the first-order and second-order dispersion equations describing the stability of liquid jet with cavitation bubbles in a coaxial swirling compressible airstream are built, respectively, and the dispersion equation and its solving method are verified by the data in the literature. On this basis, the developments of first-order and second-order disturbance are analyzed, and the spatial evolutions of liquid jet are compared under linear and nonlinear stability theories. The results show that the wavelength and amplitude of the second-order disturbance are much smaller than those of the first-order disturbance. The disturbance development on jet surface is mainly dominated by the development of the first-order disturbance along the axial direction. With the increasing of axial distance, the second-order disturbance gradually begins to play a role in the developing of disturbance. The role of second-order disturbance is mainly reflected in three aspects, i. e., obviously increasing the disturbance amplitude at wave crest, reducing the disturbance amplitude at wave trough (sometimes ups and downs occur), and changing the waveform to a certain degree. The dominant disturbance mode on jet surface will not change under two kinds of theories. By using the nonlinear stability theory, satellite droplets which are found on jet surface in experiments can be reflected, and the shape of main droplet changes obviously from the ellipsoid to sphere. Also, the change of dimensionless radius of liquid jet is greater by nonlinear stability theory than by linear stability theory, which indicates that the oscillation extent of jet surface increases due to considering the second-order disturbance. Therefore, compared with the linear stability theory, the nonlinear stability theory has the advantage that it considers the effects of high-order disturbance on the spatial evolution of liquid jet in addition to the first-order disturbance on jet surface. The nonlinear stability theory can predict the spatial development of liquid jet in more detail than the linear stability theory.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
2016, 65 (16): 167201.
doi: 10.7498/aps.65.167201
Abstract +
We develop a new method to determine the edge and screw dislocation density in wurtzite n-GaN film. The method is to fit the van der Pauw variable temperature Hall-effect measurements with a analytic expression of low-field electron mobility in n-GaN. Our calculations take the comprehensive effect between the dislocation line and the shallow-donor defects as the main cause to depress the carrier mobility. Because of the crystal distortion near the dislocation line, the energy is so high that shallow-donor defects in the GaN crystal can be captured near the dislocation line. In other words, the shallow-donor defects distribute in lines along the dislocation line, but the shallow-donor defects along the screw and edge dislocation line have different energy levels. The shallow-donor defects take energy from lattice and the carrier, which is in relaxation process, then deliver the energy through ionizing. So, it is found that the following assumptions need to be made in order to obtain the model function for the mobility over a wide temperature range: i) there are 6 shallow-donor defect lines around one dislocation line; ii) two donor energy levels belonging to the screw and edge dislocation respectively must be taken into account; iii) the exchange energy between the carrier and the shallow-donor defect is ħωLO, the energy value of polar optical phonon. Under these assumptions, experiments indicate that our calculation function can fit the experimental curve best. The values of dislocation density from our model and others determined by x-ray diffraction or by chemical etching method are in good agreement, and the values of donor energy levels from our model and Rode iterative method to solve the Boltzmann equation are also in good accordance with each other. This method is applicable for the wurtzite n-GaN films grown by various preparation technologies under any condition, which is for the sample with the peak-mobility temperature about or under 200 K, not for the sample with the peak-mobility temperature about or above 300 K, which room-temperature mobility usually is about or less than 100 cm2/(V·s).
2016, 65 (16): 167301.
doi: 10.7498/aps.65.167301
Abstract +
Electron dephasing process is important and interesting in disordered conductors. In general three-dimensional (3D) disordered metals, the electron-electron (e-e) scattering is negligibly weak compared with the electron-phonon (e-ph) scattering. Thus, the theoretical prediction concerning the e-e scattering rate 1/τee as a function of temperature T in 3D disordered conductor has not been fully tested so far, though it was proposed four decades ago. In the frame of free-electron-like model, the e-ph relaxation rate 1/τep is proportional to carrier concentration n, while the small-and large-energy-transfer e-e scattering rate obey the laws 1/τeeS ∝ n-4/3 and 1/τeeL ∝ n-2/3, respectively. In other words, e-e scattering may dominate the dephasing processes in 3D disordered metals with sufficient low carrier concentrations. In the present work, we systematically investigate the electronic transport properties of amorphous indium gallium zinc oxide (a-IGZO) prepared by the radio frequency sputtering method. The carrier concentrations of the highly degenerate IGZO films are all ～ 5×1019 cm-3, which are 3-4 orders of magnitude lower than those of typical metals. Our thick films (～ 800 nm) are 3D systems with respect to weak-localization (WL) effect and e-e scattering. X-ray diffraction patterns of the films indicate that our films are all amorphous. For each film, the resistivity increases with the increase of the temperature in the high temperature region (T ≥ 200 K) and the carrier concentration is almost invariable in the whole measured temperature range. This indicates that the films possess metal-like transport properties. By comparing the low-field magnetoconductivity versus magnetic field data σ (B) with that from the 3D WL theory, we extract the electron dephasing rate 1/τφ at different temperatures in the low temperature region. It is found that 1/τφ varies linearly with T3/2 for each film. The T3/2 behavior of 1/τφ can be quantitatively described by the 3D small-energy-transfer e-e scattering theory. The e-ph scattering rate 1/τep and large-energy-transfer e-e scattering rate 1/τeeL are negligibly weak in this low-carrier-concentration conductor. Thus, we can observe the T3/2 behavior of 1/τφ.
2016, 65 (16): 167501.
doi: 10.7498/aps.65.167501
Abstract +
Magnetoelectric (ME) composite as one kind of ME material that can yield a strong coupling effect between magnetic and electric fields at room temperature, has drawn widespread attention for decades due to its rich physics contents and significant technological prospect. Except for traditional magnetostrictive/piezoelectric based ME composites, other ME composites have been reported, among which the magnet/piezo-cantilever composites show super strong ME coupling effect. The magnet/piezo-cantilever composite is generally composed of a piezoelectric cantilever and magnets attached at the free end of the cantilever, which realizes ME coupling by force moment-mediated magnetic torque effect and piezoelectric effect. Recently, various configurations of the magnet/piezo-cantilever composites for obtaining higher ME coupling coefficients have been proposed and demonstrated experimentally. However, few theoretical researches of these magnet/piezo-cantilever composites of different configurations have been carried out, which is of great importance for optimizing the design of ME coupling property of the magnet/piezo-cantilever composites. Here in this paper, a theoretical expression for the low-frequency ME coupling coefficient in the magnet/piezo-cantilever composite is deduced based on piezoelectric constitutive equations by using the theory of elastic mechanics. The typical magnet/bimorph composite is chosen as the theoretical model. Based on the deduced theoretical expression, the dependences of the lowfrequency ME coupling coefficients in the magnet/bimorph composite on material and structural parameters are numerically calculated. The results show that there are optimal thickness values of the piezoelectric layers in the magnet/bimorph composite with different metal thickness values and material constituents for achieving maximal lowfrequency ME coupling coefficients. The thicker the metal layer in the magnet/bimorph composite, the less insensitive the low-frequency ME coupling coefficient to the thickness of the piezoelectric layer will be. And the low-frequency ME coupling coefficient of the magnet/bimorph composite decreases when a metal with higher elastic module is selected for bimorph. For the magnet/bimorph composite consisting of hard piezoelectric ceramics (PZT-4), the low-frequency ME coupling coefficient is higher than that of the composite consisting of the soft counterpart ones (PZT-5 H), which is due to the hard piezoelectric ceramics with higher piezoelectric voltage coefficient than the soft counterpart ones. What is more interesting is that when the piezoelectric material in the magnet/bimorph composite is changed into relaxor ferroelectric single crystals Pb(Zn1/3 Nb2/3)O3-PbTiO3 (PZN-PT), an extremely high low-frequency ME coupling coefficient can be obtained, which is 3.8 and 5 times those of the 13 composites with hard and soft piezoelectric ceramics, respectively. This research gives a theoretical guidance for optimal design and practical applications of the magnet/Bimorph composite.
2016, 65 (16): 167801.
doi: 10.7498/aps.65.167801
Abstract +
Pressure-dependent refractive index of semiconductor germanium (Ge) near the absorption edge has not been well studied theoretically and experimentally to date. In this paper, we present a pressure-dependent refractive index of Ge film near its absorption threshold (about 1550 nm), deduced from the reflectivity of high crystalline Ge film coated on a fiber end. The thin Ge layer is deposited on one end of an optical fiber by using an E-beam evaporation machine equipped with a substrate heater of a quartz halogen lamp. In order to obtain high crystalline film, the quartz halogen lamp heater provides a constant substrate temperature of 450 ℃ during film deposition. After the film forming, the sample is transferred into a muffle furnace with a nitrogen atmosphere and annealed at 600 ℃ for 20 h to guarantee the formation of higher crystalline film. The process of light propagating through the optical fiber and reflecting from the Ge thin-film involves multi-beam interference. An abnormal dispersion is observed in the refractive index spectra of the polycrystalline Ge near the absorption edge. A comparison shows that the refractive index spectrum of the amorphous Ge is normal dispersion. Unlike previously reported results that the pressure-dependent refractive index had a negative value, in our experiment it is observed to be a positive value near the absorption edge. To better understand this phenomenon, we use a critical point model including the pressure effect to successfully fit the experimental data. We obtain an abnormal dispersion range of 1505-1585 nm and a range of negative value of pressure-dependent refractive index of 1500-1580 nm from the critical point model. In this paper, we adopt the method of high crystalline Ge film coated on a fiber end. This method has the advantages of small volume, high precision and strong stability, which can be used to measure the optical properties of many thin film materials under the different conditions (temperature, electric field or magnetic field, etc.). In this work, we obtain the refractive index of the Ge film and its pressure dependence near the C-band of the optical fiber communication, and these results are conducive to optimized design of the Ge-based optical systems near the C-band of the optical fiber communication.
2016, 65 (16): 167802.
doi: 10.7498/aps.65.167802
Abstract +
Metal materials play an important role in many domains, which are significant to the national economy. However, different kinds of metal defects, such as cracks, contraction cavities, impurities, will be generated in the process of production and service. These defects will affect the metal service life and mechanical properties directly, and even cause serious economic loss. Therefore, it is vital to detect the metal defects.
Numerous nondestructive testing (NDT) methods have been proposed for detecting metal defects, such as ultrasonic (US) testing, eddy current testing, photoacoustic (PA) testing, magnetic particle testing, etc. However, each of them uses a single modal signal, which leads to a limited detection range. A nondestructive detecting method for metal material defects based on multimodal signals is proposed to expand the scope of detection and obtain more complete information. Specifically, optical signal, PA signal and US signal are combined together in this method, with the consideration of their complementarities.
Simulation and experiments are conducted to validate the effectiveness of the proposed method. Firstly, finite element simulation is employed to analyze the relationship between material parameters and the absorption of laser energy. Meanwhile, the influence of defect size on PA surface wave is simulated and analyzed. Then, a multimodal NDT platform is established to collect and process optical, PA and US signals of the metal defects. These three modal signals contain information about metal surface, shallow surface and internal defects respectively. Eventually, the information, including the location, appearance on the surface, depth, extension path in the material, is obtained.
As demonstrated in the results, the nondestructive detecting method based on multimodal signals can detect the metal defects accurately and comprehensively. This method improves the existing methods in terms of detection range and quantitative detection. Additionally, it provides a new way for the quantitative detection and comprehensive diagnosis of metal defects.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
2016, 65 (16): 168101.
doi: 10.7498/aps.65.168101
Abstract +
Detection of the terahertz (THz) electromagnetic spectrum(wavelengths range 0.03-3 mm) is a promising technique for a large variety of strategic applications, such as biomedical diagnostics and process, quality control, homeland security, and environmental monitoring, etc. Graphene has been recognized internationally to have dominant advantages in photodetectors operating due to its high carrier mobility, gapless spectrum, and frequency-independent absorption coefficient. Graphene photodetector operating in the THz region has been extensively studied with great interests.
A graphene microcavity photodetector with THz electromagnetic spectrum is demonstrated in this paper, and its responsivity and detectivity under THz electromagnetic spectrum are evaluated. In the designed device, we adopt a distributed bragger reflection (DBR) consisting of two semiconductor materials SiO2 and TiO2 to form an alternating cavity with high-finesse planar, sandwich the absorbing graphene layer between the cavitys top and bottom layers, and design the DBRs reflectivity by the optical transmission matrix method. The monolayer graphenes optical absorption mechanism of the THz radiation spectrum is studied by the conductivity matrix and Maxwells equations with the electromagnetic boundary conditions. Graphenes transfer matrix and absorption coefficient equation are further derived. It is found that at THz region, graphenes conductivity plays an important role in its absorptionand its absorption is 9-22 times enhanced compared with that at the visible region. An optical absorption model of microcavity-enhanced graphene photodetector at THz region is established. The photodetectors absorption rate and responsitivity are analyzed specifically. Theoretical analysis shows that absorption rate is symmetrical to the microcavitys center position and changes periodically, and the shift of the microcavity length influences the period numbers. The maximum rate of the photodetectors absorption reaches 0.965 at 0.12 THz, which increases 93% compared with its maximum absorption rate 0.5 with no cavity. The optimal structure parameters for the designed photodetector are as follows, the top and bottom mirrors reflectivity are 0.928 and 0.998 respectively, the microcavity length is 2.5 mm, the graphene is 0.035 mm away from the top mirror. Under the optimal structure, the photodetectors responsivity reaches 236.7 A/W, and its full width at half maximum reaches 0.035 THz. The designed graphene microcavity photodetector can exhibit high responsivity and detectivity in THz radiation spectrum.
2016, 65 (16): 168102.
doi: 10.7498/aps.65.168102
Abstract +
Explosive emission cathode (EEC) is a pivotal component in high power microwave source (HPMS), of which the ultimate properties are significantly dependent on the quality of electron beams generated from the cathode. Short lifetime and poor emission uniformity are the persistent drawbacks of conventional field EEC. Improvement of cathode material by changing its compositions and modifying surface micromorphology, is a feasible way to solve this problem. Graphite is one of the frequently used materials for EECs due to its long life-time and sturdy performance under high voltage and repetition frequency. Meanwhile silicon carbide (SiC) whiskers are distinguished by high aspect ratio (ratio of height to diameter) and low work function which is in favor of the fast onset of electron emission. In this work, the novel composites, composed of SiC whiskers, pitch and major graphite powders, are prepared by the conventional mingling and sintering. The cathodes are installed on TPG1000 system with a parameterized pulse of 970 kV, 9.2 kA, and 50 ns. By analyzing the changes of the rise edge of measured diode current and output microwave pulse duration, the effects of material composition and surface micromorphology on electron emission properties for the cathode are disclosed in detail. The results, based on the comparison of emission properties between graphite cathodes with and without SiC whiskers doped, reveal that SiC whiskers play an important role in accelerating the field emission of cathode, which is demonstrated by the eclipse of displacement current peak on the rise edge of measured current waveform after doping. Meanwhile, the duration of output microwave pulse is enhanced by about 11% after doping, which could be explained by the lower expansion speed of Si plasma. Moreover, the surface micro-protrusions of graphite cathode doped by SiC whiskers are constantly “polished” by heating effect and cathode plasma as the number of emission pulses increases to 11000. This is in quite good agreement with the appearance of the displacement current peak on the rise edge of measured current curves after 6000 and 11000 pulses treatment. These changes imply that the initial speed of field emission from cathode is slowed down gradually. The output microwave pulse starts early, which is benefited from the homogeneous surface micromorphology of the cathode due to “polishing” effect. The quantity of releasing absorbed gases, including water and vacuum pump oil vapor, decreases with increasing emission pulses. Then the pulse shortening phenomenon is restrained and the falling edge of output microwave pulse is extended. The duration of output microwave pulse is increased by about 5%, for graphite cathode doped by SiC whiskers after experiencing 11000 pulses. In conclusion, the reaction mechanism of SiC whiskers in the process of explosive electron emission (EEE) is considered as being due to accelerating the onset of felid emission and reducing the expansion speed of cathode plasma. Therefore, combination with SiC whiskers is an effective way to improve the electron emission properties of graphite EECs, especially in the output microwave pulse width and energy conversion efficiency of HPMS.
2016, 65 (16): 168501.
doi: 10.7498/aps.65.168501
Abstract +
In this paper, the damage process and mechanism of the typical high electron mobility transistor by injecting high power microwave signals are studied by simulation and experiment methods. By using the device simulator software Sentaurus-TCAD, a typical two-dimensional electro-thermal model of high electron mobility transistor is established with considering the high-field saturation mobility, Shockley-Read-Hall generation-recombination and avalanche breakdown. The simulation is carried out by injecting the 14.9 GHz, 20 V equivalent voltage signals into the gate electrode. Then, the distributions of the space charge density, electric field, current density and temperature with time are analyzed.
During the positive half cycle, a conduction channel appears beneath the gate electrode near the source side within device. It is found that the electric field is extremely strong and the current density is very large. Therefore, the temperature increases mainly occurs beneath the gate electrode near the source side. During the negative half cycle, because of the concentration of the large number of carriers induced by avalanche breakdown, the electric field is stronger than that in the positive half cycle. But the current density is lower than that in positive half cycle. Therefore, the increase of temperature is dominated by the electric field. With the effects of both strong electric field and high current density, the temperature of the transistor rises in the whole signal cycle. In addition, temperature in the positive half-cycle rises faster than that in the negative half-cycle.Furthermore, the peak temperature appears at the location beneath gate electrode near the source side because the electric field and current density are strongest in this area. When the temperature within the device is higher than 750 K, intrinsic breakdown occurs in GaAs material, so the heating process becomes quicker. With the temperature increases, the GaAs reaches its melting point, and the device fails permanently.
Furthermore, taking the original phase of 0 and for example, we discuss the influences of different original phases on damage process. It is shown that when original phase is zero, the temperature increase rate is faster, and the burn-out time is shorter.Failure analysis of high electron mobility transistor devices damaged by microwaves is carried out with scanning electron microscope, and the simulation results are well consistent with the experimental results. The conclusion may provide guidance for studying high power microwave defense of low noise amplifier and rugged design of high electron mobility transistor in fabrication technology.
2016, 65 (16): 168502.
doi: 10.7498/aps.65.168502
Abstract +
Negative bias temperature instability (NBTI) is a p-channel metal-oxide-semiconductor (PMOS) degradation mechanism, which becomes one of the important reliability concerns. The NBTI drastically influences device performance and circuit lifetime. On the other hand, the circuit performance is also affected by the fabrication-induced process variation when the transistor size shrinks to a nanometer-scale. In the presence of the fabrication-induced random variations, the NBTI aging process and its influence on PMOS device become a random process. In this paper, the joint effects of NBTI and process variations on PMOS device are investigated. Firstly, the influence of process variation on NBTI aging is analyzed based on the reaction-diffusion (R-D) mechanism. The NBTI-induced PMOS threshold voltage degradation depends not only on stress time but also on fabrication-determined process parameters, such as the initial threshold voltage and oxide thickness. Then the statistical model is proposed to model NBTI-induced aging under process variation, which captures the threshold voltage variation and oxide thickness variation as random vectors with normal distributions. For 100-times Monte-Carlo simulation based on 65 nm technology, the voltage error and oxide thickness error of the PMOS device are obtained. Applying these process errors to the statistical model, the results show that mean value of threshold voltages is increased along the negative direction with the stress time going on under the process variation and NBTI effect interaction. Meanwhile the standard deviation of threshold voltage is reduced, which represents that the matching between those PMOS devices becomes better. The proposed statistical model accuracy is verified by R-D model theoretical solutions. The maximum relative error of the mean value and of the standard deviations for the threshold voltages degradation of the PMOS device are only 0.058% and 0.91% respectively in 104 s. The distribution characteristic of PMOS NBTI effect is more serious to analog circuit, because analog circuit is more sensitive to device mismatch. For current steering digital-to-analog converter (DAC), PMOS device is always adopted as current source due to its good isolating properties. The PMOS current source requires good matching, and mismatch error could cause circuit failure. To realize aging simulation on DAC circuit in Spectre environment, the above statistical NBTI model is realized by Verilog-ASM language as the subcircuit module to PMOS device. Finally, this module is applied to the current steering DAC. Considering the NBTI effect under process variations, the simulation results show that the DAC gain error is increased with the stress time going on, while its linearity error is gradually reduced.
2016, 65 (16): 168701.
doi: 10.7498/aps.65.168701
Abstract +
Original image is directly processed by the existing image segmentation algorithms, which is easily affected by noise. A bi-dimensional ensemble empirical mode decomposition (BEEMD) method is introduced to improve the accuracy of MR image segmentation by distance regularized level set (DRLSE) method. The BEEMD method is the extension of one-dimensional noise assisted data analysis from ensemble empirical mode decomposition (EEMD). The key points of BEEMD are as follows. four-neighborhood optimization is used to find extermum; three-spline interpolation is used to obtain the envelope; amplitude standard of added white noise is restricted; a certain time of integration is used to avoid modality aliasing problem. The main steps of the proposed method are as follows. Firstly, the MR image is decomposed into a number of two-dimensional intrinsic mode functions (BIMF) by BEEMD method; different weighting coefficients are endued to BIMF for image reconstruction to enhance the segmentation target. Secondly, part of BIMF components are added into edge indicator function of DRLSE to recover the blurring boundary caused by Gauss smooth operation. Then DRLSE is used to segment the reconstructed MR image. High accuracy and robustness of proposed algorithm are obtained in both simulations and clinical MR images. However, compared with DRLSE, the proposed method is complex and time consuming because using BEEMD for preprocessing the segmentation image.
2016, 65 (16): 168901.
doi: 10.7498/aps.65.168901
Abstract +
Influential nodes in large-scale complex networks are very important for accelerating information propagation, understanding hierarchical community structure and controlling rumors spreading. Classic centralities such as degree, betweenness and closeness, can be used to measure the node influence. Other systemic metrics, such as k-shell and H-index, take network structure into account to identify influential nodes. However, these methods suffer some drawbacks. For example, betweenness is an effective index to identify influential nodes. However, computing betweenness is a high time complexity task and some nodes with high degree are not highly influential nodes. Presented in this paper is a simple and effective node influence measure index model based on a triangular structure between a node and its neighbor nodes (local triangle centrality (LTC)). The model considers not only the triangle structure between nodes, but also the degree of the surrounding neighbor nodes. However, in complex networks the numbers of triangles for a pair of nodes are extremely unbalanced, a sigmoid function is introduced to bound the number of triangles for each pair of nodes between 0 and 1. The LTC model is very flexible and can be used to measure the node influence on weighted complex networks. We detailedly compare the influential nodes produced by different approaches in Karata network. Results show that LTC can effectively identify the influential nodes. Comprehensive experiments are conducted based on six real complex networks with different network scales. We select highly influential nodes produced by five benchmark approaches and LTC model to run spreading processes by the SIR model, thus we can evaluate the efficacies of different approaches. The experimental results of the SIR model show that LTC metric can more accurately identify highly influential nodes in most real complex networks than other indicators. We also conduct network robustness experiment on four selected networks by computing the ratio of nodes in giant component to remaining nodes after removing highly influential nodes. The experimental results also show that LTC model outperforms other methods.
2016, 65 (16): 168801.
doi: 10.7498/aps.65.168801
Abstract +
Now, the traditional four-coil magnetic coupling systems have been used in the wireless charging of mobile electronic devices and electric vehicles. However, the system efficiency is difficult to improve due to the divergence of spatial distribution of magnetic field. To overcome this disadvantage, we propose an efficient system based on the toroidal metamaterials, which support a resonant electromagnetic mode that is dominated by the toroidal moment. The toroidal moment is produced by currents flowing on the surface of a torus along its meridian. It presents remarkable ability to localize the field and suppress the radiation.
This new toroidal magnetic mode system (TMMS) consists of four asymmetric split resonant rings (ASRRs). Pairs of ASRRs in the same unit (transmit unit and receiver unit) have mirror symmetry about the yz plane. Pairs of ASRRs in different units have 180 rotational symmetry about the x axis. These four rings support the toroidal magnetic resonant mode (dominated by toroidal moment). For comparison, we also construct two symmetric split resonant rings to imitate the four-coil system (FCS). It supports parallel magnetic mode (dominated by magnetic dipole moment) and antiparallel magnetic mode (dominated by magnetic dipole moment and magnetic quadrupole moment).
To confirm the improvement of efficiency, we compare the transmission of the TMMS with that of the FCS at the same transfer distance (10 mm). The TMMS presents a higher transmission and the increase in simulation (experiment) is 81% (40%). The toroidal magnetic mode in the TMMS also exhibits low metal loss, which is reflected in these spectra. The simulated distributions of magnetic field line corresponding to the resonantly magnetic modes in both systems are provided in this article. Instead of divergence in FCS, the magnetic field lines of TMMS are well constrained around the four rings and form closed loops along these rings. The density of the field line and the magnitude of field near the receiving coil are both enhanced. So the system efficiency, which is determined by the magnetic flux of the receiving coil, is improved. The dispersions of radiation power for various induced multipole moments from the two systems are also calculated. The dominance of toroidal moment corresponding to the resonant mode in TMMS is verified and the radiation is suppressed to 1/4 of FCS. Finally, the transmissions of two systems at different transfer distances are presented. The toroidal magnetic mode system presents a higher efficiency at strong coupling area (0-25 mm). The average increase of the transmission in simulation (experiment) is 73% (46%).
In summary, the proposed new system exhibits the properties of high efficiency, low metal loss and low radiation loss with the multiport output. It would have broad prospects of practical application in WPT.
REVIEW
2016, 65 (16): 168201.
doi: 10.7498/aps.65.168201
Abstract +
From the aspects of both experimental studies and first-principles calculation, we review the research progress of improving the electrochromic performances of WO3, and analyze the transformation tendency in applied field, performance requirement and research focus. Due to the low color-switching, the application field of WO3 shifts from display devices to smart windows or other energy-saving devices. According to the requirement for electrochromic performance, the concerned WO3 morphology changes from amorphous form to nanostructure. For the high desire of smart windows in large-area curtain walls, the solid state inorganic electrochromic materials with lithium ion conductors are used as substitutes for the organic electrochromic films in hydrogen ion electrolytic solution. Correspondingly, response time and cycle life are regarded as the most important performance indices. Doping and synthesizing nanostructure are considered to be the main methods to improve electrochromic performance by introducing the pores into the crystals as the ion diffusion path. Especially, the nano-crystalline WO3 attracts much attention, due to its high stability and quick color switching. In the respect of the first-principles calculation, the simple cubic WO3 is a widely used model for calculation, because of its simple structure and high symmetry. However, there always occur the underestimation of band gap and the incorrect relationship between the cell sizes of WO3 and LiWO3. In response to the problem, by analyzing the Li-intercalated WO3 configuration, it is found that the lattice parameter is closely associated with the interaction between lithium and oxygen. The large discrepancy between the experimental and calculated band gaps is primarily due to the omission of the structural distortion in the calculation, including tilting of WO6 octahedra, as well as the off-centering of W in octahedral caused by the second-order Jahn-Teller effect. According to this, we propose a distorted cubic WO3 model (Im3 space group) to better explain the relevant experimental results. In light of the achieved results and the encountered problems in recent researches, it is generally received that the industrialization of nano-crystalline WO3 and systematic calculation on the lithium diffusion in WO3 deserve the serious consideration. In addition, possessing the function of blocking near-infrared and visible light selectively is the trend for the next generation electrochromic materials. Therefore, the noteworthy development directions on the aspect of both experimental studies and first-principles calculation are pointed out to provide some valuable references for the further researches.
