###### GENERAL

2018, 67 (7): 070303.
doi: 10.7498/aps.67.20172697

Abstract +

Quantum entanglement, as an indispensable resource in quantum communication and quantum computation, is widely used in the field of quantum information. However, people's understanding on entanglement is quite limited both theoretically and experimentally. How to determine whether a given quantum state is entangled is still an important task. The entanglement witness is a kind of special self-adjoint operator, it can be used to determine whether a quantum state is an entangled state. This provides a new direction for the determination of entangled states. Entanglement witness has its own unique characteristics in various kinds of entanglement criterion. It is the most effective tool for detecting multipartite entanglement, and the most useful method to detect entanglement in experiments. In the background of quantum theory, we use theory of operators to make a thorough and systematic study of the construction of entanglement witness in this paper. First, from the definition of an entanglement witness, a general method is given to construct an entanglement witness. It is proved that when the maximal expectation CA of an observable A in the separable pure states is strictly less than its biggest eigenvalue max(A), the operator WC=CI-A is an entanglement witness provided that CA C max(A). Although the entanglement witness WCA can detect more entangled states than WC, but it is difficult to calculate the exact value of CA, and the estimate of the upper bound of CA is easier. Therefore, it is more convenient to construct entanglement witness WC than WCA.
In quantum computation, a graph state is a special kind of multi-qubit state that can be represented by a graph. Each qubit is represented by a vertex of the graph, and there is an edge between every interacting pair of qubits. Graph states play a crucial role in many applications of quantum information theory, such as quantum error correcting codes, measurement-based quantum computation, and quantum simulation. Consequently, a significant effort is devoted to the creation and investigation of graph states. In the last part of this paper, as applications of our method, a series of methods for constructing an entanglement witness is obtained in the stabilizer formalism. It is also proved that how entanglement witnesses can be derived for a given graph state, provided some stabilizing operators of the graph state are known. Especially, when A is made up of some stabilizing operators of a graph state, entanglement witness WCA becomes one in literature.

2018, 67 (7): 070601.
doi: 10.7498/aps.67.20172759

Abstract +

We demonstrate a spin-polarized clock transition spectrum of the 87Sr optical lattice clock. The clock transition 5s2 1S05s5p 3P0 of isotope 87Sr has a hyperfine structure due to non-zero nuclear spin, inducing ten -polarized transitions from each individual mF state under the condition of a bias magnetic field along the probing polarization axis. In this experiment, atoms are driven to a certain mF state by a circular-polarization pump light to maximize the atomic population, which is beneficial to the stability and uncertainty evaluation of the optical lattice clock. After two stages cooling and trapping, about 3.5106 atoms are trapped in the red magneto-optical trap with a temperature of 3.9 K. A grating-feedback external cavity diode laser with a tapered amplifier is used to build the optical lattice with a magic-wavelength of 813.426 nm. Both waists of the counter-propagating lattice beam along the horizontal direction are overlapped to form a one-dimensional (1D) optical lattice. The lifetime of the atoms trapped in the 1D optical lattice is 1600 ms. The clock laser at 698 nm is a grating-feedback diode laser, which is locked to an ultra-low expansion cavity by the Pound-Drever-Hall technique to stabilize the frequency and phase. As a result, the linewidth of clock laser is narrowed to Hz level. By the normalized shelving method, we obtain a resolved sideband spectrum of 87Sr 5s2 1S05s5p 3P0 transition. According to the spectrum, the lattice temperature along the longitudinal direction is approximately 4.2 K. After that a linewidth of 6.7 Hz of the degenerate clock transition is obtained at a probing time of 150 ms by utilizing a three-dimensional (3D) bias magnetic field, which is used to eliminate the stray magnetic fields. Then a small bias magnetic field of 300 mGs is applied along the polarization axis of the lattice light to achieve the spectrum of Zeeman magnetic sublevels of the clock transition. Furthermore, the mF=+9/2 and mF=-9/2 magnetic sublevels are picked to be respectively pumped by the +-polarized and --polarized light at 689 nm, a variable liquid crystal wave plate is employed to switch on both polarizations. Finally, the spin polarized clock transition spectrum is obtained at the interrogating pulse of 150 ms, and the linewidths of the mF=+9/2, mF=-9/2 magnetic sublevel transitions are 6.8 Hz and 6.2 Hz respectively.

2018, 67 (7): 070703.
doi: 10.7498/aps.67.20172576

Abstract +

Based on the two-dimensional finite element method, the magnetic field of circular composite magnetron sputtering cathode is calculated by COMSOL software. The genetic algorithm and simulated annealing algorithm combined with Matlab optimization toolbox are used to optimize the structure of circular composite magnetron sputtering cathode, and the structural parameters with the largest utilization rate of target are obtained. For the resulting optimized magnetron cathode, based on the self-consistent particle simulation method, the discharge characteristics under different working conditions are simulated by VSim software. It is found that with the increase of non-equilibrium degree of magnetic field, the cathode surface potential drops to the maximum position and the location of the plasma aggregation, moving from the outer surface of the cathode to the center, the intensity of the magnetic field on the cathode surface decreases When the two coils have no currents flowing, the density of the plasma is largest and the thickness of the sheath is smallest In the two coils there flow reverse 5 A currents, the non-equilibrium magnetic field reaches a maximum value and the thickness of sheath is largest, the corresponding electric field strength is weak, which is not conducive to the impact ionization, so the plasma density is smallest However, in the two coils there flow positive 5 A currents, and the non-equilibrium magnetic field is smallest, the plasma density and the sheath thickness are not only related to the non-equilibrium magnetic field, but also to the magnetic field strength. Finally, according to the results of particle simulation, the target erosion depth of the magnetron cathode is studied. Combined with the sputtering yield curve, the curve of etching depth of the cathode target surface is obtained. It is found that the erosion range of the target extends from 60 mm to 76.2 mm (target radius) before and after optimization. By adjusting the magnitudes and directions of currents in the two coils, all the target surfaces can be etched, which greatly improves the target utilization.

2018, 67 (7): 070204.
doi: 10.7498/aps.67.20172198

Abstract +

The effect of window acoustic impedance on the wave profile of phase transition of zirconium under ramp wave compression is investigated in experiment and simulation. In the experiments, a ramp wave driven by magnetic pressure is applied to the zirconium samples backed windows with different acoustic impedances such as LiF, sapphire and free surface based on the compact pulsed power generator CQ-4. The experimental wave profiles measured by an advanced laser interference velocimeter show that the characteristic particle velocity of the onset phase transition from to is about 331.0 m/s in the conditions of LiF widow and free surface with low acoustic impedance, and it is approximately 301.9 m/s for the sapphire window with higher acoustic impedance. The corresponding onset pressure of phase transition varies from about 9.14 GPa to 8.27 GPa. The result shows that this onset pressure of phase transition, which is affected by diverse factors, is not the inherent value of phase transition belonging to the material properties. In order to describe these dynamic responses in experiment well, the numerical simulation of phase transition dynamics of zirconium is conducted in one-dimensional hydrodynamic code, in which included are the muti-phase equation of state based on Helmholtz free energy, the equation of non-equilibrium phase transition dynamics, and Steinberg constitutive relationship. The simulated results show that they can reflect the physical processes of elasto-plastic transition and - phase transition as well, which are excellently consistent with the experimental data. The relaxation times of - phase transition in three different acoustic impedance experiments are nearly the same (30 ns), and their finishing times of phase transition are all about 100 ns. The calculated quasi-isentrope of zirconium below 20 GPa in the pressure-volume and temperature-pressure thermodynamic planes shows that the isentrope and shock adiabat exhibit tiny difference before phase transition, and then separate gradually with the increase of pressure. The isentrope lies below the shock adiabat after the onset of phase transition. At about 20 GPa, the temperature of zirconium under ramp wave loading is bout 100 K lower than that under shock loading. Meanwhile, the abrupt change of volume at phase transition causes the Lagrange sound speed to reduce about 7% and then comes back to the bulk sound speed again after the phase transition has been finished.

2018, 67 (7): 070301.
doi: 10.7498/aps.67.20172546

Abstract +

Quantum entanglement is one of most remarkable features of quantum mechanics,and in recent years it has played a more and more important role in quantum information.However,real quantum system inevitably interacts with the environment,resulting in the entanglement decay or even entanglement sudden death,so it is necessary to study the entanglement dynamical properties of an open system under different environments.In this paper,we investigate the entanglement dynamic behaviors of three interacting two-level atoms in an optical cavity which is coupled to a structured zero-temperature bosonic reservoir.Laplace transform,LBC and other methods are utilized,through numerical method we analyze the entanglement dynamic behavios of tripartite of three atoms and bipartite of cavity and reservoir.We also discuss how the coupling parameters affect the entanglement dynamics.Results show that in a short time,the entanglement of tripartite increases with coupling strength of three atoms increasing,and a periodic oscillation appears, but entanglement of bipartite decreases.The entanglement of tripartite decreases with the coupling strength between atoms and cavity increasing and damping oscillation appears,but the entanglement of bipartite increases.In a long-time limit,the entanglement approaches to a steady value.The non-Markovian dynamics of the qubits is determined by both the coupling strength and the spectral width.The strong system-reservoir coupling regime results in the non-Markovian dynamics of system.As the spectral width increases,the system of three atoms transforms from non-Markovian regime to Markovian regime.The increasing of spectral width results in the Markovian dynamic behavior of system,but the system of the atoms falls into the non-Markovian regime once more.When the coupling between the cavity and reservoir is weak,the entanglement of three atoms increases as the detuning of the cavity and reservoir increases,but it is not obvious.When the coupling between the cavity and reservoir is strong,the entanglement of three atoms increases and a periodic oscillation appears with increasing the detuning between the cavity and reservoir,so we can effectively restrain the effects of dissipation of reservoir on entanglement decay by adjusting the detuning between the cavity and reservoir.

2018, 67 (7): 070501.
doi: 10.7498/aps.67.20172144

Abstract +

In the past two decades,the ensemble forecasting has gained considerable attention.The atmosphere is a chaotic system,and a small error in the initial conditions will result in an enormous forecast uncertainty with time.It is impossible to precisely predict the future state of the atmosphere by a single (or control) forecasting.The ensemble forecasting is a feasible method to reduce the forecast uncertainty and to provide the reliability information about forecast.Many studies showed that because of the nonlinear filtering effect,the ensemble forecasting is more skillful than the single forecasting according to the statistical average over a large number of numerical experimental cases. However,the forecast skill can vary widely from day to day according to the specific synoptic events.The dependence of the ensemble forecasting on specific event has not been fully addressed in previous studies.Therefore,the performances of the ensemble forecasting in specific experimental cases should be further studied,which is important for improving the forecast skill in weather and climate events.In this paper,the nonlinear local Lyapunov vectors (NLLVs),which indicate orthogonal directions in phase space with different perturbation growth rates,are introduced to generate the initial perturbations for the ensemble forecasting.The NLLVs span the fast-growing perturbation subspace efficiently, and thus may grasp more components in analysis errors than other ensemble methods.Meanwhile,the bred growing mode (BGM) method,which indicates the fastest growing perturbation mode,is also used for the ensemble forecasting. Based on the NLLV and BGM methods,the forecast performances of the ensemble forecasting and single forecasting are compared in the Lorenz63 and Lorenz96 models for specific experimental cases.Additionally,two practical measures, namely the root mean square error (RMSE) and pattern anomaly correlation (PAC),are used to assess the performances of the ensemble forecasting.The results indicate that each ensemble mean forecasting is more skillful than its single forecasting in terms of RMSE and PAC.For each experimental case,the proportion of the ensemble forecasting better than single forecasting gradually increases with time in Lorenz63(Lorenz96) model by both NLLV and BGM methods, respectively.In addition,the variation of probability distribution of the ensemble mean states might be the reason why the forecast error of ensemble forecasting is less than that of the single forecast.The results based on simple model could provide a new perspective to understand ensemble forecasting and may be conducive to the weather and climate prediction.

## EDITOR'S SUGGESTION

2018, 67 (7): 070201.
doi: 10.7498/aps.67.20171774

Abstract +

Over the last decades, manipulating polarization has received much attention due to its wide applications in science and technology. In this paper, a half-waveplate based on a field transformation (FT) method is proposed and investigated in order to convert polarization, which works at millimeter-wave band with a wide incident angle and broad working bandwidth.Inspired by the FT method, we confine our attention to a two-dimensional (2D) case of in-plane wave propagation on the x-y plane, with both material properties and fields unchanged in the z direction. The fields are denoted with a subscript “(0)” in the virtual space. Then a series of theoretical calculations is analyzed in detail. Under the guidance of theoretical analysis, it is shown that the main job for realizing this half-wavepalate is to obtain a material with specific permittivity and permeability. The proposed waveplate is composed of periodically arranged two dielectric layers each with sub-wavelength in height. By using the effective medium theory, the effective electromagnetic (EM) parameters of the waveplate can be tuned by manipulating the heights of the two dielectric layers. Among them one layer is a material with a permittivity of 10 and height of 0.68 mm, and another layer material has a permittivity of 1, and height of 5 mm. We alternately arrange the two materials along one direction periodically to obtain a waveplate for realizing the TE-to-TM and LCP-to-RCP conversion. The thickness of whole waveplate is 5.5 mm. A broadband EM half-waveplate is achieved in millimeter-wave region, which possesses a nearly 90% conversion efficiency across the frequency band from 24 GHz to 37 GHz. At the same time, we also find that when the incident angle gradually increases from 0° to 60°, the performances of polarization conversion efficiency and working bandwidth are still good. For the incident angle of 60°, a 3-dB bandwidth over 26-33 GHz is still achieved. The performance of the waveplate is verified through both full-wave simulation and experimental measurement, which are in good agreement with each other. Meanwhile, three-dimensional (3D) printing technology makes the sample fabricated more easily. Another advantage of our design is that the 3D printing technology can be used to carry out the experimental fabrication, which may pave a new way to manufacturing more microwave devices.

2018, 67 (7): 070202.
doi: 10.7498/aps.67.20171671

Abstract +

Although Carlson fractal-lattice fractance approximation circuit belongs to the ideal approximation, it can only have operational performance of fractional operator of negative half-order. When series of this circuit increases, the approximation benefit decreases. Even though the fractance approximation circuit of -1/2n (n is an integer greater than or equal to 2) order can be obtained by using nested structures, the structure of this kind of circuit is complicated and fractional operation of arbitrary order cannot be achieved by this circuit. The Liu-Kaplan fractal-chain fractance class, which can be regarded as scaling extension circuits of the Oldham fractal-chain fractance class, has high approximation benefit and can realize operational performance of arbitrary fractional order. Based on analogy, arbitrary order scaling fractal-lattice franctance approximation circuits of high approximation benefit and corresponding lattice type scaling equation can be achieved through respectively making scaling extension to the Carlson fractal-lattice franctance approximation circuit and its normalized iterating equation. There exists the possibility to verify the validity of this scaling extension and scaling fractal-lattice fractance approximation circuits with operational performance of arbitrary order in different ways, including the transmission parameter matrix algorithm, the iterating matrix algorithm and the coefficient vector iterating algorithm. Arbitrary order scaling fractal-lattice franctance approximation circuits can be realized by adjusting both the resistance progressive-ratio and the capacitance progressive-ratio parameters. The approximation benefit of scaling fractal-lattice franctance approximation circuit of arbitrary order is determined by both the scaling factor and the circuit series. The introduced extension benefit function is to be used in performance analyses. Besides, performance comparisons have been made between the Carlson fractal-lattice franctance approximation circuit of five series and the scaling fractal-lattice franctance approximation circuit of negative half-order. With the increasing of the value of the scaling factor, approximation efficiency of the scaling fractal-lattice franctance approximation circuits gradually increases, which are higher than those of the Carlson fractal-lattice franctance approximation circuits. The Carlson fractal-lattice franctance approximation circuit and the scaling fractal-lattice franctance approximation circuit of five series are designed to be used in the active differential operational circuit of half-order to construct experimental testing systems. The approximation performances of both circuits are investigated from the aspects of order-frequency characteristic and F-frequency characteristic. The approximation performance of the scaling fractal-lattice franctance approximation circuit outperforms that of the Carlson fractal-lattice franctance approximation circuit. As the successful application case, the active differential operational circuit designed by the scaling fractal-lattice franctance approximation circuit is used to do the half-order calculus of triangular and square wave signals. This paper is merely an incipient work on scaling fractal-lattice franctance approximation circuits of arbitrary order and irregular lattice type scaling equations.

2018, 67 (7): 070203.
doi: 10.7498/aps.67.20172288

Abstract +

Silicon, as the next-generation cathode material in lithium-ion batteries, exhibits excellent electrochemical performances compared with traditional cathode material, such as high capacity and cheap price. However, its cycling performances are greatly affected by the volume change of silicon due to the insertion of Li atoms. Lots of work focuses on the analysis of diffusion-induced stresses in electrode, but the convection term is seldom considered in analyzing the diffusion-induced stress in an electrode. In this paper, a mathematical model is established, where the convection term is taken into consideration in the diffusion process. The mechanics equations and diffusion equation are derived based on continuum mechanics and the diffusion theory. Diffusion-induced stress, axial reaction force and the critical buckling time in a hollow cylindrical electrode under galvanostatic charging are calculated. The effects of local velocity, ratio of the outer radius to inner radius, charging rate, material parameters and lithiation induced softening factors on stress field and the critical buckling time are studied. According to the results, it is found that the influence of local velocity on stress distribution increases with the increasing of Li concentration, and the contribution of local velocity to axial reaction force is insignificant. Compared with the results without local velocity, the tensile hoop stress of inner surface is large, and compressive stress at the outer surface is small. The axial reaction force and the critical buckling time are calculated with different ratios of outer radius to inner radius. As the radius ratio increases, the axial reaction force and critical buckling time decrease. The effects of three main material parameters (elastic modulus, diffusion coefficient, partial molar volume) on axial reaction force are discussed. The dimensionless force is independent of elastic modulus due to stress varying linearly with Young's modulus. The critical time is inversely proportional to diffusion coefficient. As the partial molar volume increases, which indicates larger volume change induced by the intercalation of the same quantity of Li-ions, the critical buckling time drops and the effect of local velocity on stress field increases. It takes less time for axial reaction force to reach the critical buckling force at a higher charging rate. The elastic properties of silicon in the lithiation process should be a function of Li concentration due to the formation of Li-Si alloy. The elastic modulus is assumed to be a linear function of Li concentration. The hollow cylindrical electrodes with increasing absolute value of lithiation induced softening factor have lower maximum axial reaction force. However, the lithiation induced softening factor has a limited effect on the critical buckling time due to the fact that the Li concentration at critical buckling time is relatively small.

2018, 67 (7): 070302.
doi: 10.7498/aps.67.20172634

Abstract +

Quantum entanglement is a kernel of quantum computation and quantum communication. We introduce a theoretical scheme to achieve the entanglement between two separated quantum nodes in a hybrid system. The proposed hybrid system based on diamond nitrogen-vacancy (NV) center spin ensemble is coherently coupled to a superconducting quantum circuit consisting of two quantum nodes and a quantum channel. Each node in our setup is composed of an NV center spin ensemble magnetically coupled to a superconducting coplanar resonator. The NV center spin ensemble composed of N identical and non-interacting NV spins, is placed in the magnetic field antinode of the superconducting coplanar resonator where the coupling is maximized. An array of superconducting quantum interference devices (SQUIDs) is inserted in the central conductor of resonator to make its frequency tunable with the magnetic flux threading through the SQUID loops. This flux is generated by passing current through an on-chip wire, so that the resonator can be brought in resonance with the NV center spins without changing their Zeeman splitting. Quantum qubits encoded into two separate nodes are connected by a vacuum superconducting coplanar resonator that is used as a quantum channel. This setup can potentially take the best elements of each individual system:NV center spin ensemble with longer coherence time capable of preparing, storing and releasing photonic quantum information, and the superconducting quantum circuits are easy to manipulate externally and can perform quantum logic gates to control quantum information rapidly. In order to realize the entanglement between two separated quantum nodes, firstly, we make a canonical transformation and obtain the Hamiltonian of the system that is reduced to two NV center spin ensembles resonantly coupled to a single mode of the superconducting coplanar resonator. Then we put forward the hybrid NV center spin-photon qubit encoding. In this hybrid encoding, the NV center spin and photon degrees of freedom enter on an equal footing into the definition of the qubit, in which case, quantum channel will switch on when three superconducting coplanar resonators are in resonance with each other, and all the manipulations can perform simply by tuning the frequencies of the superconducting coplanar resonators. Under the precise control of the evolution time, high fidelity entanglement between two separated quantum nodes is achieved. We show that this proposal can provide high fidelity quantum entanglement under realistic conditions, both in the resonant and the dispersive interaction cases. This hybrid quantum system will exhibit long coherence time and possess features like easy fabrication, integratability, and potential scalability. Furthermore, the quantum node composed of an NV center spin ensemble magnetically coupled to a superconducting coplanar resonator can be respectively integrated, which has practical applications in the realization of quantum information transmission and quantum entanglement among multiple quantum nodes.

2018, 67 (7): 070701.
doi: 10.7498/aps.67.20172596

Abstract +

The curved light-emitting diode (LED) array has so many advantages over conventional planar micro LED array such as wider viewing angles, and convenience in its actual applications:curved mobile phone screen, curved smart watch screen, and wide-angle communication illumination light source, etc. Irradiance uniformity is considered to be one of the momentous parameters for evaluating the degree of display or communication lighting devices. In order to improve the untilization of micro-curved LED array in display illumination, we focus on uniform irradiance of cylindrical and spherical micro-LED array by the method of ray-tracing. The calculation results show that the curved radius R and LED radiation parameter m are main factors affecting the uniform irradiance of the cylindrical array. We can improve the energy utilization efficiency by arranging the array pixel positions rationally. The simulation of 1010 cylindrical array with bending radius R=5 cm shows that the uniformity of maximum irradiance can reach 90.5% when detection distance z=300 cm and the detection area is defined as {(x, y)|-100 x 100, -100 y 100}. Furthermore, the irradiance distribution of spherical array is calculated and the results show that the irradiance uniformity of the single spherical array is unrelated to the number of pixels when it surpasses three. The main factors that affect the irradiance distribution of the multi-ring LED array are the ring distribution coefficient K, the normal angle 0, and the luminous flux ratio of each ring . Also the two-ring LED array model is calculated when the pixel number of the first ring is set to be 6 and the second ring is assumed to be 12. And the simulation results show that the maximum irradiance uniformity of the two-ring LED array can reach 94.8% in which the value of 0 is set to be 20, the ring distribution coefficient K=0.5 and the two ring pixel unit luminous flux ratio =20. Experimentally, we adopt the approach of the two micro LEDs to confirm the accuracy of the theory. And the results show that the irradiance distributions of two LEDs with the values of angle =13, 15 and 17 are consistent with the theoretical calculations. Thus, the theoretical and the experimental results in the paper can offer references for curved-LED display and multi-mode intelligent illumination.

2018, 67 (7): 070702.
doi: 10.7498/aps.67.20172154

Abstract +

In practical applications such as radar, sonar, and mobile communications, transmitted signals are often affected by the scattering and reflection phenomena, which causes the signal energy received by the antenna array to be distributed into a certain space. In this case, a distributed source model will be more applicable. In general, the distributed sources have been classified as coherently distributed (CD) source and incoherently distributed (ID) source, which prove to be suitable for the cases of slowly time-varying and rapidly time-varying channels, respectively.In this paper, we consider the two-dimensional direction of arrival (DOA) estimation of distributed sources (including CD source or ID source). Specifically, uniform circular array (UCA) is widely used because of its ability to measure full azimuth angle and high resolution. However, the existing estimation algorithms all require spectral peak searching and the eigenvalue decomposition, which can bring a large computational complexity. To solve this problem, a decoupled rapid two-dimensional DOA estimation algorithm is proposed based on vectoring differential phases considering the two cases of single CD source and ID source. Firstly, based on spatial frequency approximation model, it is proved that none of differential phases between the received signals of different sensors in the UCA is affected by angle spread parameters when there is only a single distributed source. Under the premise of such a property, the central DOAs can be decoupled through obtaining the differential phases. Next, we can obtain the phase angles of strictly upper triangular elements in the sample covariance matrix, which correspond to differential phases between different sensors. Finally, by vectoring these differential phases, the central azimuth and elevation DOAs are estimated in the closed form from a least-squared problem, where the spectral peak searching and eigenvalue decomposition can be avoided, hence the computational complexity is reduced greatly. Theoretical analysis and simulation results show that the proposed algorithm has higher estimation accuracy and does not require prior information about the distribution of angular signals. With both low computational complexity and low hardware complexity, the proposed algorithm is beneficial to the engineering practice of array direction finding in complex environment.

###### ATOMIC AND MOLECULAR PHYSICS

2018, 67 (7): 073202.
doi: 10.7498/aps.67.20172440

Abstract +

With the development of high harmonic generation and the free electron laser,one can obtain the laser pulses whose frequencies range from XUV to X ray.Using these novel light sources,one can investigate the electron dynamics with attosecond resolution.With the increase of intensity,a lot of nonlinear processes have been found,such as high harmonic generation, above threshold ionization and dynamic stabilization of atomic ionization.When the atom is irradiated by an ultra-intense short laser pulse,many additional sub-peaks appear in the original photoelectron peaks.The original peaks of the photoelectron spectra are formed by the ionization interference from different optical cycles.The formation of sub-peaks are attributed to the shift energy level by the action of strong laser electric field.In previous studies,the sub-peak phenomenon was mainly observed in the short pulse.In this work,we investigate the duration effect of laser pulse on this phenomenon.The photoelectron is calculated from the time-dependent wavefunction in momentum by using generalized time dependent pseudo spectral scheme.At small laser intensity,there is only main photoelectron peak near the position whose energy is the difference between the central frequency of the laser and ionization energy.As the laser duration decreases,the width of the photoelectron peak gradually increases.For the higher laser intensity,many sub-peaks appear in the photoelectron spectra.The width of the sub-peak is also decreasing with the increase of the laser pulse's duration. The amplitude of these sub-peaks is decreasing with the increasing of the duration of laser pulse.For the longer pulse (50 optical cycles),these sub-peaks disappear.The variation of the amplitude and energy position for the first sub-peak with the laser intensity is analyzed.As the increase of laser pulse width,the energy of the sub-peak increased.Comparing with the longer pulse,the short pulse has a larger enhancement.In order to understand the profiles of the photoelectron spectra,we investigate the time-dependent ionization profile of the atom.The results show that the ionization occurs in the whole duration of the laser pulse for small incident intensity.The ionization mainly occurs at the raising edge of the laser pulse for the large laser intensity.For the longer pulse,the gradient of laser intensity is small.Its energy level shift effects on the ground state of the atom is small.Thus, one can not observe any sub-peak in the photoelectron spectrum of atom irradiated by the long laser pulse.

2018, 67 (7): 073201.
doi: 10.7498/aps.67.20172636

Abstract +

Significant progress has been made in atom-based measurements of length, time, gravity and electromagnetic fields in recently years. Rydberg atom-based microwave electric field measurement, using electromagnetically induced transparency (EIT) in room temperature alkali-metal vapors, has been extensively investigated and aroused the broad interest. This approach may establish a new standard for the measurements of microwave (MW) and radio frequency (RF) electric fields.In this review, we describe the work on a new method of measuring electric fields based on quantum interference by using either cesium or rubidium atoms contained in a dielectric vapor cell. Rydberg atoms with principal quantum number n >>1 have large direct current (DC) polarizabilities and microwave transition dipole moments, thereby making them extremely sensitive to external electric fields. Using the Rydberg three-level EIT to detect the level splitting and shift that is induced by the external field, we can realize a rapid and robust self-calibration method of measuring the electric field in a frequency range from 0.01 GHz to 1000 GHz. For the MW electric field (frequency range > 1 GHz), the MW field causes the Rydberg states to split, known as an Autler-Townes splitting (A-T) effect when the applied microwave can resonate with adjacent Rydberg states. The MW coupled A-T splitting is proportional to the applied electric field strength, from which the field strength is measured. Using the EIT window, a high sensitivity of 3 μV·cm-1·Hz-1/2 and small electric field of 1 μV/cm are expected to be achieved with a modest setup, and the limitations of the sensitivity are also addressed in the review. For the RF field at frequency mj EIT lines, and avoided crossings formed with the fine-structure levels of equal mj and different J's, which is used to calibrate and measure the RF field amplitude. On the other hand, the dependence of the EIT-line strength on the RF field polarization provides a fast and robust polarization measurement of RF fields based on matching experimental data with a theoretical simulation. The measurements of minimum strengths and sensitivity of RF fields based on Rydberg atoms are one order magnitude below the values obtained by traditional antenna methods. The atom-based field measurement paves the way for determining fields through calibration-free, invariable atomic properties and miniaturization. We also propose its various potential applications in the future.

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

2018, 67 (7): 074201.
doi: 10.7498/aps.67.20171932

Abstract +

High-performance lasers with high-quality beam laser and high-stability power are widely used for laser machining, laser precision measuring, etc. Reflection curved mirrors are widely used in lasers to provide several small intracavity focal spots and reduce the dispersion and volume of the laser. The primary disadvantage of using reflection curved mirrors in folded resonators is that relatively large angles of incidence deform the circular transverse pattern of the output beam and limit laser performance. In addition, in high power lasers or ultra short pulse lasers, the gain medium thermal lens focal length fluctuation is the primary cause of the instability of laser output power. This paper focuses on beam quality and power stability of laser, and an effective method of solving the two problems, i.e., astigmatism and instability power of laser, is presented. The laser resonator with high-quality laser beam and high-stability power is very easy and intuitive to design by this method, in which the resonator transform circle graphic theory is used and the thermal lens and astigmatism compensation is taken into account. The theoretical investigation shows that the astigmatism in two terminal arms of the folded laser resonator can be successfully eliminated by using this method, and the experimental measurements of the pattern of the laser output beam show that the deformations of spot intensity profiles in the two terminal arms can be simultaneously compensated for completely in the cavity, which is in good agreement with the analytical prediction. When the focal length of the laser crystal thermal lens varies, the variations of radii of spots not only at some key position, but also at all locations of the laser resonance designed by this method, are overtly smaller than the variation of the normal resonant cavity. The stability of the output laser power of the laser is better than that of a universal laser resonator under the same external conditions.

2018, 67 (7): 074202.
doi: 10.7498/aps.67.20172079

Abstract +

Based on the measurement principle of pulse time-of-flight, non-cooperative target ranging technology using a pulsed laser diode (LD) as a light source has received widespread attention in recent years. Using leading edge timing method to directly detect pulses, its measuring range is about a few tens of meters and only a cm-level single-shot accuracy could be reached due to the limitations of its pulse width and eye-safe laser power of the LD, which cannot meet the needs of most applications. Especially, in order to increase its receiver channel bandwidth from hundreds of MHz to even a few GHz to reduce its work error, its distance measurement accuracy and ranging distance are significantly degraded as its signal-to-noise ratio (SNR) decreases. When a target is out of its measuring range, the back diffused laser pulse signal with an SNR of much less than 1 will be too weak to be extracted even with digital correlation processing technology.In this paper, using a pre-detection with high frequency resonance and multi-pulse correlation processing, a new ranging method to solve long ranging targets with high precision is proposed for the first time. Through the pre-detection circuit with high frequency resonance, a pulsed photocurrent signal is amplified and filtered, and then converted into a bipolar attenuation oscillation signal. Thereafter, its SNR is further improved by a new pulse function constructed through multi-pulse correlation processing. The peak of the new pulse is constant and its zero crossing point is found to be the timing point to calculate the target distance. The method has a better SNR and a high timing accuracy. And the detected ranging distance could be increased over one thousand meters or more. Theoretical calculation results show that the minimum detectable peak current of light pulse is around 17 nA in the method. Comparing with the direct pulse detection method, its SNR can increase 60 times. When a received peak of a photocurrent pulse is within a dynamic range of 1:10000, its work error is less than 0.1 ps. A pulsed laser rangefinder is developed based on the principle. And its average laser emission power is about 1 mW. Its measurement ranging without cooperative target is greater than 2000 m. Its distance measurement accuracy increases up to ± (3 mm+2 ppm) in a range of 1.5-300 m. For a long ranging target, its distance measurement accuracy is ± (10 mm+10 ppm). The rangefinder system is used in a total station product and can be used to measure large-scale engineering structures (such as roads, bridges, dams, tunnels, subways, etc.), building structures and industrial sites.

2018, 67 (7): 074302.
doi: 10.7498/aps.67.20172265

Abstract +

Adhesively bonded structures possess various industrial applications, such as safety-critical structures in the aerospace and automotive industries. With the increasing using of adhesive joints, corresponding methods of evaluating and testing the structural integrity and quality of bonded joints have been widely investigated and developed for the structural health monitoring. Studies show that the damage and degradation of material are closely related to the nonlinearity of ultrasonic waves propagating within the material. In this paper, for the evaluating of the damage to bonding interface under cyclic temperature fatigue, acoustic nonlinear parameters (ANPs) of specimens made of aluminum alloy 6061 and modified acrylate adhesive are measured experimentally by using the nonlinear ultrasonic technique; and thus the variations of the ANPs with the fatigue time under high and low cyclic temperature are obtained for the bonded specimens. The study shows that the ANP, which serves as an indicator of material properties, remains nearly unchanged in the initial stage of high temperature cyclic fatigue test, and the ANP obviously increases with temperature cyclic time increasing. For low temperature cyclic fatigue test, the ANP increases rapidly with the increase of temperature cyclic time in the initial stage, and its value growth slows down in the later stage. Further discussion shows that the increase of third order elastic constant is the main reason for the change of ANP for high temperature cyclic fatigue, and that the change of the tensile stiffness of the bonding interface is the main source for the change of the ANP for low temperature cyclic fatigue. It is shown that the ANP based on the theoretical model increases consistently with the experimentally measured values. The present research is expected to provide a promising way of characterizing and monitoring the damage to bonding interface under cyclic temperature fatigue.

2018, 67 (7): 074301.
doi: 10.7498/aps.67.20172383

Abstract +

Noise reduction is an interesting and important subject in the piping systems of many applications, in order to suppress noise in the pipe, many significative researches have been done. In recent years, the acoustic wave propagation in the phononic crystal pipe has received increasing attention. The characteristic band gaps in phononic crystal pipe can forbid wave to propagate within the band-gap frequency range, which provides a new way to control the noise in piping system. In this paper, the acoustic properties of phononic crystal pipe consisting of expansion chambers with the extended inlet/outlet are investigated theoretically and numerically. By combining the two-dimensional mode matching method and the transfer matrix method, the band structure and transmission loss, especially the band-gap properties of the phononic crystal structure are presented. The obtained results exhibit excellent agreement with the results from the finite element method. Then, this theoretical method is compared with the one-dimensional plane wave method, and it is found that the results from the proposed method are more accurate within the studied frequency range. Further, the effect of modal order in the band-gap frequency range is analyzed, which shows that the mode matching method has a good convergence.The wave scattering and resonance of the chamber will induce the Bragg and locally-resonant band gaps in the periodic pipe, respectively. Further analysis on the transmission coefficient in a band gap is conducted. It shows that the transmission coefficient decays exponentially with the periodic number increasing, which demonstrates that the suppression of the wave propagation in phononic crystal pipe is caused by the band-gap rather than the impedance mismatch. Then the effects of variable parameters including the lattice constant and the length of the insertion on the location and width of the band gaps are investigated. The results show that the lattice constant mainly controls the Bragg band gaps and the length of the insertion exerts a significant influence on the locally-resonant band gaps. Finally, the coupling behaviors of band gaps are studied to expand their widths. It is found that the Bragg band gaps can be coupled with the locally-resonant band gaps via changing the lattice constant and the length of the insertion, which can give rise to wider band gaps. Furthermore, the coupling between two locally-resonant band gaps is proposed by changing the length of the insertion, which also produces wider band gaps.This study can provide new ideas for designing the phononic crystal pipe to suppress the noise in piping system.

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

2018, 67 (7): 075201.
doi: 10.7498/aps.67.20172205

Abstract +

Atmospheric pressure glow discharge above liquid electrode has extensive application potentials in biomedicine, chemical degradation,environmental protection,etc.In this paper,such a kind of discharge excited by a direct current voltage is generated through using a metal rod above water surface.Results show that the discharge has a ring shape on the water surface when the current is low.With increasing the discharge current,its diameter first increases,and then decreases after reaching a maximum,and finally slightly increases.In this process,the discharge transits from a conical shape to a column.Fast photography indicates that the conical discharge actually originates from the rotation of a discharge filament,which can be attributed to the effect of electronegative particles generated in the discharge channel. These electronegative particles,mainly including NO,NO2,NO3,O,O3 and OH,can increase electron attachment coefficient β,resulting in extinguishment of the original discharge channel.Due to a similar field value and a normal β coefficient,the breakdown conditions can be satisfied in a region adjacent to the original channel.Therefore,the discharge will move into the new region.Further investigation indicates that both the conical discharge and the column discharge are in a normal glow regime.By optical emission spectroscopy,it is found that the vibrational temperature,the rotational temperature and the intensity ratio of I391.4/I337.1 increase with increasing the current.Electron mobility decreases in the conical discharge due to voltage decreasing with the current.Hence,electrons have an increased possibility with which they are attracted by the electronegative particles to form negative ions.Consequently,with increasing the discharge current,more negative ions will be accumulated not only near the conical center,but also in the vicinity of the discharge channel.Obviously,there is repulsive force between the negative ions in the two regions.The repulsive force increases with increasing the discharge current,which leads to the ring diameter increasing with the current.Besides the negative ions,gas temperature plays another important role in the discharge.It increases with current increasing,leading to the decrease of gas density in the discharge channel.Hence,electrons have a reduced probability with which they are attached by electronegative particles.This factor will lead to a reduced force between less negative ions in the two regions.Consequently,after reaching its maximum,the ring diameter decreases with current increasing.If the current is high enough,the discharge channel will have a sufficiently high temperature and an adequately lower gas density, resulting in an increased electron energy as well as an increased α(the first Townsend ionization coefficient).Therefore, the discharge will be self-sustained in the original region,other than move into an adjacent region.Consequently,the column discharge appears with the current increasing to some extent.In the column discharge,more negative ions will be accumulated above the water surface with increasing the current.These negative ions extend along the water surface,which contributes to the slight diameter increase of the luminous column.These experimental results are of great significance for theoretically studying liquid anode discharge.

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

2018, 67 (7): 076801.
doi: 10.7498/aps.67.20172581

Abstract +

Gallium nitride (GaN) has great potential applications in high-power and high-frequency electrical devices due to its superior physical properties.High dislocation density of GaN grown on a foreign substrate leads to poor crystal quality and device reliability.The homo-epitaxial growth of GaN material has low dislocation density,which is the foundation of high performance of AlGaN/GaN highelectronic mobility transistor.However,it is difficult to prepare flat surface of GaN template or GaN substrate in thermal treatment process under the metal-organic chemical vapor deposition (MOCVD) ambient condition in which hydrogen (H2) is commonly used to clean the substrate surface,i.e.,to remove impurities from the substrate surface,since H2 would greatly enhance GaN decomposition in MOCVD high-temperature condition and etch GaN into roughness surfaceIn this work,an alternation gas model of ammonia/hydrogen (NH3/H2) mixed gas and H2 gas is designed.This technique is used in a thermal treatment process of GaN template and substrate by MOCVD.Then,we in-situ grow AlGaN/GaN HEMTs (high electron mobility transistors) on GaN template and GaN substrate,respectively.A series of alternation gas samples with various H2 treatment times is investigated.Optical microscope and atomic force microscope are used to observe the morphologies of GaN template and AlGaN/GaN HEMTs and two-dimensional electron gas (2DEG) mobility and density of AlGaN/GaN HEMTs are measured by contactless Hall measurement.Optical properties of AlGaN/GaN HEMTs are analyzed by photoluminescence at room temperature.The residual impurities of C and O in the GaN epilayer and the interfacial region between GaN epilayer and GaN substrate are analyzed by secondary ion mass spectrometry.The study results show that H2 enhances GaN decomposition in MOCVD at high temperature,and GaN decomposition greatly strengthens with H2 treatment time increasing leading to rough surface and the decrease of 2DEG mobility.The NH3/2 mixed gas could suppress GaN decomposition and avoid roughn surface,but go against cleaning out the purity from GaN surface,and the relativive intensity of the yellow band is higher.The NH3/2 mixed gas and 2 gas alternate thermal treatment model with proper 2 treatment time on GaN template or GaN substrate,not only obtains atomically flat surface of GaN template and HEMT structure,but also cleans out the purity from GaN surface,which is conducive to the increase of the electric properties of HEMT material.The highest 2DEG mobility reaches to 2136 cm2/V·s with 1 min 2 treatment in the alternate gas thermal treatment process grown on GaN templates and the electrical properties of HEMT material turn excellent.Finally,an alternate model with 5 min NH3/2 mixed gas followed by 1 min 2 and then 4 min mixed gas of thermal treatment process is used,the surface morphology of HEMT grown on GaN substrate shows highly uniform atomically steps and the root-mean-square value is 0.126 nm for 2 μm×2 μm scan area;the HEMT 2DEG mobility 2113 cm2/V·s grown on GaN substrate shows good electric properties,the residual impurities of C and O in the interfacial region between GaN epilayer and GaN substrates are below 1×1017 cm-3,showing clean interfacial.

2018, 67 (7): 076802.
doi: 10.7498/aps.67.20172654

Abstract +

Graphene films grown on metallic substrates by chemical vapor deposition have wide potential applications, such as serving as transparent electrodes, transistors, sensors, etc. The coverage of graphene on metal surface can influence many performance parameters, such as square resistance and transparence, after it has been transferred to other substrates. As most of the performance parameters cannot be measured while graphene is still on the metal, it is very useful to evaluate the coverage of graphene before further actions. In this paper, we present a method to measure the coverage of graphene on metal by using scanning electron microscopy and image processing software. We also calculate and measure the uncertainty of the measured coverage. There are two main factors, namely the determination of the boundary between the covered areas and the uncovered areas, and the number of the graphene islands or vacancy islands in view, which can bring uncertainty to the coverage. The former factor raises the uncertainty of the coverage while the number of graphene (vacancy) islands in view is higher, because the more the islands in view, the smaller the islands are, therefore the total boundaries become more. The latter factor reduces uncertainty with the number of islands increasing, because of the quantum fluctuation. The uncertainty of the latter factor is proportional to 1/√N, where N is the number of islands in view. As we can see, the number of islands in view is the key parameter to balance the two factors. We measure the graphene coverage with different graphene islands in view, and also measure the uncertainty by using the statistics knowledge. Meanwhile, we also build a model to calculate the uncertainty under different numbers of islands in view. The experiments and the calculations accord with each other reasonably well. By these carefully modeling and experimentations, we optimize and balance the two faces and suggest the number of islands in view to reduce the uncertainty of the measured coverage to a lowest value. The use of these measured data can ensure the accuracy of the graphene coverage measurement with minimal time cost.

2018, 67 (7): 076301.
doi: 10.7498/aps.67.20172491

Abstract +

Based on the first-principles method of density functional theory, a systematic research is conducted on the electron mechanism of the effect of deformation, electric field action and combined action on the electrical properties of graphene. The research results show that the energy gap and density of states of graphene are both 0 at the Fermi level, indicating semi-metallic character, which implies that the calculation model and the parameter setting are reasonable in this paper. After some deformation actions, such as shear, stretch, torsion and bending deformation on the graphene, it is found that shear and torsion exert an obvious effect on opening the energy gap of graphene, but the effects of tensile and bending deformation on the energy gap of graphene are negligible. Therefore, shear deformation and torsion deformation are a preferred alternative to controlling the energy gap of graphene. By adding the electric field to the graphene in different directions, it is found that the , and direction electric fields which are parallel to the plane of graphene exert a strong effect on opening the energy gap of graphene, but the effect of direction electric field which is perpendicular to the plane of graphene is weak. Especially, the direction electric field has the strongest effect on opening the energy gap of the graphene because the positive value of the population of graphene C–C atoms in the direction is relatively large and bond energy is high while the negative value is small and the antibond energy is low. In order to investigate the influence of electric field strength on energy gap of graphene, the electric field strength is increased linearly from 0.1 eV/Å/e to 0.5 eV/Å/e. It can be observed that the energy gap of graphene increases in turn, and shows a linear growth. Under the action of 0.1 eV/Å/e electric field strength, shear deformation, stretch deformation, torsion deformation and bending deformation take place on the grapheme. It is found that under the combined action of deformation and electric field, the electric field improves the effect of deformation on the energy gap, but the effect is not so good asunder the superposition of two fields.

## COVER ARTICLE

2018, 67 (7): 076302.
doi: 10.7498/aps.67.20172407

Abstract +

In traditional physics, phonon is widely regarded as being linearly polarized, which means that phonon carries zero angular momentum. Thus the angular momentum of lattice related to mechanical rotation only reflects the lattice rigid-body motion. Recently, in a magnetic system with time reversal symmetry broken by spin-phonon interaction, one found that the phonon angular momentum is nonzero and an odd function of magnetization. At zero temperature, phonon was reported to have a zero-point angular momentum and zero-point energy. Thus the gyromagnetic ratio obtained through the Einstein-de Haas effect needs correcting by considering the nonzero phonon angular momentum. As is well known, if phonon has nonzero angular momentum, which means that phonon can have rotation, it can be right-handed or left-handed, that is, the phonon is chiral. Actually, we can define the polarization of phonon to represent the phonon chirality, which comes from the circular vibration of sublattices. When the phonon polarization is larger (less) than zero, the phonon is right (left)-handed. In non-magnetic honeycomb AB lattices, with inversion symmetrybrocken, the chiral phonons are found to be of valley contrasting circular polarization and concentrated in Brillouin-zone corners. At valleys, there is a three-fold rotational symmetry endowing phonons with quantized pseudo angular momentum. Then conversation of pseudo angular momentum, which determines the selection rules in phonon-involved intervalley scattering of electrons, must be satisfied. Chiral valley phonons can be measured by polarized infrared absorption or emission. In addition, since the phonon Berry curvature is reported to be nonzero at valley, it can distort phonon transport under a strain gradient, which can act as an effective magnetic field. Thus, a valley phonon Hall effect is theoretically predicted, which is probably a method of measuring chiral valley phonons. In consideration of phonons angular momentum and chiral phonons, photon helicity changed by phonons at Gamma point will be explained reasonably. In conclusion, chiral phonons are present in systems that break time reversal or spatial inversion symmetries. In a magnetic system, where time reversal symmetry is broken, phonons generally carry a nonzero angular momentum, which can influence the classic Einstein-de Haas effect. In a nonequilibrium system, the phonon Hall effect can be observed due to the chiral phonons. In a non-magnetic crystal, with inversion symmetry brocken, phonons in the Brillouin-zone center and corners are chiral and have a quantized pseudo angular momentum, providing an alternative to valleytronics in insulators. We believe that the findings of the phonon angular momentum and the chiral phonons together with phonon pseudoangular momentum, selection rules, and valley phonon Hall effect will lead to the relevant exploration and new development of phonon related subject in condensed matter physics.

2018, 67 (7): 076101.
doi: 10.7498/aps.67.20180311

Abstract +

In this paper, the influences of hydroxyl groups between interfaces on friction and energy dissipation are investigated by molecular dynamics simulations. The simulation systems include horizontal oriented carbon nanotube and Si substrate. The hydroxyl groups are grafted only on the substrates or between interfaces in different cases. The simulation procedure is as follows. First, the structure of the simulation system is optimized through energy minimization. Then the relaxation is conducted to ensure the the system reaches an equilibrium state. Finally, carbon nanotube moves at a constant speed along the x direction on the Si substrate. The results show that the average friction on carbon nanotube increases significantly due to the formation of hydrogen bonds between interfaces. The number of hydrogen bonds between interfaces increases with hydroxyl group ratio increasing, which is similar to the trend of friction. The chiral angle of carbon nanotube has a certain effect on friction. The friction on the armchair carbon nanotube is larger than on other types of carbon nanotubes. The diameter has an obvious influence on friction. The friction between the interfaces increases with the diameter of carbon nanotube increasing. The reason is that carbon nanotube with a large diameter becomes flattened at the bottom, which leads to the increase of contact area between interfaces. New peaks appear in the phonon state density of simulation system due to the introduction of hydroxyl groups. With the increase of hydroxyl groups ratio, the values of corresponding peaks of hydroxyl groups in the phonon state density become higher, which indicates that the vibration of hydroxyl groups plays a more important role in energy dissipation. When the hydroxyl group ratio on the carbon nanotube and Si substrate reach 10% and 20% respectively, most energy dissipates through the vibration of hydroxyl groups rather than the vibration of the carbon nanotube and Si substrate. The total energy of the system increases with hydroxyl group ratio increasing, and the potential energy of carbon nanotube also increases with the augment of hydroxyl group ratio on the carbon nanotube. However, when the hydroxyl group ratio on the carbon nanotube remains constant, the potential energy of carbon nanotube decreases with the increase of hydroxyl group ratio on Si substrate. This phenomenon becomes obvious when the hydroxyl group ratio is high. The reason can be attributed to the larger interaction between the carbon nanotube and Si substrate. In general, the energy dissipation of the system is related to the total energy, but the energy dissipating through the carbon nanotube may become less with the increase of total energy.

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

## EDITOR'S SUGGESTION

2018, 67 (7): 077701.
doi: 10.7498/aps.67.20172710

Abstract +

There is no relevant research on the relationship between the piezoelectric behavior of superlattice and the internal cations.In this paper,by the first-principles method of density-functional theory,we study the polarizations and piezoelectric contributions of cations A and B in three lead-free tetragonal perovskite ferroelectric superlattices (BaTiO3/SrTiO3,KNbO3/KTaO3 and BaTiO3/KNbO3).By calculating atomic structures and atomic Born effective charges of three superlattices under different axial strain conditions (-0.15-0.15),the polarization and piezoelectric coefficients of superlattices and internal cations are obtained.With the axial compressive strain changing from -0.15 to 0,the variations of displacements D(A) and D(B) of cations A and B in lead-free superlattices are very small, and displacements D(A) and D(B) significantly increase as the axial tensile strain (0-0.15) is applied,indicating that the axial compressive strain is not beneficial to the ferroelectric displacement in the tetragonal superlattice,especially in BaTiO3/SrTiO3 nor KNbO3/KTaO3 superlattices.The tetragonal ferroelectric superlattices BaTiO3/SrTiO3 and KNbO3/KTaO3 may be unstable under the condition of the axial compressive strain,and only the axial tensile strain can promote the existence of tetragonal phase in superlattice.As the axial strain is applied,Born effective changes of A-site cations in three lead-free tetragonal superlattices are small,and Z33*(B) gradually declines,and Zxy*(B) continually rises.The axial strain induced charges are transferred from the B-site cations to O atoms along the c-axis,and the charges are transferred from O atoms to B-site cations along the xy direction.The variation rate of Born effective charges under the condition of the axial tensile strain is greater than under the condition of the axial compressive strain, especially in superlattices BaTiO3/SrTiO3 and KNbO3/KTaO3,showing that the axial tensile strain is more beneficial to the redistribution of atomic charges in the superlattices.Under the condition of the axial compressive strain,the total polarizations of superlattices BaTiO3/SrTiO3 and KNbO3/KTaO3 are close to zero;while polarizations of superlattices BaTiO3/KNbO3 gradually increase with the axial compressive strain varying from -0.15 to 0.There are atomic ferroelectric displacements in superlattice BaTiO3/KNbO3,and the interaction between BaTiO3 ferroelectric layer and KNbO3 ferroelectric layer contributes to the generation of ferroelectric behavior.When the axial tensile strain (0-0.15) is applied,the polarization contributions of B-site cations in superlattices BaTiO3/SrTiO3 and KNbO3/KTaO3 increase significantly,especially the polarization contributions of B-site cations Ti,Nb and Ta,and the total polarization is obviously improved.The effect of the tensile strain on polarization of BaTiO3/KNbO3 is smaller than on polarizations of BaTiO3/SrTiO3 and KNbO3/KTaO3.The interaction between two ferroelectric layers in BaTiO3/KNbO3 contributes to the redistribution of atomic charges,and alleviates ferroelectric displacements of atoms to some extent.The polarization contribution of B-site cations is largest,because of their large Born effective charges and ferroelectric displacements. When the tensile strain reaches a certain threshold,tetragonal superlattices will present obvious piezoelectric behavior. With the tensile strain increasing,total piezoelectric coefficient d33 and piezoelectric contributions of A,B-site cations both increase.The piezoelectric behaviors of lead-free superlattices are mainly attributed to the B-site cations.

2018, 67 (7): 077801.
doi: 10.7498/aps.67.20172438

Abstract +

Bulk viscosity is an important parameter to understand gas viscosity in micro perspective. The traditional ultrasound absorbtion method with acoustic frequencies in a megahertz range cannot be directly applied to high frequencies field, where acoustic waves are in the gigahertz domain. However, gas bulk viscosity at high frequency can be measured by spontaneous Rayleigh-Brillouin scattering (SRBS) and coherent Rayleigh-Brillouin scattering (CRBS). Recent researches show that the bulk viscosity of nitrogen measured by CRBS at a wavelength of 532 nm is obviously different from the values from SRBS in the near-ultraviolet region. In order to obtain accurate bulk viscosity of nitrogen at the wavelength of 532 nm, the SRBS spectra of nitrogen excited by a 532 nm laser are measured in a pressure range from 1 bar to 9 bar at the constant room temperature. The measured SRBS spectrum at the pressure of 7 bar is compared with the theoretical spectrum to obtain optimal scattering angle by using the principle of minimum value of χ2. The theoretical spectrum is calculated by convolving the Tenti S6 model with the instrument transmission function of measurement system. Given that the effect of pressure on the bulk viscosity is negligible, the bulk viscosity value (1.46±0.14)×10-5 kg·m-1-1 of nitrogen at a temperature of 299 K is acquired by averaging the values of bulk viscosity under different pressures (4-9 bar), each value is obtained by comparing the measured spectra at different pressures with the theoretical spectra by using the optimal scattering angle and the principle of minimum value of χ2. The values of bulk viscosity of nitrogen over the pressure of 1-3 bar are not considered because of its big deviation compared with the values under higher pressures (4-9 bar). The results show that the average value of bulk viscosity obtained in our experiment is close to that from the theoretical calculation and SRBS experiments reported in the literature but different obviously from the bulk viscosity obtained by CRBS. In order to testify the bulk viscosity of nitrogen measured in our experiment, it is used to retrieve temperature of nitrogen under pressure ranging from 1 bar to 9 bar. The results show that the absolute error between the retrieved temperature and the reference temperature under different pressures are all below 2.50 K and the difference between the average temperature and the reference temperature is less than 0.15 K. This demonstrates that the measured bulk viscosity of nitrogen in our experiment is accurate and reliable for the gas parameters retrieved by SRBS.

2018, 67 (7): 077501.
doi: 10.7498/aps.67.20172250

Abstract +

Magnetic refrigeration materials based on magnetocaloric effect (MCE) attract wide attention.In the past decades, magnetic materials with MCE have been extensively studied due to their enormous potential applications in magnetic refrigeration fields.Among these materials,La (Fe,Al)13 compound is perceived to be one of the promising candidates as high-performance magnetic refrigerant because of its giant magnetic entropy change,tunable Curie temperature,low cost and toxin-free.For LaFe13-xAlx compounds,previous studies showed that the TC can increase by substituting Co for Fe,which leads to the value of maximum magnetic entropy change (-△SM) decreasing.In addition,the interstitial atom (N,H,C and B) can cause the lattice to expand,which shifts the anti-ferromagnetic (AFM) ground state to the ferromagnetic (FM) state.The TC increases with doping the interstitial atoms,accompanied by a remarkable change in the magnetic properties related to the magneto-volume effect.In this paper,the magnetic properties and the magnetocaloric effects of LaFe11.5Al1.5Hx(x=0,0.12,0.6 and 1.3), LaFe11.5Al1.5By(y=0.1,0.2 and 0.3) and LaFe11.5Al1.5Cz(z=0.1,0.2,0.3,0.4 and 0.5) intermetallic compounds are studied.The H,B or C atoms are inserted into the LaFe11.5Al1.5 compounds by gas-solid or solid-solid reaction.All the compounds crystallize into the cubic NaZn13-type structure.The magnetic ground state changes from the AFM to the FM state due to the introduction of interstitial atoms.Unlike the patent compound LaFe11.5Al1.5,all of the hydrides,borides and carbides display a typical FM state,which easily reach saturation under a magnetic field of 1 T.In addition,the saturation magnetization (MS) slightly increases and the Curie temperature (TC) significantly is enhanced with increasing the interstitial atom (H,B or C) content.It is attractive that the magnetic transition changes from the second-order to the weakly first-order with increasing hydrogen content,which is in contrast with the magnetic transition from the weakly first-order to the second-order with increasing boron or carbon content.All the compounds of LaFe11.5Al1.5 hydrides, borides and carbides exhibit a considerable magnetic entropy change.The values of maximum magnetic entropy change (-△SM) reach 12.3 J/kg·K for LaFe11.5Al1.5H1.3,9.6 J/kg·K for LaFe11.5Al1.5B0.1 and 10.8 J/kg·K for LaFe11.5Al1.5C0.2 under a magnetic field change of 0-5 T,respectively.And the values of refrigerant capacity (RC) reach 259.2 J/kg for LaFe11.5Al1.5H0.6,116.4 J/kg for LaFe11.5Al1.5B0.1,and 230.4 J/kg for LaFe11.5Al1.5C0.1 under a magnetic field change of 0-5 T,respectively,indicating that LaFe11.5Al1.5H0.6 compound is a promising candidate for magnetic refrigerants.

2018, 67 (7): 077702.
doi: 10.7498/aps.67.20172307

Abstract +

Based on the finite element analysis software COMSOL5.0,a three-dimensional (3D) model of cantilever beam composed of magnetostrictive/piezoelectric/magnetostrictive laminated composites is established using the piezoelectric module and magnetic field module.The magneto electro coupling coefficient αME of the composite is analyzed.The effect of geometrical parameter on magnetoelectric coefficient is studied,and the geometrical parameters are optimized. Firstly,the stress,strain,displacement and potential distributions of the magnetoelectric layered structure are analyzed by the steady-state solver.The stress and strain concentrate on the fixed terminal while the maximum displacement exists in the free end of the structure.As a result,the potential appears between the upper and lower surface of the piezoelectric layer and the voltage distribution is not uniform.The output voltage in the fixed terminal is larger than that in the free end,which is about 49 V compared with 42 V in the free end.And the dynamic distributions of various variables in magnetoelectric composite structure are analyzed by transient solution.Secondly,the resonance frequency of the structure and the influence of the bias magnetic field on the output voltage are studied by small signal analysis in frequency domain.The results show that the output voltage decreases with the increase of Hdc.Also,the maximum output voltage is about 3.36 V at the second-order resonance frequency,which is far higher than the voltage at the first-order resonant frequency in the condition of bias magnetic fields Hdc=200 Oe and alternating magnetic fields Hac=1 Oe.The reason is that the composite structure has a larger deformation at the second-order resonance frequency.Furthermore,the effect of thickness ratio between magnetostrictive and piezoelectric layers tm/tp on coupling coefficient is analyzed by changing the thickness of magnetostrictive layer and piezoelectric layer,respectively.The results show that the magnetoelectric coefficient increases with the augment of the thickness ratio,but the increasing rate decreases gradually.The research also shows that it has a greater influence on magnetoelectric coefficient to change tp rather than tm.Finally,the variations of magnetoelectric coefficient with the area of composite structure and the aspect ratio are analyzed.The results show that the magnetoelectric coefficient increases gradually with the augment of magnetoelectric composite area,but the increasing rate declines gradually.With the constant composite area,the magnetoelectric coefficient first increases and then drops with the increase of aspect ratio L/W,demonstrating the existence of an optimized value.Besides,the width W acts more importantly than length L because strain concentrates on the fixed terminal along

2018, 67 (7): 077401.
doi: 10.7498/aps.67.20172418

Abstract +

The influence of inner diameter of hollow cylindrical permanent magnet on the levitation force of single domain GdBCO bulk superconductor is investigated by measuring the levitation force between the hollow cylindrical permanent magnet and the single domain GdBCO bulk superconductor. The results show that the levitation force is closely related to the inner diameter of the hollow cylindrical permanent magnet when the inner diameter (d) increases from 0 mm to 26 mm (minimum measuring gap distance Z=2 mm), and all the superconducting magnetic levitation force curve shows magnetic hysteresis phenomenon. With the increase of the inner diameter of the hollow cylindrical permanent magnet, the levitation force at a minimum distance decreases gradually from 14.8 N at d=0 mm to -0.1 N at d=26 mm. The levitation force at the minimum gap distance is negative when d ≥ 20 mm. When 0 mm ≤ dd ≥ 5 mm. The larger magnetic field strength of the superconductor can be obtained, and the levitation force can be effectively improved by the scientific and reasonable designing of the permanent magnet structure. The results have certain guiding significance for designing and optimizing the magnetic suspension bearing system, ring track and superconductor.

###### GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS

2018, 67 (7): 079401.
doi: 10.7498/aps.67.20172575

Abstract +

Ionosperic sporadic-E layer (Es layer) is the irregular structure in ionosphere which often occurs in summer of China, but the current model of height estimation with high frequency rays does not consider the Es layer, which often makes a large error in the estimation of the target height. In this paper, the parameters of the actual ionosphere are analyzed by using the measured data of the ionospheric vertical measurement station and the information about the variation of the ionosphere in southeastern China which was obtained in recent years. The measured data indicate that the probability of occurrence of Es in China is relatively high, especially in summer. When Es appears in summer, the probability of its cut-off frequency greater than 4.5 MHz reaches up to 83.6%, therefore, it is necessary to study the target height measurement model and algorithm when the ionosphere contains Es. Firstly, on the basis of the quasi-parabolic segments ionosphere model and real ionosphere parameters, the ionosphere model containing the Es layer is established. In this model, Es layer and its connection layer with the E layer are represented by parabola and reverse parabola respectively. Then, the high frequency transmission characteristics of the target micro multipath are analyzed based on Es model. The simulation shows that 4 multipath echoes can be simulated by the characteristics of different slant ranges and Doppler frequencies in the multiple echoes of the target. By matching the simulated 4 multipath echoes with the actual high frequency echo of the target, when the matching degree reaches a maximum value, the estimated height value can be obtained. Finally, based on the micro multipath difference between high frequency rays and the ionospheric model with Es layer, a height estimation method using matched-field processing and hill climbing search algorithm is proposed. This method can greatly reduce the search time for obtaining the real height value. Through theoretical analysis and experimental verification, the relationships between the ionospheric plasma frequency and height, between the transmission path of high frequency rays and the elevation angle/transmitting frequency, and between the micro path characteristics of high frequency rays and the height of target are obtained. Ionospheric model with the Es layer and the new target height measurement method based on the matched-field processing can accurately estimate the height of the target and have a faster calculation speed.

###### NUCLEAR PHYSICS

2018, 67 (7): 072901.
doi: 10.7498/aps.67.20172618

Abstract +

As a multipurpose reflectometer device, the two-dimensional (2D) position resolution neutron detector with a 200 mm×200 mm effective area is developed for China Spallation Neutron Source (CSNS) in Dongguang, China. Due to the requirements for the specific parameters of the multipurpose reflectometer, it should be designed to have a more than 50% (@2 Å) detection efficiency, better than 2 mm position resolution and 3 times n/γ resolution ability during the whole operation period of 10 years. The high pressure multi-wire proportional chamber (MWPC) neutron detector filling 3He gas is used as a key detector. Some simulation results and the experimental results show that the optimized thickness of the neutron entrance window should be 9 mm with using the 7075 aluminum alloy, the high pressure chamber should be sealed by the aluminous ring and a gas mixture should be filled with 6 bar 3He+2.5 bar C3H8. The assembled detector can achieve a more than 54% (@2 Å) detection efficiency in the normal operation.With the 100 μm wide collimator slit, the position resolution for X-rays is about 0.235 mm. Therefore, the position resolution for neutron is about 1.4 mm when 2.5 bar propane is used as the stopping gas for proton and triton. In the chamber, the water vapor, the oxygen and the organic impurity gases will reduce the gas gain, cause the detector electrodes to break down and the detector to speed up aging. To solve the outgassing effect of the detector components and keep the stable operation, the recycled device is designed to have the purification function for the working gases. It could purify the working gas at a flow rate of 2 L/min to remove the oxygen, the water vapor and the organic impurity gases. The detector gain increases about 27% with the purification function. Finally, the n/γ resolution and 2D imaging ability of the detector are tested with the 252Cf neutron source in Institute of High Energy Physics, Chinese Academy of Sciences, the peak ratio of neutron to gamma is obtained to be above 5 from the energy spectra and the detecor has good 2D imaging ability. The performance of the high-pressure MWPC neutron detector could meet the requirements for the multipurpose reflectometer, and the detector will be mounted in the CSNS in this year.