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Vol.69 No.14
20200720

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20200705

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20200605
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GENERAL
2020, 69 (14): 140201.
doi: 10.7498/aps.69.20200154
Abstract +
$ {\gamma }' $ phase with L1_{2} structure and of $ \theta $ phase with DO_{22} structure.The two ordered phases constitute a pseudo binary system. The L1_{0} phase precipitates at the same time as DO_{22}, and the L1_{0} phase gradually transforms into the L1_{2}γ′ phase, while the traditional Ni_{75}Al_{7.5}V_{17.5} alloy first precipitates L1_{0} phase, and then the DO_{22} phase precipitates at the boundary of antiphase domain of L1_{2} phase. In the transition from L1_{0} to L1_{2}, α position of fcc lattice is occupied by Ni atom, and the β position is occupied by Al atom and V atom. The congruent ordering of atoms results in the formation of θ singlephase ordered domain of DO_{22} structure, followed by spinodal decomposition; the nonclassical nucleation of L1_{0} structure gradually transforms into L1_{2}γ′ phase and spinodal decomposition. The interaction potential between the firstnearestneighbor atoms of NiAl increases linearly with temperature, and increases gradually with the increase of long range order parameters. The incubation period of Ni_{60}Al_{20}V_{20} medium entropy alloy lengthens with temperature increasing. This study can be applied to the design of NiAlV medium entropy alloy.">Medium entropy alloys have attracted much attention because of their excellent physical and chemical properties. Nanoscaled L1_{2} structure ordered phase plays an important role in strengthening the mechanical properties of medium entropy alloys, and its local atomic arrangement plays a decisive role in yield strength of medium entropy alloys. In this paper, the microscopic mechanism of the precipitation process of Ni_{60}Al_{20}V_{20} medium entropy alloy is studied by using the micro diffusion phase field dynamics model, in which the probability of atoms to occupy the lattice position is taken as a field variable to describe the configuration of atoms and the morphology of precipitates. In this model, the shape and concentration of precipitate phase, the position and appearance of new phase cannot be set in advance. Combined with the inversion algorithm, the precipitation mechanism of ordered phases of γ' (L1_{2}Ni_{3}Al) and θ (DO_{22}Ni_{3}V) is discussed by analyzing the evolution of atomic images, the change of order parameters and volume fraction. The result shows that two kinds of ordered phases are precipitated in the kinetical process of disordered phase ordering into Ni_{60}Al_{20}V_{20} medium entropy alloys, which are of $ {\gamma }' $ phase with L1_{2} structure and of $ \theta $ phase with DO_{22} structure.The two ordered phases constitute a pseudo binary system. The L1_{0} phase precipitates at the same time as DO_{22}, and the L1_{0} phase gradually transforms into the L1_{2}γ′ phase, while the traditional Ni_{75}Al_{7.5}V_{17.5} alloy first precipitates L1_{0} phase, and then the DO_{22} phase precipitates at the boundary of antiphase domain of L1_{2} phase. In the transition from L1_{0} to L1_{2}, α position of fcc lattice is occupied by Ni atom, and the β position is occupied by Al atom and V atom. The congruent ordering of atoms results in the formation of θ singlephase ordered domain of DO_{22} structure, followed by spinodal decomposition; the nonclassical nucleation of L1_{0} structure gradually transforms into L1_{2}γ′ phase and spinodal decomposition. The interaction potential between the firstnearestneighbor atoms of NiAl increases linearly with temperature, and increases gradually with the increase of long range order parameters. The incubation period of Ni_{60}Al_{20}V_{20} medium entropy alloy lengthens with temperature increasing. This study can be applied to the design of NiAlV medium entropy alloy.
GENERAL
2020, 69 (14): 140301.
doi: 10.7498/aps.69.20200372
Abstract +
Realization of spinor BoseEinstein condensate in an optical trap has made it possible to create a variety of topological nontrivial structures, due to the vector character of the order parameter. Recently, artificial spinorbit coupling in the spinor BoseEinstein condensate, owing to coupling between the spin and the centerofmass motion of the atom, provides an unprecedented opportunity to search for novel quantum states. As is well known, the potential well in the BoseEinstein condensate is adjustable. The toroidal trap is an important model potential because of its simplicity and richness in physics. In particular, the spinor BoseEinstein condensate under the toroidal trap has brought an ideal platform for studying fascinating properties of a superfluid, such as persistent flow and symmetrybreaking localization. For the case of the spinorbitcoupled BoseEinstein condensate, the previous studies of the toroidal trap mainly focused on the twocomponent or antiferromagnetic case. However, in the presence of a toroidal trap, there remains an open question whether the combined effects of the spinorbit coupling and rotation can produce previously unknown types of topological excitations in the ferromagnetic BoseEinstein condensate. In this work, by using quasi twodimensional GrossPitaevskii equations, we study the ground state structure of spinorbit coupled rotating ferromagnetic BoseEinstein condensate in the toroidal trap. We concentrate on the effects of the spinorbit coupling and the rotation on the ground states. The numerical results show that in the presence of a toroidal trap, the ground state structure is displayed as halfskyrmion chain with circular distribution. Adjusting the strength of spinorbit coupling not only changes the number of halfskyrmion in the system, but also controls the symmetry of halfskyrmion with circular distribution. As the rotation frequency increases, the system undergoes the transitions from the plane wave to the halfskyrmion chain with circular distribution, and eventually developing the halfskyrmion phase of triangular lattice. Next, we examine the effect of spinindependent interaction on spinorbit coupled rotating spinor BoseEinstein condensate. As the spinindependent interaction increases, the topological defects in the condensate increase due to the variation of the local magnetic order. We also discuss the influence of well shape on the ground state structure. These topological structures can be detected via the timeofflight absorption imaging technique. The spinorbit coupled spinor BoseEinstein condensate in the toroidal trap is an important quantum platform, which not only opens up a new avenue for exploring the exotic topological structures, but also is crucial for realizing the transitions among different ground states. This work paves the way for futureexploring the topological defects and the corresponding dynamical stability in quantum system subjected to the toroidal trap.
GENERAL
2020, 69 (14): 140501.
doi: 10.7498/aps.69.20191657
Abstract +
In classic statistical physics, an isolated system corresponds to a constant energy shell in the phase space, which can be described by the microcanonical ensemble. While, for an isolated quantum system, the conventional treatment is to subject the system to a narrow energy window in the Hilbert space instead of the energy shell in classical phase space, and then confine the participating eigen states of system wave function in the narrow window, so that the microcanonical ensemble can be recovered in the framework of quantum mechanics. Apart from the traditional theory, there is a more selfconsistent description for the isolated quantum system, that is, the quantum microcanonical (QMC) ensemble. The QMC ensemble abandons the narrow energy window assumption, and allows all the eigen states to contribute to the system wave function on condition that the system average energy is fixed at a given value. At the same time, the total occupation probability of these eigen states is conserved to unity. The most probable probability distribution obtained in the Hilbert space for an isolated quantum system according to the constraints specified above is called the QMC statistics. There is a clear difference between the QMC distribution and the traditional Gibbs distribution having an exponential form. Through the external periodic drives, an isolated quantum system may produce the QMC distribution, which is a consequence of the interplay between internal origins and external drives. In this paper, we investigate the conditions for the formation and suppression of QMC distribution by using the exact diagonalization method based on the onedimensional Ising model. We start with the onedimensional Ising model and focus on three different cases of periodic drives: systems under vertical (along the z axis), horizontal (along the x axis), horizontal magnetic field together with random internal (along the y axis) magnetic field. For all these three cases, the external magnetic fields are set to be ordinary rectangular pulses and the Gibbs distributions are taken as the initial states. We then study the evolutions and their asymptotic tendencies to the QMC distributions of the eigen state occupation probability under the effect of external periodic magnetic field. The results show that under the vertical magnetic field, the eigen state occupation probability does not change, and the system cannot produce the QMC distribution; under the horizontal magnetic field, the system tends to display a QMC distribution, but only partly; under horizontal and random internal magnetic fields at the same time, the transition to QMC distribution can be fully realized, and finally the system is almost completely thermalized. In order to clarify the different behaviors of the Ising model in the three cases, we also calculate the information entropy of the eigen state of Floquet operator in the eigen representation of the unperturbed Hamiltonian. We find that as the information entropy of the Floquet eigen state increases, the convergence to the QMC distribution in the Hilbert space is improved. We also notice that the mechanism for the emergence of QMC distribution is closely related to the thermalization effect of the isolated quantum system. Our analyses show that when the magnetic field is vertical, it cannot trigger the thermalization of the system. When the magnetic field is horizontal, the system becomes partly, but not completely, thermalized. When we add a horizontal periodic magnetic field and a random internal magnetic field at the same time, the system can be completely thermalized to infinite temperature. Thus, the asymptotic behavior towards the QMC statistics is a reflection of the fact that the isolated quantum system is thermalizable under periodic drives.
GENERAL
2020, 69 (14): 140502.
doi: 10.7498/aps.69.20191981
Abstract +
INVITED REVIEW
2020, 69 (14): 140503.
doi: 10.7498/aps.69.20200161
Abstract +
Selforganization represents a ubiquitous transition from disorder to order. It plays a critical role in forming crystalline materials and functional structures in biology. Functional structures are generally hybrid on a multiple scale in which nanostructures are often organized in a specific way such that they can perform functions. There are two typical functional structures: static equilibrium structures and dynamic nonequilibrium structures. In this review, recent advances in understanding and mimicking functional structures are summarized. Although great advances have been achieved, it is still a big challenge to realize dynamic nonequilibrium structures. In this case, we suggest that the controlling of selforganization in active systems may be a route toward interactive and adaptive structures.
GENERAL
2020, 69 (14): 140504.
doi: 10.7498/aps.69.20200133
Abstract +
Grain boundary affects the microstructure of metal material, and thus further its macroscopic properties. As is well known, under the action of applied stress, the grain boundary migrates. The structures and arrangements of grain boundary dislocations at different misorientation angles are very different, which affects the macrophysical and chemical properties of metal crystal. Therefore, it is of great theoretical and practical significance to study the dislocation structure and reaction mechanism of grain boundary under different misorientations for further studying the material properties.The phase field crystal method is used to simulate the lowangle asymmetric tilt grain boundary structure and dislocation motion on a nanoscale. From the perspective of the change of the position of the grain boundary dislocation motion under the applied stress and the change of the free energy of the crystal system, the influences of the misorientation angle on the lowangle asymmetric tilt grain boundary structure and the motion of the grain boundary dislocation are analyzed. The results show that the types of dislocation pairs of lowangle asymmetric tilt grain boundaries at different misorientation angles are the same. With the increase of misorientation angle, the grain boundary dislocation pairs increase, and n1n2 and n4n5 type dislocation pairs are more easily formed at the grain boundaries. Under the action of applied stress, the initial movement states of the grain boundary dislocation pairs at different misorientation angles are all climbing along the grain boundaries. As the system energy accumulates, the larger the misorientation angle is, the more the number of decomposed grain boundary dislocation pairs decomposed will be, and only in the dislocation pairs of n1n2 and n4n5 type there occurs decomposition reaction. There are four stages in the free energy curve of the lowangle asymmetric tilt grain boundary system at different misorientation angles, which correspond to the dislocation pairs climbing, dislocation pairs sliding and decomposition, dislocation pairs reaction to form single crystal, and the free energy rising process of the system. Further research shows that as the misorientation angle increases, the time for the single crystal system formed by the dislocation of grain boundary pairs to annihilate is required to be long.
GENERAL
2020, 69 (14): 140601.
doi: 10.7498/aps.69.20191800
Abstract +
Cold collision frequency shift is one of the major systematic effects which limit the frequency uncertainty of the cesium fountain atomic clock. It is proportional to the effective atomic density, which is defined as the average density over the initial spacial and velocity distribution. The measurement of the frequency shift is based on a differential method, in which the fountain clock is operated with two different atomic densities, i.e. high density and low density, in turn. The clock frequency without collision shift can be achieved by linear extrapolation with the frequencies and density ratios of two states. For the density ratio is estimated with the atom number, it plays a crucial role in generating atoms with same density distribution for reducing systematic uncertainty in cold collision frequency shift estimation. The rapid adiabatic passage method is used in Cesium fountain clock to realize homogeneous transition probability, which modulates the amplitude and frequency of microwave continuously to prepare atom sample. To investigate the precision of this method, theoretical analysis and experimental measurement are both used here. An equation of deviation is derived from the time evolution of Bloch vector. The vector rotates at angular speed Ω with the rotation axis processing at lower angular speed. The deviations in the two directions on the surface of Bloch sphere are determined by the equations which are similar to wave equations, and can be simplified into wave equations when the deviations are sufficiently small. It is shown in the equations that the deviations are stimulated by angular velocity and angular acceleration of the precession, and is inversely proportional to the square of Ω. Further calculation shows that the deviation becomes smaller when the amplitude of microwave frequency and Rabi frequency are close to each other. It is then confirmed experimentally. The effects of some other parameters, such as the pulse length and time delay, on transition probability are also measured, showing that the RAP method is insensitive to these parameters up to a large scope. The precision of RAP method is dominated by three factors. The first factor is the product of rotating angular speed Ω and pulse length T, i.e. ΩT: The increase of ΩT can reduce the uncertainty to a satisfactory degree. The second factor is the uncertainty of resonant frequency, so the measurement is required to be precise. The third factor is the unexpected atoms which are not selected by the microwave, and may be attributed to pulling light. After optimizing the parameters, the ratio of low density to high density can approach to 0.5 with 3 × 10^{–3} uncertainty, which leads to a systematic relative uncertainty of cold collision shift up to 1.6 × 10^{–16}.
ELECTROMAGNETISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
2020, 69 (14): 144201.
doi: 10.7498/aps.69.20200125
Abstract +
In this paper, a nearinfrared laser heterodyne spectrometer developed by the laboratory is used to investigate the inversion of greenhouse gas column concentration and approximately evaluate the system measurement errors based on the optimal estimation algorithm. Firstly, the spectral database and the calculation results from the reference forward model are compared with the groundbased FTIR results, thereby selecting the detection window, the corresponding laser and detector. Secondly, the optimal estimation concentration inversion algorithm based on the reference forward model is established, and the LevenbergMarquardt (LM) iterative method is adopted to realize the inversion of the concentration and vertical distribution profile of atmospheric CO_{2} column in the whole layer, and the longterm observation comparative experiment is carried out to verify the feasibility of this algorithm. Finally, by simulating the selected detection window spectrum in different white noise, the approximate corresponding relationship between the system signalnoiseratio (SNR) and CO_{2} column concentration measuring error is eventually obtained. This research is an indispensable theoretical calculation part of the detection system and will conduce to improving the application of laser heterodyne technology in atmospheric observations.
ELECTROMAGNETISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
2020, 69 (14): 144202.
doi: 10.7498/aps.69.20200496
Abstract +
Recent years, with the rapid development of information technology, the information security has received more and more attention. A variety of encryption methods to protect the information have been reported. Visual cryptography is one of the encryption methods, which has highly security because of its threshold feature. And the cryptographic information can be explained by a naked eye in the decryption process. In the application of visual cryptography, however, each shared image is limited to transparency films and overlapping on computer. In our previous work, we proposed the scheme of invisible visual cryptography and developed the visualcryptographybased optical hiding system (VCOH), which transformed the conventional visual cryptography shares into diffraction optical elements (DOEs). It not only increases the application range of visual cryptography, but also enhances security. In this paper, we propose an optical information hiding system based on the extended visual cryptography, which inherits the concept of invisible visual cryptography. In contrast to our previous work, the method proposed in this work can hide a meaningful image instead of text messages. Meanwhile, the capacity and imperceptibility of the method are greatly increased. The hiding process of the system contains two steps. Firstly, the secret image is converted into meaningful shares through the extended visual cryptography algorithm. Secondly, the meaningful shares are able to hide in phasekeys through an iterative phase retrieval algorithm, such as GerchbergSaxton algorithm and YangGu iterative algorithm. Then the phasekeys can be made into diffraction optical elements (DOEs) to store and transport in a physical way. In the decryption process, DOEs are illuminated with the laser beam to reconstruct the meaningful shares. The secret image can be explained by the direct overlapping of the reconstructed shares without any optical or cryptographic knowledge. The simulation and optical experimental results show that the proposed method has good performance of security and validate the feasibility of the proposed method. Besides, in this paper the robustness and security issues are also analyzed. This system has a high security because of its indistinguishability under adaptive chosen ciphertext attack (INDCCA2) security. Additionally, this system is relatively less robust than the VCOH because it shares meaningful images with highly complex and detailed structures.
ELECTROMAGNETISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
2020, 69 (14): 144203.
doi: 10.7498/aps.69.20200243
Abstract +
Vortex beams with orbital angular momenta with different mode numbers are mutually orthogonal to each other, which makes it possible to improve the information transmission efficiency in space optical communication system. Nevertheless, the implementation of this strategy is limited by the orbital angular momentum crosstalk caused by atmospheric turbulence. Focused LaguerreGaussian vortex beams are less affected by atmospheric turbulence due to their lager intensity density. Consequently, focused LaguerreGaussian vortex beams can be used as the carriers to reduce the orbit angular momentum crosstalk and increase the channel capacity of information transmission. In this paper, based on the spiral spectrum analysis theory, the analytical expression of spiral spectrum of focused Laguerre Gaussian beam propagating in anisotropic atmospheric turbulence is derived. The influences of turbulence and beam parameters on the received power of focused and unfocused Laguerre Gaussian beam are investigated via numerical calculations. Finally, the multiphase screen method is used for verificating the simulation. The research findings are as follows. First, with the increase of transmission distance, turbulence intensity and topological charge, the receiving power of orbital angular momentum decreases, that is, the orbital angular momentum crosstalk turns more serious. Second, the larger the turbulence innerscale, anisotropy index and beam wavelength are, the smaller the orbital angular momentum crosstalk is. Third, when the receiving aperture reaches a certain value, its influence on the orbit angular momentum crosstalk is very small. Fourth, different parameters have different effects on crosstalk, and the orbit angular momentum crosstalk of the focused vortex beam is less than that of the unfocused vortex beam. Therefore, in the vortex optical communication, the focused vortex beams can be used as the signal light to reduce the crosstalk between the orbit angular momentum modes, and thus improving the communication quality. These results have some theoretical reference values for reducing crosstalk in freespace optical communication.
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