In this study, the electronic structure and magnetism of the Heusler alloy Fe2CrGe are investigated using first-principle calculations. Results show that the ground state of Fe2CrGe is antiferromagnetic metal in which Fe ion and Cr ion are in low- and high-spin states of $ S=0 $ and $ S=1 $ , respectively. The energy of the antiferromagnetic state is approximately 0.103 eV less than that of the ferromagnetic state. In addition, when a tetragonal strain is applied to Fe2CrGe, a transition from antiferromagnetic to ferromagnetic material occurs at +1.7% and –1.7% strains, and Fe2CrGe becomes a ferromagnetic half-metal. A half-metal energy gap of approximately 0.2 eV occurs when the strain reaches ±5%. The Curie temperature of Fe2CrGe is estimated to be 393 K, which is much higher than room temperature, indicating that Fe2CrGe may be a potential candidate for spintronic applications.
For a polar molecule subjected to an external electric field, its molecular axis will oscillate around the direction of the electric field, forming pendular states. Taking the two lowest-lying pendular states with magnetic quantum number $M=0 $ as qubit states, we study quantum entanglement of polar molecular arrays trapped in a one-dimensional optical lattice. We evaluate pairwise concurrence and global entanglement as functions of three dimensionless variables related to external field intensity–permanent dipole moment, a rotation constant, dipole-dipole interaction, and temperature —thus revealing the properties of the entangled molecular dipole arrays.
Plasma gratings are important in physics because they do not break down in strong fields. Using particle-in-cell (PIC) simulations, a new mechanism to generate plasma grating was developed based on the interactions between picosecond intense laser pulses (the magnitude of $ I $ is ${10}^{15}\;\mathrm{W}/{\mathrm{c}\mathrm{m}}^{2}$ ) and overcritical solid-density plasma (particle number density $n \approx 10{n}_{\rm{c}}$ ). This plasma grating results from the interference of plasma waves excited by strong laser fields in solids. Hence, one laser beam is sufficient for generating the gratings. The method produced a nanometer spatial period, which is significantly different from the micrometer spatial period produced by traditional methods that use two counter-propagating lasers in gas-density plasma. This finding may be useful for manipulating strong x-band frequency laser fields.
Based on multiple scattering theory, the effect of the configuration of complex unit cells on transmissivity in the negative first diffraction order is studied when negative transmission occurs in metagratings with a complex unit cell; that is, when transmitted beams lie on the same side of the normal as incident beams occurs in such metagratings. The complex unit cells are composed of two dielectric nanorods of different radii that constitute a metagrating when they are arranged in a line. Our calculations show that no stringent requirements on the radius of the smaller rods and their position in a complex unit cell are required in order for the negative transmission to be perfectly efficient. This implies that the configuration of complex unit cells is robust to the negative transmission, and hence, that it is easier to construct a high-efficiency dielectric metagrating with a complex unit cell.
The Kraus operator-sum representation method for mixed-state evolution was used to analyze the change in the fidelity and von Neumann entropy of the final state after decoherent time evolution. The analysis was based on the Jaynes-Cummings model for the initial state set in the depolarization mode. The results show that the fidelity of the quantum state undergoing decoherent evolution exhibits decaying oscillations with time until it becomes stable, while the von Neumann entropy exhibits oscillations of decreasing amplitude with time.
Temporal modes are a set of orthogonal wave-packet modes that can be used to characterize temporal multi-mode quantum light fields. They provide a complete alternate theoretical framework for the description of quantum systems. This study is based on light-induced seeding as an input to stimulated Raman scattering (SRS), whose output, the Stokes field, is the input seed field of the next SRS ; thus, the process of a continuous iterative SRS system is realized. The pump light field is then fixed in a Gaussian waveform and super-Gaussian waveform, and the temporal waveform evolution characteristics of the output Stokes light field under the input of Gaussian waveform seed light with various structures are studied. The seed light injection can obtain the same stable waveform output through iteration, and the FWHM (full-width at the half of the maximum) of the output light field waveform depends on the pump light field. Furthermore, Schmidt mode decomposition is applied to the final stable output waveform, and the eigenvalues of the final output Stokes field are all concentrated in the fundamental mode by numerical calculation. The research on the temporal mode properties of light presented in this paper provides theoretical guidance and experimental reference for the further development and utilization of the quantum resource of temporal modes.
Based on the Monte Carlo algorithm, photon detection during ultrafast evolutionary light emission was simulated and analyzed in this study. In addition, a method for calculating the time-resolved photon second-order correlation function was developed, and the effects of various errors on the photon second-order correlation function were investigated. The results show that the synchronous time jitter (initial time drift) significantly increases the time-resolved second-order correlation function when the light intensity sharply increases at the initial time, and the value of the time-integrated second-order correlation function is high at any delay. The existence of background photon counts causes the value of the zero-delay second-order correlation function of thermal light to approach unity. This study proposes a simplified simulation method for the theoretical study of photon second-order correlation functions in complicated light fields. Furthermore, this study provides theoretical support and numerical analysis methods for the subsequent experimental measurements of second-order correlation functions with ultrahigh time resolution.
The optical response characteristics of a subwavelength lithium niobate film guided-mode resonance metasurface were investigated via simulations. The influences of parameters such as the period, filling factor and etching depth of the etched micro–nano structure on the transmission spectrum were examined, and the effects of light sources with different polarization states and incidence angles on the spectral linewidth were imvestigated. Because of the asymmetric grating structure design, the bound states in the continuum (BIC) decay into a quasi-BIC mode with a high Q value (>10 000), and the second harmonic conversion efficiency of the subwavelength lithium niobate film increases by five orders of magnitude as a result of the local field enhancement effect of the bound state. The simulation results show that a high-efficiency conversion of the second harmonic can be realized in the ultraviolet band when the peak power density of the incident fundamental wave is on the order of ~1 GW/cm2, that is, the ultraviolet second harmonic conversion efficiency emitted after a single pass through the subwavelength lithium niobate film is up to 10–3 orders of magnitude. This study affords ideas and design schemes for improving the nonlinear response characteristics of a micro–nano structure and optical table interface system.
Based on the closed-form t matrix of a two-body system in low-energy short-distance effective field theory, the approximate closed-form three-body T matrix for a zero-spin three-boson system is obtained using the Faddeev equation under two-body contact interactions. In momentum representation, the contact potentials are polynomials, and the Lippmann-Schwinger equation can be simplified to algebraic equations using a factorization trick, facilitating nonperturbative renormalization. However, it is impossible to apply such a factorization trick directly to the Faddeev equation. Therefore, the momenta dependence of the T matrix is “split” such that the factorization trick can still be applied. The closed-form T matrices are then obtained as nonperturbative approximate solutions of the Faddeev equation under the leading and next-to-leading order contact potentials with verified consistency. As in a two-body case, such a closed-form T matrix also facilitates the convenient implementation of the nonperturbative renormalization.
Electron vortex beams were first discovered in systems that have a conservable orbital angular momentum; for systems where orbital angular momentum is not conserved, the existence of the electron vortices is uncertain. This article takes the electrons in the central field as examples and, in case of relativity, constructs a case where the orbital angular momentum is not conserved while the total angular momentum is conserved. When the electrons that carry a fixed total angular momentum propagate along the z-axis, the perturbation solution of the electron vortex beams corresponding to the system at this time is calculated and combined with the Foldy-Wouthuysen (F-W) transformation. Accordingly, we can prove, in the case of relativity central field, that the vortex solution does exist when the electrons with orbital angular momentum propagate along the z-axis. Consequently, the corresponding vortex wave solution and spiral isophase surface are shown in this article.
Theories of gravity with torsion indicate that spin-torsion coupling impacts the propagation of spin particles in a torsion field background. This paper encompasses the Dirac action in a curved spacetime with torsion, generalized geometrical hydrodynamics method from the flat Riemann spacetime to Robertson-Walker spacetime, and obtained semiclassical equations including the flow conservation equation, dynamic equation, and spin evolution equation, which describe the behavior of a spin 1/2 particle in arbitrary curved spacetime with torsion. These equations show that spin-torsion interaction usually causes the particles to deviate from the geodesic. Moreover, a solution for the cosmological background is found with definitive cosmological torsion; it concludes that the motion of the particle under the torsion field is a spiral motion along the propagation path, which the particle would take without torsion. This effect, subsequently, verifies the cosmological models established on theories of gravity with torsion.
Based on previous studies on the scattering of Majorana and Dirac fermions in Schwarzschild spacetime and the effects of the torsion on the scattering of the two fermions, under the weak field approximation of gravity and the lowest order approximation of the perturbation of the gravitational field scattering of fermions, this study decomposes the spin connection into a vector-like part under parity transformation and separately analyzes the effects of the two parts on the scattering matrix elements of the two fermions. A difference is found to exist between the general gravitational field on the quantum scattering matrix elements of the two fermions, where the difference derives from the vector-like part. These findings are then verified in the context of the Kerr gravitational field, where the difference between the scattering matrix elements of the two fermions is determined to be related to the mass and angular momentum of the gravitational source. The difference diminishes in the case of Schwarzschild spacetime when the angular momentum is zero.
The (2 + 1)-dimensional Dirac oscillator is a fundamental model used to study the relativistic extensions of quantum effects and principles. Due to the influence of relativistic effects, including the non-equidistant and negative excitation spectrum and the spin-orbit coupling, the eigenstates are complicated dressed states composed of spin and angular momentum state vectors; in turn, this renders theoretical research difficult. In this work, we decouple the spin and angular momentum state vectors and separate the spin-up and -down components into positive- and negative-energy states, respectively, using the Foldy-Wouthuysen (F-W) transformation. The Hamiltonian and eigenstates of the Dirac oscillator are then largely simplified in the F-W representation; nevertheless, we find the forms of the operators for spin and angular momentum in the same representation with complex combinations of each other. The results are useful in advancing research in relativistic quantum mechanics and spin-orbit coupling.
In this paper, corrections to the octet baryon masses based on the strange quark contribution are calculated using the SU(3) covariant chiral effective theory. We find that the items violating chiral power counting rules are local and can be subtracted by local counterterms which leads to extended minimal subtraction ${(\text{E}}\overline {{\text{MS}}})$ scheme. In addition to the chiral contribution, relativistic correction items are also retained, which is meaningful for accurately calculating the corrections to baryon masses and analytical extrapolation.
In this paper, bandgap tuning of C3N through the stacking pattern, layer number, and external electric field were investigated by employing first-principles density functional theory (DFT) calculations. Four stacking structures—namely AA-1, AA-2, AB-1, and AB-2—were investigated in our study; the calculation results showed that the AB-2 structure was the most energetically favorable. Accurate calculations of the bandgap by the HSE06 hybrid functional revealed a large bandgap difference between the C3N bilayers with AA and AB stacking; specifically, structures with AA stacking had much smaller bandgap than those with AB stacking. Moreover, we found that the bandgap of C3N decreases from 1.21 eV for a single layer to 0.69 eV for the AB-2 bulk structure. By applying a vertical electric field, the bandgap of a C3N bilayer, tri-layer, and four-layer with AB-2 stacking can be tuned to a nearly metallic state.
In this study, we investigated the dynamic behavior of quantum Fisher information (QFI) for the qubit-qutrit system suffering from noisy environments by considering quantum memory; the qubit is located near the event horizon of the Garfinkle-Horowitz-Strominger (GHS) dilation black hole and the qutrit stays at the asymptotically flat region. We proposed an effective strategy to protect QFI under the influence of noise by employing weak measurement and reversal measurement. The results show that QFI decays as the amplitude damping strength increases; meanwhile, QFI is nearly constant with an increase in the phase damping strength. QFI can be improved with the selection of appropriate values for measurement strengths and reversal strengths.
In this paper, a method for testing the Bell correlation between two spatially separated two-mode squeezed Bose-Einstein condensates (BECs) is proposed. Using the referenced method, violation of the Clauser-Horne-Shimony-Holt (CHSH) Bell inequality can be observed. First, the method for producing the required physical states is introduced, and then the Bell correlation is tested by calculating the relevant factors using the normalized expected value of the particle number operator. It is shown that violation of the Bell inequality can be observed when $r \lesssim 0.49$ . One of biggest violations occurs, furthermore, when $r \to 0$ and $B = 2\sqrt 2 $ . The method is highly robust in the presence of noise.
In this paper, we present a new type of atom-light accelerometer (ALA) based on use of an atom-light hybrid interferometer. At present, all-optical accelerometers are the most commonly used and most stable accelerometers on the market, owing to their small size and high accuracy. However, due to measurement bandwidth limitations, their practical application range is limited. Hence, we designed a new type of accelerometer to address this challenge. The atom-light hybrid interferometer is first constructed in the atomic system through the stimulated Raman scattering (SRS) process, and the elastic mass of the accelerometer is formed by a mirror. When the mass is subjected to acceleration on the experimental platform, it will perceive the change in external displacement, thereby introducing the phase into the interferometer. Through the change of the interference fringe, the change of the external phase and the displacement can be determined; hence, the magnitude of the acceleration can be obtained. The primary advantage of the atom-light accelerometer is that the Stokes field generated by the SRS process is phase related to the atomic spin-wave, which ensures the stability of the device phase. Secondly, the adjustable bandwidth of the device increases its scope of application. Finally, theoretical calculations show that its measurement accuracy exceeds the standard quantum limit (SQL) under ideal conditions.
Research on complex networks has given birth to models for understanding evolution dynamics and structure formation; their respective degree growth fluctuations, however, behave very differently. To test the validity of existing models, we carry out an empirical study on two real networks. The results show that both their fluctuation exponents decrease linearly with the observation interval, presenting an interval-dependent picture that has not been predicted by any of the existing models. By exploring the response of the fluctuation to shuffling data, we deduce the interval dependence from the reinforcement of the internal temporal correlation. These results reveal not only the limitations of the existing models, but the complex dynamics of the correlation itself, which is significant for further understanding the underlying mechanism of network evolution.
This paper studies the use of quantum nondemolition (QND) measurement to produce a spin squeezed atomic Bose-Einstein condensate (BEC) in a double-well trap. The spin squeezed atomic Bose-Einstein condensate is performed by putting the BECs of a double well in the two arms of a Mach Zehnder interferometer and performing a QND measurement. The dynamics of the light-atom system are solved using an exact wave-function approach, in contrast to previous approaches where approximations were made using techniques like the Holstein-Primakoff approximation. The backaction of the measurement on atoms is minimized by monitoring the condensate at zero detection current and the identical coherent beams. At the weak atom-light interaction limit, we find that the average spin direction is relatively unaffected by observing the conditional probability distribution and the Q function distribution. The spin variance is squeezed along the axis of optical coupling.
中国综合性科技类核心期刊(北大核心)
