Vol. 74, No. 2 (2025)
2025-01-20
GENERAL
2025, 74 (2): 020701.
doi: 10.7498/aps.74.20241542
Abstract +
Terahertz thickness measurement is very important in materials research and industrial test. And it can beused to measure various materials such as wood, paper, ceramics, plastics, and composite materials. Atomic superheterodyne terahertz detector has extremely high sensitivity. The sensitivity of terahertz electric field strength measurement can reach 5.76 μV/(cm·Hz1/2). Simultaneously, the linear dynamic range is better than 60 dB. So, it can be used to precisely measure the thickness of materials through the terahertz transmission efficiency. The experiments in this work demonstrate the thickness measurement of sapphire crystal and organic materials PTFE. The terahertz signal is shown in Fig. (a) sapphire material and Fig. (b) PTFE material. The thickness can be calculated from the transmittance, and the result is consistent with the result measured directly with a vernier caliper. Furthermore, single-layer graphene and few-layer graphene can be clearly distinguished from terahertz transmission signals as shown in Fig. (c) graphene material. Even for niobium meta thin films with thickness of 1 μm, very weak terahertz signal can be well distinguished due to the high sensitivity of atomic superheterodyne terahertz detector. In summary, the technology developed for terahertz thickness measurement based on atomic superheterodyne detection is very important for detecting defects, checking coating, and measuring the parameters of materials.
2025, 74 (2): 020201.
doi: 10.7498/aps.74.20241275
Abstract +
Lithium-ion batteries (LIBs) are widely used in portable electronic devices, electric vehicles, and other fields. With the rapid development of its application fields, there is an urgent need to further improve its energy density and safety. In the charging/discharging process of the LIBs, the diffusion of Li will cause local volumetric change in the electrode material. The degradation and damage of the electrode material structure caused by diffusion-induced deformation is a major obstacle to the development of LIBs. Generally speaking, the electrode materials in LIBs are always subject to specific external constraints, including both inevitable passive structural constraints within the battery and external active constraints that may be imposed by emerging technology application scenarios, which can also affect the mechanical properties of the electrode materials. Therefore, a more in-depth understanding of the diffusion-induced stress and Li concentration changes in the electrode material is an engineering requirement for developing new material design paradigms to improve the overall performance of LIBs. In this work, a two-way diffusion-stress coupling model is used to discuss the effects of the four different levels of idealized deformation constraints on the Li concentration and stress in the bilayer plate electrode in the charging process through the numerical solution. From a mechanical perspective, the bilayer plate electrode structure has two degrees of freedom: lateral expansion and bending deformation. Weakened constraint conditions can partially or completely activate these stress release mechanisms, thereby reducing the overall stress level of the electrode structure and improving its mechanical stability. However, from an electrochemical perspective, the stress gradient generated by the forward bending deformation of the electrode structure can hinder the Li intercalation process. Enhanced constraints can partially or completely suppress the forward bending of the electrode, making the Li concentration in the active layer more uniform and thus improving the capacity utilization efficiency of the active layer. These results not only provide theoretical references for further understanding the chemical-mechanical response of the bilayer electrodes under more realistic or extreme service conditions, but also indicate from a design perspective that compromised external constraints are beneficial for balancing the structural durability and electrochemical performance of electrodes.
2025, 74 (2): 020301.
doi: 10.7498/aps.74.20241592
Abstract +
Anderson localization is a profound phenomenon in condensed matter physics, representing a fundamental transition in eigenstates, which is triggered off by disorder. The one-dimensional Aubry-André-Harper (AAH) model, an iconic quasiperiodic lattice model, is one of the simplest models that demonstrate the Anderson localization transition. Recently, with the growth of interest in quantum lattice models in curved spacetime (CST), the AAH model in CST has been proposed to explore the interplay between Anderson localization and CST physics. Several CST lattice models have been realized in optical waveguide systems to date, but there are still significant challenges to the experimental preparation and measurement of states, primarily due to the difficulty in dynamically modulating the lattices in such systems. In this work, we propose an experimental scheme using a momentum-state lattice (MSL) in an ultracold atom system to realize the AAH model in CST and study the Anderson localization in this context. Due to the individually controllable coupling between adjacent momentum states in each pair, the coupling amplitude in the MSL can be encoded as a power-law position-dependent $J_n \propto n^{\sigma}$, which is conducive to the effective simulation of CST. The numerical calculation results of the MSL Hamiltonian show that the phase separation appears in a 34-site AAH chain in CST, where wave packet dynamics exhibit the localized behavior on one side of the critical site and the extended behavior on the other side. The critical site of phase separation is identified by extracting the turning points of the evolving fractal dimension and wave packet width from the evolution simulations. Furthermore, by modulating the spacetime curvature parameter σ, we propose a method of preparing the eigenstates of the AAH chain in CST, and perform numerical simulations in the MSL. By calculating the fractal dimension of eigenstates prepared using the aforementioned method, we analyze the localization properties of eigenstates under various quasiperiodic modulation phases, confirming the coexistence of localized phase, swing phase, and extended phase in the energy spectrum. Unlike traditional localized and extended phases, eigenstates in the swing phase of the AAH model in CST exhibit different localization properties under different modulation phases, indicating the existence of a swing mobility edge. Our results provide a feasible experimental method for studying Anderson localization in CST and presents a new platform for realizing quantum lattice models in curved spacetime.
THE PHYSICS OF ELEMENTARY PARTICLES AND FIELDS
2025, 74 (2): 021401.
doi: 10.7498/aps.74.20241529
Abstract +
Fast neutron multiplicity measurement technology is an important non-destructive testing technology in the field of arms control verification. In the technique, the liquid scintillation detector is used to detect the fission neutron and combined with the time correlation analysis method to extract multiplicity counting rates from the pulse signals. This technique is commonly used to measure the mass of nuclear materials, however, it is based on the point model that assumes that the neutron multiplication coefficient keeps constant in the whole spatial volume, which will lead to overestimation of the multiplication coefficient and result in system deviation. To correct the deviation and improve the measurement accuracy, the fast neutron multiplicity simulation measurements are carried out on spherical and cylindrical samples in this work. The relationship among the position of neutron generation, absorption and net growth in the space volume of the material is obtained. According to the definition of the leakage multiplication coefficient, the leakage multiplication coefficients at different positions in the space volume of the material are calculated. On this basis, a method based on spatial multiplication coefficient correction is proposed according to the functional relationship between neutron multiplicity factorial moments and the unknown parameters. In this method, the n-order multiplication coefficient is modified by introducing a weight factor $ {g_n} $, and the fast neutron multiplicity weighted point model equation is derived. To verify the accuracy of this method, a set of fast neutron multiplicity detection model is built by Geant4, and the fast neutron multiplicity simulation measurement is carried out on the spherical and cylindrical samples. The results show that the solution accuracy of the weighted point model equation is higher than that of the standard point model equation, and the measurement deviation is reduced to less than 6 %. This work provides an optimization method for solving plutonium samples with several kilograms in mass, and promotes the development of the fast neutron multiplicity measurement technology.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
2025, 74 (2): 028101.
doi: 10.7498/aps.74.20241294
Abstract +
Although phosphorescent organic light-emitting devices (OLEDs) can have an internal quantum efficiency (IQE) of 100%, the IQE usually decays at high current densities due to triplet-triplet annihilation. Phosphor-sensitized fluorescence can realize the energy transfer between phosphorescent emitter and fluorescent emitter, and can be used to suppress the efficiency fluctuations and adjust the color of the device. With this in mind, white light emission including different colors of phosphorescent emitter and fluorescent emitter can be expected. Herein, phosphor-sensitized fluorescent white OLEDs are fabricated by combining ultra-thin layer insertion and doping, in which laser dyes DCM (4-(Dicyanomethylene)-2-methyl-6-(4-dimethyl-aminostyryl)-4H-pyran), iridium complexes Ir(ppy)3 (tris(2-phenylpyridine)iridium), and biphenyl ethylene derivatives BCzVB (1,4-bis[2- (3-N-ethylcarbazoryl)vinyl]benzene) are used as red, green and blue emitters, respectively. By adjusting the doping concentration of Ir(ppy)3 phosphorescent green emitter in CBP (4,4’-N,N’-dicarbazole-biphyenyl) host, with ultra-thin layers of BCzVB fluorescent blue emitter on both sides of CBP:Ir(ppy)3 doping system and with ultra-thin layer of DCM fluorescent red emitter inserting in CBP:Ir(ppy)3 layer, the three colors can be balanced. White emissions are obtained in the device, the highest external quantum efficiency is 2.5% (current efficiency of 5.1 cd/A), the maximum brightness is 12400 cd/m2, and Commission Internationale de l'Eclairage (CIE) co-ordinates can reach the ideal white light equilibrium point (0.33, 0.33) at a current density of 1 mA/cm2. The acquisition of white light is attributed to the appropriate doping ratio of Ir(ppy)3 and the position of DCM, which effectively balances the emission ratio of three primary colors: red, green, and blue. The results indicate that the partially energy transfer of triplet excitons to singlet excitons by phosphor-sensitized fluorescence scheme can be used to realize high-efficiency white organic electroluminescent devices, thereby reducing energy consumption and providing more room for promoting OLED applications.
2025, 74 (2): 028103.
doi: 10.7498/aps.74.20241581
Abstract +
Shrinkage cavities and porosity are the main defects generated in the solidification process of castings. These defects are caused by the alloy’s contraction during solidification, with the final solidified area not being effectively compensated for by the liquid metal, resulting in cavitation defects. Shrinkage cavities and porosity significantly reduce the mechanical properties of castings and shorten their service lives, thus necessitating appropriate process to eliminate them. Utilizing numerical simulation technology can effectively predict the shrinkage of castings during solidification and optimize the process based on simulation results, thereby reducing the occurrence of shrinkage defects, which is a low-cost and high-efficiency method. In this work, a machine learning-driven dynamic mesh model is established to simulate the dynamic shrinkage behavior of castings during solidification. Cellular automata are used to simulate the solidification process of castings, dynamically marking the displacement of boundary points and calculating the displacement of other grids using RBF neural network algorithms and support vector machine algorithms, thereby achieving the dynamic simulation of the solidification process. The model is used to simulate the shrinkage cavity morphology of the Al-4.7%Cu alloy solidification process, and corresponding casting experiments are designed for verification. Comparisons between simulation results and experimental results indicate that this coupled method can effectively capture the casting deformation caused by solidification shrinkage, the evolution of complex solid-liquid interface morphologies, and the deformation of internal grids within the castings. Compared with the experimental results, the simulation results have an error of no more than 2%, providing a new approach for numerically simulating the solidification process.
2025, 74 (2): 028701.
doi: 10.7498/aps.74.20241465
Abstract +
2025, 74 (2): 028401.
doi: 10.7498/aps.74.20241549
Abstract +
Normal (n-i-p) perovskite solar cells (PSCs) have received increasing attention due to their advantages such as high conversion efficiency and good stability. Tin dioxide is an ideal electron transport layer material for normal perovskite solar cells. Among various available electron transport layers, tin dioxide stands out because of its excellent stability, low density of defect states, and appropriate energy levels. The interface defects between tin dioxide and perovskite are the key factors restricting the improvement of the conversion efficiency in perovskite solar cells. Therefore, a method of fabricating normal perovskite solar cells based on the buried interface modification strategy is proposed in this work. By doping methylammonium bromide into tin dioxide to form a buried interface, the interface defects between tin dioxide and perovskite are reduced, the electron mobility of tin dioxide is enhanced, and the growth of high-quality perovskite materials is promoted. The conversion efficiency of the normal perovskite solar cells reaches 23.12%, providing an effective strategy for fabricating high-efficiency normal perovskite solar cells.
2025, 74 (2): 028102.
doi: 10.7498/aps.74.20241438
Abstract +
Diamond nitrogen vacancy (NV) color centers have good stability at room temperature and long electron spin coherence time, and can be manipulated by lasers and microwaves, thereby becoming the most promising structure in the field of quantum detection. Within a certain range, the higher the concentration of NV color centers, the higher the sensitivity of detecting physical quantities is. Therefore, it is necessary to dope sufficient nitrogen atoms into diamond single crystals to form high-concentration NV color centers. In this study, diamond single crystals with different nitrogen content are prepared by microwave plasma chemical vapor deposition (MPCVD) to construct high-concentration NV color centers. By doping different amounts of nitrogen atoms into the precursor gas, many problems encountered during long-time growth of diamond single crystals under high nitrogen conditions are solved. Diamond single crystals with nitrogen content of about 0.205, 5, 8, 11, 15, 36, and 54 ppm (1 ppm = 10–6) are prepared. As the nitrogen content increases, the width of the step flow on the surface of the diamond single crystal gradually widens, eventually the step flow gradually disappears and the surface becomes smooth. Under the experimental conditions in this study, it is preliminarily determined that the average ratio of the nitrogen content in the precursor gas to the nitrogen atom content introduced into the diamond single crystal lattice is about 11. Fourier transform infrared spectroscopy shows that as the nitrogen content inside the CVD diamond single crystal increases, the density of vacancy defects also increases. Therefore, the color of CVD high nitrogen diamond single crystals ranges from light brown to brownish black. Compared with HPHT diamond single crystal, the CVD high nitrogen diamond single crystal has a weak intensity of absorption peak at 1130 cm–1 and no absorption peak at 1280 cm–1. Three obvious nitrogen-related absorption peaks at 1371, 1353, and 1332 cm–1 of the CVD diamond single crystal are displayed. Nitrogen atoms mainly exist in the form of aggregated nitrogen and single substitutional N+ in diamond single crystals, rather than in the form of C-defect. The PL spectrum results show that defects such as vacancies inside the diamond single crystal with nitrogen content of 54 ppm are significantly increased after electron irradiation, leading to a remarkable increase in the concentration of NV color centers. The magnetic detection performance of the NV color center material after irradiation is verified, and the fluorescence intensity is uniformly distributed in the sample surface. The diamond single crystal with nitrogen content of 54 ppm has good microwave spin manipulation, and its longitudinal relaxation time is about 3.37 ms.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
2025, 74 (2): 027501.
doi: 10.7498/aps.74.20241340
Abstract +
Rare-earth elements share similar ground-state electronic properties, and their unique lanthanide contraction effect can lower the mixing enthalpy of rare-earth elements in high-entropy materials, which is of great significance for fabricating low-cost and high-performance high-entropy rare-earth intermetallic compounds. In this work, the magnetization reversal mechanisms of rapidly quenched ribbons such as Nd11.76Fe82.36B5.88 (NdFeB) and the relevant high-entropy rare-earth permanent magnet alloy compounds (La0.2Pr0.2Nd0.2Gd0.2Dy0.2)11.76Fe82.36B5.88 and (La0.2Pr0.2Nd0.2Gd0.2Tb0.2)11.76Fe82.36B5.88 are studied by analyzing the magnetization and demagnetization curves, supplemented by Henkel curves and magnetic viscosity coefficient S. Compared with the pure NdFeB sample, the high-entropy rare-earth permanent magnet has the inter-grain exchange coupling significantly enhanced and the magnetic dipole interaction weakened, indicating that the element diffusion mechanism in heavy rare-earth containing high-entropy material homogenizes the sample, and significantly increases the coercivity. The mechanism of the coercivity is the nucleation of magnetization reversal domains in the grains of the hard magnetic phase. The magnetization mechanism is dominated by pinning at low magnetic fields and by nucleation at high magnetic fields, which is different from the magnetization mechanism of pure NdFeB and has some similarities with the self-pinning mechanism. The magnetic viscosity coefficient of (La0.2Pr0.2Nd0.2Gd0.2Dy0.2)11.76Fe82.36B5.88 is larger than that of pure NdFeB. Due to the asynchrony of hard magnetic phase reversal and intergranular magnetic coupling in (La0.2Pr0.2Nd0.2Gd0.2Tb0.2)11.76Fe82.36B5.88, the magnetic viscosity coefficient is small but the anisotropy field is large. This indicates that high-entropy sample reduces the magnetocrystalline anisotropy field barrier but increases the magnetocrystalline coupling length. This suggests that the magnetization reversal of high-entropy rare-earth permanent magnet material is significantly different from that of conventional rare earth permanent magnet material and it is worthy of further in-depth research.
DATA PAPERS
2025, 74 (2): 023102.
doi: 10.7498/aps.74.20241325
Abstract +
The effects of different concentrations of Fe doping on the photoelectric properties of two-dimensional (2D) CuI semiconductor are studied based on the first-principles calculation method. The results show that both intrinsic 2D CuI and Fe-doped 2D CuI are direct band gap semiconductors. The total state density and partial wave state density of 2D CuI doped with different concentrations of Fe show that the increase in the number of energy bands at Fermi level is due to the influence of Fe-d and Fe-p orbital contributions after Fe doping, which can improve the conductivity of 2D CuI. With the increase of Fe doping concentration, the peak value of ε1 decreases gradually, and the peak value moves toward the high-energy end near the relatively high energy 3 eV and 6 eV, and the greater the concentration, the more obvious the shift is. These results indicate that Fe doping can enhance the high temperature resistance of 2D CuI. When a small amount of Fe is doped, the ε2 peak value increases, indicating that the ability of material to absorb electromagnetic waves is enhanced, which can stimulate more conductive electrons, and with the increase of Fe doping concentration, the absorption capability decreases, so the conductivity of 2D CuI is inhibited. The absorption coefficient of intrinsic 2D CuI and Fe-doped 2D CuI indicate that the semiconductor has strong ability to absorb photons in the ultraviolet region. The 2D CuI reflection coefficient of doped Fe atoms increases gradually with the increase of metallic properties of doped elements. This study provides theoretical reference for applying the 2D semiconductor materials and 2D CuI to optoelectronic devices. All the data presented in this paper are openly available at https://doi.org/10.57760/sciencedb.j00213.00060 .
ELECTROMAGNETISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
2025, 74 (2): 024702.
doi: 10.7498/aps.74.20241487
Abstract +
2025, 74 (2): 024201.
doi: 10.7498/aps.74.20241371
Abstract +
2025, 74 (2): 024701.
doi: 10.7498/aps.74.20241307
Abstract +
In the re-entry process of the vehicle into the atmosphere, the high-temperature environment, induced by the compression of the strong shock wave and viscous retardation, is created around the head of a vehicle. These generate a conductive plasma flow field, which provides a direct working environment for the application of magnetohydrodynaimic (MHD) control technology. Numerical simulations based on thermochemical non-equilibrium MHD model are adopted to analyze the surface heat flux of an orbital reentry experiment (OREX) vehicle. The influences of wall catalytic conditions on the aerothermal environment under different flight conditions are discussed. In addition, the control mechanism of an external magnetic field on high-temperature thermochemical non-equilibrium flow field is analyzed. The results show that the distribution of surface heat flux monotonically increases with the catalytic recombination coefficient increasing, and the surface heat flux rises and then drops with the flight altitude decreasing. Moreover, the wall catalytic properties significantly affect the efficiency of MHD control technology, and the total heat flux is closely related to the accumulation of atomic components, diffusion gradient and temperature gradient near the wall region. With an external magnetic field applied, the accumulation of oxygen atoms and nitrogen atoms near the wall can be reduced. Moreover, the Lorentz force can increase the shock standoff distance, and then reduce the component diffusion gradient and wall temperature gradient. Under three different wall catalytic conditions, the ability to control the surface heat flux MHD is ranked from strong to weak as fully catalyzed, partially catalyzed and non-catalyzed.