bluuebunny/crossref_metadata_2025 huggingface dataset:
Energy level splitting and parity oscillation in Lipkin-Meshkov-Glick model
Authors: Yu Yi-Xiang  , Song Ning-Fang  , Liu Wu-Ming  ,   Institute of Optics and Electronics, School of Instrument Science and Opto-electronics Engineering, Beihang University, Beijing 100191, China;,   Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
Published: December 2020

The Lipkin-Meshkov-Glick (LMG) model originally describes a Fermionic many-body system in nuclear physics. However, in recent years, it has been widely found in condensed matter physics, quantum information systems, and quantum optics, and it is of wider and wider interest. Previous studies on this model mainly focused on the physics under the thermal dynamical limit, such as quantum phase transitions and quantum entanglement. There are also some researches about LMG model with finite size in some special limits, but the finite-size effect on energy spectrum is not very clear yet. This is the main motivation of this work.
In this paper, the exact diagonalization method and the quantum perturbation theory are used to calculate and analyze the energy-level structure of the LMG model at a finite N. To solve it, we first map this model into the angular-momentum space to obtain a reduced LMG model. By this mapping, the dimension of Hilbert space is reduced to N+1 from 2N. The exact solution of its energy levels can be obtained easily in the U(1) limit where the total spin is conserved. We find that the levels are woven into a fishing-net structure in the U(1) limit. While away from the U(1) limit, the crossings between even and odd levels will open a gap, and the system's energy levels will be grouped into pairs with an odd and an even level, forming some bound states, called doublet states, and the parity of each doublet state will oscillate as the Zeeman field increases. This work gives the values of the critical Zeeman field for the parity crossings. These critical values shift as the interacting parameters and disappear at zero in the Z2 limit. In the Z2 limit, the system energy levels form splittings near the zero Zeeman field. In this article, we analytically calculate the relationship between these energy gaps and the Zeeman field. For odd and even number N, the parity of each state has a different behavior. Specifically, the ground state and the doublet excited states of the system with odd N will suffer a parity reversion at zero Zeeman field, while the states with even N will not. By tuning the interacting parameters, we also study the crossover from the U(1) limit to the Z2 limit. The parity oscillation we find in this system is a very important physical phenomenon, which also exists in some other systems like optical cavity quantum electrodynamics and magnetic molecule system.
Broadband absorption caused by coupling of epsilon-near-zero mode with plasmon mode
Authors: Wang Dong  , Xu Jun  , Chen Yi-Hang  ,   School of Physics and Telecommunication Engineering, South China Normal University, Guangzhou 510006, China
Published: December 2020

Epsilon-near-zero mode provides a new path for tailoring light-matter interactions on a nanoscale because of its unique characteristics and ability to be used in many scientific fields. Among these applications, broadband absorption has aroused the considerable interest in photonic research. In this paper, we first show that the surface plasmon resonance is excited by the metal disk array structure without dysprosium-doped cadmium oxide nanolayer, and the structure achieves the local effect of light at a certain wavelength. In addition, in order to be able to use this new technique to achieve a broadband absorption, we take advantage of the surface plasmon resonance to excite the epsilonnear-zero mode which cannot be excited under normal incidence but has a very large density of states. Then, we show that over one order of magnitude increase in the absorption band of a periodically patterned metal-dielectric-metal structure can be obtained by integrating a dysprosium-doped cadmium oxide material into the insulating dielectric gap region. We analyze the absorption band at mid-infrared wavelength comprising plasmonic metamaterial resonators and epsilon-near-zero modes supported by dysprosium-doped cadmium oxide material. The two resonance modes lie in the weak coupling regime and achieve a 470 nm wideband light absorption. Finally, we perform numerical simulations by using the finite-difference-time-domain method to investigate the relationship between the epsilon-near-zero mode and the surface plasmon resonance mode. It is sure that the whole broadband mightily has the local effect of light. The epsilon-near-zero mode mainly is excited at the short wavelength of the broadband, and the surface plasmon resonance mode mainly focuses on long wavelength of the broadband. The simulation demonstrates that the two resonance modes are coupled to achieve a broadband absorption. Additionally, the dielectric constants are tunable by doping density, resulting in plasma frequency change, where the real part of the dielectric constant becomes zero at plasma frequency. Broadband absorption theoretically can be realized in any band of mid-infrared wavelength due to plasma frequency changing. Broadband absorption relaxes the single wavelength condition in previous absorption studies, and compared with the narrowband absorption, broadband absorption at present has many applications, such as in absorbers, sensors, filters, coherent thermal emitters, microbolometers, photodectors, solar cells, fingerprint recognition, energy harvesting devices, etc.
First-principles study on half-metallic ferromagnetism of half-Heusler alloys VLiBi and CrLiBi
Authors: Yao Zhong-Yu  , Sun Li  , Pan Meng-Mei  , Sun Shu-Juan  , Liu Han-Jun  ,   School of Physics and Electronic Engineering, Hainan Normal University, Haikou 571158, China
Published: December 2020

The hypothetical half-Heusler alloys VLiBi and CrLiBi containing only one transition metal element are constructed. The electronic structure and magnetic properties of VLiBi and CrLiBi are investigated by using the first-principles full-potential linearized augmented plane wave method based on density functional theory. The spin-polarized calculations of electronic structure for the half-Heusler alloys VLiBi and CrLiBi are performed. The calculation results reveal that VLiBi and CrLiBi are half-metallic ferromagnets with the half-metallic gaps of 0.25 eV and 0.46 eV and the total magnetic moments of 3.00 μB and 4.00 μB per formula unit, respectively. The total magnetic moments mainly originate from the magnetic moment on V or Cr atom. Li and Bi have small atomic magnetic moments, where the atomic magnetic moment of Bi is negative. The mean field approximation method is used to estimate the Curie temperatures of the alloys. The calculated results show that the values of Curie temperature for VLiBi and CrLiBi are 1401 K and 1551 K, respectively. To study the robustness of the half-metallicity with the change of lattice constant, the electronic structures of VLiBi and CrLiBi are also calculated under their lattice constant changing from-10% to +10% relative to the equilibrium lattice constant. It is found that the VLiBi and CrLiBi can maintain their half-metallicity and retain their total magnetic moments of 3.00 μB and 4.00 μB per formula unit even when their lattice constants change from-5.6% to 10.0% and from-6.9% to 10.0%, respectively. To discuss the effect of strongly correlated interaction on the half-metallicity, the electronic structure of VLiBi and CrLiBi are calculated by the LDA+U method with U for V-3d and Cr-3d orbital. The calculation results indicate that VLiBi and CrLiBi can keep their half-metallicity and integer total magnetic moments of 3.00 μB and 4.00 μB when the value of U reaches to 5 eV. Also, the electronic structure of VLiBi and CrLiBi are recalculated by the GGA+SOC method. The calculated results show that 1) there are some spin-down bands crossing the Fermi level, 2) the spin polarizations of VLiBi and CrLiBi at the Fermi level are 98.8% and 94.3%, respectively, and 3) total magnetic moments of VLiBi and CrLiBi are 3.03 μB and 4.04 μB per formula unit, respectively. The spin-orbit coupling has a weak effect on the half-metallic of half-Heusler alloy VLiBi and the spin polarization is still high for the half-Heusler alloy CrLiBi. The half-Heusler alloys VLiBi and CrLiBi may be useful in spintronics and other applications.
Suppression of lateral vibration in rectangular ultrasonic plastic welding tool based on phononic crystal structure
Authors: Zhao Tian-Tian  , Lin Shu-Yu  , Duan Yi-Lin  ,   Shaanxi Key Laboratory of Ultrasonics, Shaanxi Normal University, Xi'an 710119, China;,   College of Science, Xi'an Shiyou University, Xi'an 710065, China
Published: December 2020

Ultrasonic welding is one of the main applications of high-power ultrasound and is used in the automotive industry and aerospace. Transducers and tool are important parts of the ultrasonic welding system. Different tools are required for different welding objects. For larger plastic welded parts, it is necessary to weld them with large-sized welding tools. Due to the large size of the welding tool, under the excitation of the transducer, the tool will produce a coupling effect of longitudinal vibration and lateral vibration. Lateral vibration will cause the radiation surface of the tool to be non-uniformly displaced, and the working efficiency and welding results of the welding system will also be affected. So, in this paper, the phononic crystal bandgap theory and coupling vibration theory are used to study the coupled vibration of large-sized rectangular plastic ultrasonic welding tools. In order to improve the work efficiency and radiation surface's displacement uniformity of the tool, the phononic crystal structure is used to suppress the lateral vibration of the large-sized plastic ultrasonic welding tool, and the lateral vibration band gap of the phononic crystal structure is calculated. The longitudinal resonance frequency of the system is designed in the band gap range of the lateral vibration of the tool. So the lateral vibration of the tool can be effectively suppressed. The longitudinal vibration displacements on the radiation surface of the rectangular tool before and after vibration suppression are analyzed and compared with each other. The vibration mode of the ultrasonic welding system is simulated by the Comsol Multiphysics finite element software. The large-scaled tool with phononic crystal structure has a radiation surface displacement compared with the tool without phononic crystal structure, and the results show that the radiation surface displacement with phononic crystal structure will increase and tend to be uniform, greatly optimize the welding effect, improve the working efficiency of the welding system, and meet the needs of practical engineering. It is concluded that the longitudinal resonance frequency of the ultrasonic plastic welding system within the lateral vibration bandgap on the phononic crystal structure can not only suppress the lateral vibration, but also make the longitudinal displacement of the radiation surface more uniform and larger. Therefore, the work efficiency is greatly improved.
Band structure model of modified Ge for optical device application
Authors: Yang Wen  , Song Jian-Jun  , Ren Yuan  , Zhang He-Ming  ,   Key Laboratory of Wide Band-Gap Semiconductor Materials and Devices, School of Microelectronics, Xidian University, Xi'an 710071, China
Published: December 2020

Ge is an indirect bandgap semiconductor, which can be converted into a direct bandgap semiconductor by using the modification techniques. The carrier radiation recombination efficiency of modified Ge is high, which can be used in optical devices. The mobility of Ge semiconductor carriers is higher than that of Si semiconductor carriers, so Ge device can work fast and have good frequency characteristics in electronic device. In view of the application advantages of modified Ge semiconductors in both optical devices and electrical devices, it has been a potential material of monolithic optoelectronic integration. The Ge and GeSn as optoelectronic device materials have a great competitive advantage, but there is no mature Ge-based monolithic photoelectric integration. In order to realize Ge-based optical interconnection, the bandgap of luminous tube, detector and waveguide active layer material must satisfy the following sequence:Eg,waveguide Eg,luminoustube Eg,detector. Therefore, in order to achieve the same layer monolithic photoelectric integration, we must modulate the energy band structure of the active layer material of the device. Unfortunately, the literature in this area is lacking.
The band structure is one of the theoretical foundations for the monolithic photoelectric integration of the modified Ge materials, but the work in this area is still inadequate. In this paper, this problem is investigated from three aspects. 1) Based on the generalized Hooke's law and the principle of deformation potential, a modified Ge bandgap type transformation model is established under different modification conditions, perfecting the theory of converting the indirect switching into direct band gap of Ge. 2) On the basis of establishing the strain tensor and deformation potential model, a modified Ge band E-k model is established, and the relevant conclusions can provide key parameters for LED and laser device simulation models. 3) Based on the theory of solid energy band, the bandgap width modulation scheme of the modified Ge under the uniaxial stress is proposed, which provides an important theoretical reference for realizing the Ge-based single-layer photoelectric integration. The results in this paper can provide an important theoretical basis for understanding the material physics of the modified Ge and designing the active layers of the light emitting devices in the Ge based optical interconnection.
Regulation of sensitivity of Yb concentration to power-dependent upconversion luminescence colors
Authors: Gao Dang-Li  , Li Lan-Xing  , Feng Xiao-Juan  , Chong Bo  , Xin Hong  , Zhao Jin  , Zhang Xiang-Yu  ,   College of Science, Xi'an University of Architecture and Technology, Xi'an 710055, China;,   College of Science, Chang'an University, Xi'an 710064, China
Published: December 2020

Controlling the power density of exciting light is a widely applied technological approach to dynamically tuning emission spectra to yield desirable luminescence properties, which is essential for various applications in laser devices, cancer cell imaging, biomarker molecule detections, thermometers and optoelectronic devices. However, most of upconversion systems are insensitive to power regulation. In this study, a series of Yb/Ho doped NaYF4 microrods with different Yb concentrations was synthesized by using a sodium citrate-assisted hydrothermal method. The dependence of upconversion characteristics of NaYF4:Yb/Ho microrods on Yb concentration and excitation power density are investigated in detail by a laser confocal microscopy system. The emission spectra exhibit discrete upconversion emission characteristic peaks that can easily be assigned to 5F3→5I8 (at about 488 nm), 5F4, 5S2→5I8 (at about 543 nm), 3K7, 5G4→5I8 (at about 580 nm) and 5F5→5I8 (at about 648 nm) transitions of Ho, respectively. The upconversion spectra and synchronous luminescence imaging patterns show that the luminescence ratio of red to green is not only dependent on the Yb concentration, but also sensitive to the excitation power. With Yb concentration increasing from 5% to 60%, the sensitivity of the power-controlled red to green luminescence ratio largely increases from 0.1% to 13.0%, corresponding to a clear luminescent color modification from green to red. These results indicate that the power-tuned red-to-green-luminescence ratio can be used as a method of measuring and evaluating Yb doping concentration. We attribute the sensitivity tuned by Yb concentration to the differences in population approach and upconversion mechanism for the red and green luminescence. By recording the slope of luminescence intensity versus exciting power density in a double-logarithmic presentation, we detect a small slope for the green emission relative to that for the red emission, especially at a high Yb concentration. These results indicate that the red upconversion process may be a three-photon process. The exciting power induced color adjusting is therefore explained by preferential three-photon population of the red emission due to the high 5S2→5G4 excitation rate, which is verified by down-conversions of emission spectra. Our present study provides a theoretical basis for the spectral tailoring of rare-earth micro/nano materials and supplies a foundation for the applications in rare-earth materials.
Current transport mechanism of Schottky contact of Pt/Au/n-InGaN
Authors: Xu Feng1\2  , Yu Guo-Hao  , Deng Xu-Guang  , Li Jun-Shuai  , Zhang Li  , Song Liang  , Fan Ya-Ming  , Zhang Bao-Shun  ,   Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123, China;,   Institute of Opto-Electronic, Nanjing University & Yangzhou, Yangzhou 225009, China
Published: December 2020

The Pt/Au Schottky contacts to InGaN samples with different background carrier concentrations are fabricated. The crystal qualities of InGaN samples are characterized by X-ray diffraction (XRD) and atomic force microscope (AFM), and the correlation between threading dislocation density of InGaN and growth temperature is further clarified. The full width at half maximum (FWHM) values of the InGaN (0002) XRD rocking curves show that the density of threading dislocations in InGaN, which can seriously deteriorate InGaN crystal quality and surface morphology, decreases rapidly with increasing growth temperature. The Hall measurements show that the background carrier concentration of InGaN increases by two orders of magnitude as growth temperature decreases from 750 to 700℃, which is due to a reduced ammonia decomposition efficiency leading to the presence of high-density donor-type nitrogen vacancy (VN) defects at lower temperature. Therefore, combining the studies of XRD, AFM and Hall, it can be concluded that the higher growth temperature is favorable for realizing the InGaN film with low density of VN defects and threading dislocations for fabricating high-quality Schottky contacts, and then the barrier characteristics and current transport mechanism of Pt/Au/n-InGaN Schottky contact are investigated by current-voltage measurements and theory analysis based on the thermionic emission (TE) model and thermionic field emission (TFE) model. The results show that Schottky characteristics for InGaN with different carrier concentrations manifest obvious differences. It is noted that the high carrier concentration leads to the Schottky barrier height and the ideality factor obtained by TE model are quite different from that by TFE model due to the presence of high density of VN defects. This discrepancy suggests that the VN defects lead to the formation of the tunneling current and further reduced Schottky barrier height. Consequently, the presence of tunneling current results in the increasing of total transport current, which means that the defects-assisted tunneling transport and TE constitute the current transport mechanism in the Schottky. However, the fitted results obtained by TE and TFE models are almost identical for the InGaN with lower carrier concentration, indicating that TE is the dominant current transport mechanism. The above studies prove that the Pt/Au/n-InGaN Schottky contact fabricated using low background carrier concentration shows better Schottky characteristics. Thus, the properly designed growth parameters can effectively suppress defects-assisted tunneling transport, which is crucial to fabricating high-quality Schottky devices.
Effect of detection efficiency on phase sensitivity in quantum-enhanced Mach-Zehnder interferometer
Authors: Li Shi-Yu  , Tian Jian-Feng  , Yang Chen  , Zuo Guan-Hua  , Zhang Yu-Chi  , Zhang Tian-Cai  ,   1. Collaborative Innovation Center of Extreme Optics, State Key Laboratory of Quantum Optics and Quantum Optics Devices, Institute of Opto-Electronics, Shanxi University, Taiyuan 030006, China;,   2. College of Physics and Electronic Engineering, Shanxi University, Taiyuan 030006, China
Published: December 2020

Three kinds of quantum light sources:Fock state, correlated Fock-state and squeezed vacuum state, which serve as the injection end of Mach-Zehnder interferometer (MZI) are investigated. The effect of detection quantum efficiency on the sensitivity of phase measurement in MZI is analyzed by using the intensity difference detection scheme. By analyzing the MZI system, the quantitative relationship between the sensitivity of phase measurement and the detection efficiency is obtained. It is found that the phase sensitivity cannot go beyond the standard quantum limit in any case when the Fock state is injected into interferometer, that is, the Fock state does not realize quantum enhanced measurement (QEM). And the injection of correlated Fock-state or squeezed vacuum state of light can go beyond the standard quantum limit, but the conditions for realizing quantum enhancement are different, quantum enhancement can only be achieved when the detection efficiency is greater than 75% for correlated Fock-state, or the squeezed vacuum state of light is injected into interferometer. There is no limitation of the minimum detection efficiency for realizing quantum enhancement on squeezed vacuum state. In principle, quantum enhancement can be achieved as long as the squeezed vacuum state is injected. The influence of detection efficiency on the phase sensitivity is investigated when the correlated Fock-state and the squeezed vacuum state are injected into the MZI. It is found that the phase sensitivity or quantum enhancement becomes better as the quantum efficiency of the detection system turns higher. And it is the squeezed vacuum state injected into the interferometer that has better quantum enhancement effect than the correlated Fock-state. In this study, the requirements for the detection efficiency for realizing QEM in experiment are given, which is of great significance for studying the QEM, when taking the real experimental system into account. In addition, the conclusions obtained from the MZI model discussed can also be used to analyze the sensitivity of detecting the gravitational wave, it explains that the improvement of detector efficiency can indeed improve the sensitivity to gravitational wave detection, which will play an important role in exploring gravitational waves and understanding the time and space to reveal the mystery of the universe in the future.
Solubility and ion-irradiation effects of uranium in Nd2Zr2O7 pyrochlore
Authors: Wang Lie-Lin  , Li Jiang-Bo  , Xie Hua  , Deng Si-Hao  , Zhang Ke-Xin  , Yi Fa-Cheng  ,   Fundamental Science on Nuclear Wastes and Environmental Safety Laboratory, Southwest University of Science and Technology, Mianyang 621010, China
Published: December 2020

Nd2Zr2O7 pyrochlore with higher physicochemical and radiation stability has been considered as a host matrix for actinide immobilization of high level radioactive wastes. Uranium is a constituent and the decay-daughter product of high level radioactive wastes. It is necessary to study the solubility and ion-irradiation effect of uranium in Nd2Zr2O7 pyrochlore. The solubility of U is studied by the A site substitution in the pyrochlore structure. A series of uranium-doped zirconate pyrochlore compositions is prepared by the sol-gel-spray pyrolysis-high temperature sintering method. The structures of immobilization are studied by using X-ray diffraction (XRD) and Raman spectroscopy. The XRD and Raman spectroscopy studies reveal that the solubility limit of uranium in Nd2Zr2O7 is estimated at 10 at%. The lattice parameter of pyrochlore decreases with U content increasing, which is due to lower ionic radius of U. The immobilization structure changes from order pyrochlore to disorder structure. Further addition of U content leads to the separation of U3O8 phase in the immobilization. The U ions with high valance may be substituted at A or B site in Nd2Zr2O7 pyrochlore, which results in the A–O and B–O bond destruction. In order to keep the balance of charge, extra O ions should enter into the vacancy site, the structure of pyrochlore maybe transforms into a disorder structure. The radiation resistance of immobilization is investigated by ion-beam irradiation with 2 MeV Kr15+ ions at room temperature. The Nd2Zr2O7 and Nd1.9U0.1Zr2O7 are irradiated at doses of 1 dpa and 3 dpa, respectively. Analyses of the XRD and Raman spectroscopy data show that the Nd2Zr2O7 pyrochlore remains full pyrochlore structure even at a higher irradiation dose, which suggests that the Nd2Zr2O7 exhibits higher radiation resistance as potential immobilization. In contrast, the Nd1.9U0.1Zr2O7 immobilization shows the weaker radiation resistance, the pyrochlore structure completely transforms into a disorder fluorite structure. The A–O and B–O bonds of Nd1.9U0.1Zr2O7 pyrochlore structure are easy to destroy under ion irradiation conditions due to the disorder of pyrochlore. At the same time, the excess O ions are rearranged in U-rich pyrochlore after irradiation. Bond destruction and ion rearrangement of pyrochlore structure result in the full disorder fluorite structure. The actinides-doped pyrochlore structure is modified due to the change in physicochemical propertyof actinide, which results in the reductionof the solubility limit and radiation resistance.
Iterative denoising of ghost imaging based on adaptive threshold method
Authors: Zhou Yang  , Zhang Hong-Wei  , Zhong Fei  , Guo Shu-Xu  ,   1. State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130012, China;,   2. School of Electrical Engineering and Information Technology, Chanchun Institute of Technology, Changchun 130012, China
Published: December 2020

Ghost imaging (GI) is an important technique in the fields of quantum imaging and classical optical imaging, and it can solve the problems which are difficult to solve by the traditional imaging techniques in the optically harsh environments. In this paper, we present the iterative denoising of GI based on an adaptive threshold method. This method is abbreviated as IDGI-AT, which takes the advantages of adaptive threshold, differential, binarization and iterative operation method, and can enhance image quality in GI. In addition, this method can reduce the number of measurements. As is well known, the enormous number of measurements and poor reconstruction quality are obstacles to the engineering application of GI. The correlation noise leads to low signal-to-noise ratio and low imaging efficiency in GI as well. Therefore, we establish a denoising model, which can reduce correlation noise and improve reconstruction quality. We first analyze the iterative denoising of ghost imaging (IDGI) theory, and use the adaptive threshold technique to calculate the ideal threshold associated with the correlation noise. It should be noted that the threshold can be obtained by this method under the condition without requiring prior knowledge of the object. Afterwards, we can construct the correlation noise in this denoising model. In the IDGI, the differential ghost imaging (DGI) image is taken as the initial iteration value. We use the adaptive threshold method, which is different from IDGI, to binarize the initial value of each iteration to make it closer to the original object's transmission coefficient. After three iterations, we can obtain a higher-quality reconstruction image. In order to demonstrate that the IDGI-AT is available, a GI experimental system with a pseudo-thermal light source is set up. The considerable simulation and experimental results show the advantage of our scheme in terms of removing reconstruction image background noise. Especially, the visual effects and peak signal-to-noise ratio values are improved in comparison with those from the traditional GI, DGI and IDGI. Besides, we demonstrate the role of binarization in our scheme. For a binary object, the iterative value binarization can achieve better image quality than that in the case without binarizing the iterative initial value. Therefore, this novel method is likely to provide an alternative mean for GI and further pave the way for the application fields of GI, such as lidar, biomedical engineering, etc.
Progress of converse magnetoelectric coupling effect in multiferroic heterostructures
Authors: Chen Ai-Tian  , Zhao Yong-Gang  ,   Department of Physics and State Key Laboratory of Low-Dimensional Quantum Physics, Tsinghua University, Beijing 100084, China;,   Physical Science and Engineering Division, King Abdullah University of Science and Technology, Thuwal 23955-6900, Kingdom of Saudi Arabia;,   Collaborative Innovation Center of Quantum Matter, Beijing 100084, China
Published: December 2020

Electric-field control of magnetism has recently received much attention because of low-power consumption, which has potential applications in low-power multifunction devices. Ferromagnetic/ferroelectric multiferroic heterostructure is a useful way to realize the electric-field control of magnetism. Strain-mediated magnetoelectric coupling with large magnetoelectric coupling coefficient at room temperature is one of the current research hotspot. In this paper, we give an overview of recent progress of strain-mediated magnetoelectric coupling in multiferroic heterostructures.This review paper consists of five parts:introduction of multiferroics, electric-field control of magnetism in multiferroic heterostructures, electrical control of magnetization reversal, electric-field control of magnetic tunnel junctions, and the future prospects of multiferroic heterostructures. The basic concepts of multiferroics and background of magnetoelectric coupling effect are introduced in the first part.In the second part, a brief review of the recent work on the Pb(Mg1/3Nb2/3)0.7Ti0.3O3 (PMN-PT) based multiferroic heterostructures is given. The PMN-PT has a FE domain structure, which plays a vital role in electric-field control of magnetism, especially the 109 domain switching. For PMN-PT (001), the importance of 109 domain switching on the nonvolatile electrical control of magnetism is discussed. For PMN-PT (011), it is shown how to obtain nonvolatile strain which induces magnetic easy axis to be rotated by 90. The work on electric-field modulation of ferromagnetic material with perpendicular magnetic anisotropy is also mentioned.Electric-field control of magnetization reversal is still a challenge and remains elusive. Combination of strain-mediated magnetoelectric coupling and exchanging bias is a promising method to reverse magnetization by electric field, and the exchange-biased system/ferroelectric structures are given in the third part. There are also some theoretical attempts and proposals to realize the electrical control of 180 magnetization reversal. Then the method to manipulate magnetic tunnel junctions by electric field is given through integrating multiferroics and spintronics. Further outlook of the multiferroic heterostructures is also presented finally.
Effect of high pressure on intercalated FeSe high-Tc superconductors
Authors: Sun Jian-Ping  , Prashant Shahi  , Zhou Hua-Xue  , Ni Shun-Li  , Wang Shao-Hua  , Lei He-Chang  , Wang Bo-Sen  , Dong Xiao-Li  , Zhao Zhong-Xian  , Cheng Jin-Guang  ,   Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China;,   School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100190, China;,   Beijing Key Laboratory of Opto-electronic Functional Materials and Micro-nano Devices, Department of Physics, Renmin University of China, Beijing 100872, China
Published: December 2020

Among the iron-based superconductors, the structural simplest FeSe and its derived materials have received much attention in recent years due to the great tunability of the superconducting transition temperature (Tc). The relatively low Tc 8.5 K of FeSe can be raised to over 40 K via the interlayer intercalations such as AxFe2-ySe2 (A=K, Rb, Cs, Tl), Lix(NH3)yFe2Se2, and (Li1-xFex)OHFeSe. Although the monolayer FeSe/SrTiO3 is reported to have a Tc as high as 65 K, none of the Tc values of these FeSe-derived bulk materials has exceeded 50 K at ambient pressure so far. In order to explore other routes to further enhance Tc of FeSe-based materials, we recently performed the detailed high-pressure study of two intercalated FeSe high-Tc superconductors, namely (Li0.84Fe0.16)OHFe0.98Se and Li036(NH3)yFe2Se2, by using a cubic anvil cell apparatus. We find that the applied high pressure first suppresses the superconducting phase (denoted as SC-I) and then induces a second high-Tc superconducting phase (denoted as SC-Ⅱ) above a critical pressure Pc (~5 GPa for (Li0.84Fe0.16)OHFe0.98Se and 2 GPa for Li036(NH3)yFe2Se2). The highest Tc values in the SC-Ⅱ phases of these two compounds can reach~52 K and 55 K, respectively, the latter of which is the highest in the FeSe-based bulk materials, and is very close to the highest Tc of FeAs-based high-Tc superconductors. Our high-precision resistivity data of (Li0.84Fe0.16)OHFe0.98Se also uncover a sharp transition of the normal state from Fermi liquid for SC-I to non-Fermi liquid for SC-Ⅱ phase. In addition, the reemergence of high-Tc SC-Ⅱ phase under pressure is found to be accompanied with a concurrent enhancement of electron carrier density. Interestingly, we find a nearly parallel scaling behavior between Tc and the inverse Hall coefficient for the SC-Ⅱ phases of both (Li0.84Fe0.16)OHFe0.98Se and Li0.36(NH3)yFe2Se2. In the case without structural transition below 10 GPa, the observed enhancement of carrier density in SC-Ⅱ should be ascribed to an electronic origin presumably associated with pressure-induced Fermi surface reconstruction. Our work demonstrates that high pressure offers a distinctive means to further raise the maximum Tc values of intercalated FeSe-based materials.
Single crystal growth and physical property study of 1111-type Fe-based superconducting system CaFeAsF
Authors: Mu Gang  , Ma Yong-Hui  ,   State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, China;,   CAS Center for Excellence in Superconducting Electronics(CENSE), Shanghai 200050, China;,   University of Chinese Academy of Sciences, Beijing 100049, China
Published: December 2020

Since Fe-based high temperature superconductor was discovered in 2008, its material exploration and physical properties have been widely and in depth studied. However, the 1111 system, which was discovered first to have the highest Tc in the bulk material, has long been lacking in large-size and high-quality single-crystalline sample. This seriously restricts the in-depth study of the physical problems relating to this material system. In recent years, the great progress of single crystal growth of the fluorine-based 1111 system CaFeAsF has been made. One has successfully grown the high-quality CaFeAsF parent phase and Co doped superconducting single crystal with millimeter size at ambient pressure by using CaAs as the flux. On this basis, several research groups have studied the physical properties of this system by different experimental means and obtained some important results. For example, Dirac Fermions have been detected in CaFeAsF single crystal by measuring the quantum oscillation and optical conductivity. A high-field-induced metal-insulator transition was reported in CaFeAsF, which is closely related to the quantum limit. This review is intended to make a preliminary summary of the progress of this area, including crystal growth, quantum oscillation, infrared spectrum, magnetoresistance under strong field, high pressure regulation, anisotropy, superconducting fluctuations, etc.
Improvement on reflectivity of tunable photonic band gap with spontaneous generated coherence
Authors: Yang Liu  , Gao Zhong-Xing  , Xue Bing  , Zhang Yong-Gang  , Cai Yong-Mao  ,   1. College of Automation, Harbin Engineering University, Harbin 150001, China;,   2. College of Science, Harbin Engineering University, Harbin 150001, China;,   3. School of Science, Northeast Electric Power University, Jilin 132012, China
Published: December 2020

The photonic band gap is a spectral range which cannot propagate in a periodic optical nanostructure, that is, the structure itself has a “forbidden band”. It has been successfully applied to the filters, amplifiers, mixers, etc. As is well known, dynamically tunable photonic band gaps in cold atomic lattices are of great importance in various research fields. However, the photonic band gaps of a traditional photonic crystal are non-tunable because the periodic structure is determined once the photonic crystal is grown. On the other hand, a majority of previous researches focused on improving the reflectivity of photonic band gap, which can only keep approaching to 1. Due to the action of the vacuum of the radiation field, near-degenerate lower level has an additional coherence term, the spontaneously generated coherence term. In this paper, we consider a three-level ∧-type atomic system driven by a strong coherent field, a weak coherent field and an incoherent pump, in which the two ground states are of hyperfine structure. The one-dimensional photonic band gaps are formed by cold atoms trapped in a one-dimensional-ordered optical lattice and this system may create two photonic band gaps (PBGs). The trapped cold atoms have a Gaussian density distribution in each period as determined by the optical potential depth and the average atomic temperature. We investigate in detail how the reflectivities of the two PBGs are influenced by the coherent effect of spontaneously generated coherence. Then, we find that the reflectivities of the two band gaps can be significantly improved by the spontaneously generated coherence. The reflectivities of such two band gaps can be dynamically manipulated by varying the intensity of incoherent driving field and the relative phase between the probe field and the coupling field, which cannot be realized in a conventional ∧-type atomic system. Besides, by adjusting the parameters appropriately, the reflectivities of these two band gaps can be higher than 1, which is because probe field gain stems from the spontaneously generated coherence. In the future, photonic transport properties can be investigated in the three-dimensional atomic lattices and this work is meaningful for the optical routing, photodiode and transistor.
Dynamic characteristics of parallel water-entry cavity based on potential flow theory
Authors: Song Wu-Chao  , Wei Ying-Jie  , Lu Li-Rui  , Wang Cong  , Lu Jia-Xing  ,   School of Astronautics, Harbin Institute of Technology, Harbin 150001, China
Published: December 2020

During the water-entry of the parallel axisymmetric bodies, the water-entry cavities are asymmetrical due to the mutual interference between the cavities. In the current study of the cavity dynamic models, the relatively perfect models of axially symmetric dynamic calculation of low speed single water entry have been established. These models mainly focus on the evolution of cavitation, and thus simplifying the flow pattern. However, due to the particularity of parallel water, the fluid forms a relative flow during the evolution of the cavitation in the inner region of the axisymmetric body axis. As a result, the flow is no longer axisymmetric but develops into a complex three-dimensional flow with strong nonlinearity, making the the theoretical model more difficult to establish. In order to analyze the evolution of the parallel cavities of the parallel axisymmetric body water-entry, the flow of the water-entry cavity interference region is simplified by the existing single water-entry calculation model based on the potential flow theory. The constraint of the relative flow to the cavity is simplified into a constraint potential, and the variation of cavity shape is analyzed. Based on the nonlinear hypothesis, the influence function is introduced to establish the calculation model of three-dimensional cavity and the three-dimensional evolution characteristics of parallel cavities are analyzed. The obtained results show that the velocity potential of the axisymmetric body water-entry can be regarded as the superposition of a point source and a line source located on the axis of the cavity. The expansion of the cavity is affected mainly by the point source, while the shrinkage of the cavity is influenced mainly by the line source. During the parallel water-entry of the axisymmetric body, the evolution of the parallel cavities in space is mirror symmetric and the mutual interference between cavities can be analyzed by introducing the potential wall surface. The potential wall has an inhibitory effect on the evolution of the cavities. The variation of the parallel water-entry cavity radius with the polar angle is related to the depth of the cavity cross section. In the inhibition evolution area near the closed point, the cavity radius decreases gradually with the increase of the polar angle, and the void section radius in the inhibition evolution region far from the closed point increases with the polar angle increasing, and is opposite to the radius law. In the shallower depth of the water-entry, the excessive evolution is formed in the expansion process of the cavity, and the excessive evolution will gradually weaken and disappear in the contraction process of the cavity.
Progress on the precision measurement of the Newtonian gravitational constant G
Authors: Liu Jian-Ping  , Wu Jun-Fei  , Li Qing  , Xue Chao  , Mao De-Kai  , Yang Shan-Qing  , Shao Cheng-Gang  , Tu Liang-Cheng  , Hu Zhong-Kun  , Luo Jun  ,   MOE Key Laboratory of Fundamental Physical Quantities Measurements, Hubei Key Laboratory of Gravitation and Quantum Physics, School of Physics, Huazhong University of Science and Technology, Wuhan 430074, China;,   TianQin Research Center for Gravitational Physics, Sun Yat-sen University, Zhuhai 519082, China
Published: December 2020

The Newtonian gravitational constant G is the first fundamental physics constant introduced by human beings. It plays an important role in many fields, such as theoretical physics, astrophysics, and geophysics. Its precision measurement and related research is of great significance to the whole experimental physics. However, the measuring accuracy of G is the worst among all fundamental physical constants, which reflects the great complexity and difficulty in determining G. This paper briefly reviews the history of G measurement, and also introduces the current research progress in this field by a summary of the recent three precision measurements of G. At the end of the paper, the latest developments of the G measurement in the center of gravitational experiments at Huazhong University of Science and Technology are introduced.
Effect of phase angle on plasma characteristics in electrically asymmetric capacitive discharge
Authors: Hu Yan-Ting  , Zhang Yu-Ru  , Song Yuan-Hong  , Wang You-Nian  ,   Key Laboratory of Materials Modification by Laser, Ion, and Electron Beams(Ministry of Education, School of Physics, Dalian University of Technology, Dalian 116024, China
Published: December 2020

In addition to the separate control of the ion energy and ion flux, the so-called electrical asymmetry effect (EAE) also plays an important role in improving the plasma radial uniformity. In this work, a two-dimensional fluid model combined with a full set of Maxwell equations is used to investigate the plasma characteristics in an electrically asymmetric capacitive discharge sustained by multiple consecutive harmonics. The effects of the phase angle θn on the dc self-bias (Vdc) and on the plasma radial uniformity for different numbers of consecutive harmonics k are discussed. The simulation results indicate that the phase angles of different harmonics θn have different influences on the dc self-bias Vdc. For instance, Vdc varies almost linearly with θ1 with a period π in dual frequency discharge, and the period is 2π for other discharge conditions. Besides, the modulation of Vdc becomes less obvious by changing the phase angle of the highest harmonic θk, especially for k>3. In addition, both the axial component of the power density Pz and the radial component of the power density Pr vary with θn, thus the plasma radial uniformity can be adjusted. When the total power density at the radial edge becomes comparable to that in the discharge center, the plasma distribution becomes uniform. For instance, when k=2, the plasma radial uniformity is the best at the phase angle θ1=π/2 and θ2=π. However, for k=3, the best radial uniformity is observed at θ1=3π/2, and the nonuniformity degree α is only 0.41% under this condition. It is worth noting that at k=8, the maximum of α is seven times higher than the minimum by changing the phase angles θ1 and θ2, which means that the plasma radial uniformity can be adjusted effectively. However, the modulation induced by θk(k>3) becomes less obvious, especially for k=8. Indeed, the electron density shows an edge-high profile, and the radial uniformity is always bad for all θ8 investigated. The results obtained in this work can help us to gain an insight into the optimization the plasma process by utilizing the EAE.