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Symmetry-Protected Scattering in Non-Hermitian Linear Systems
L. Jin and Z. Song
Chin. Phys. Lett.    2021, 38 (2): 024202 .   DOI: 10.1088/0256-307X/38/2/024202
Abstract   HTML   PDF (2449KB)
Symmetry plays fundamental role in physics and the nature of symmetry changes in non-Hermitian physics. Here the symmetry-protected scattering in non-Hermitian linear systems is investigated by employing the discrete symmetries that classify the random matrices. The even-parity symmetries impose strict constraints on the scattering coefficients: the time-reversal ($C$ and $K$) symmetries protect the symmetric transmission or reflection; the pseudo-Hermiticity ($Q$ symmetry) or the inversion ($P$) symmetry protects the symmetric transmission and reflection. For the inversion-combined time-reversal symmetries, the symmetric features on the transmission and reflection interchange. The odd-parity symmetries including the particle-hole symmetry, chiral symmetry, and sublattice symmetry cannot ensure the scattering to be symmetric. These guiding principles are valid for both Hermitian and non-Hermitian linear systems. Our findings provide fundamental insights into symmetry and scattering ranging from condensed matter physics to quantum physics and optics.
Classical-Noise-Free Sensing Based on Quantum Correlation Measurement
Ping Wang , Chong Chen , and Ren-Bao Liu
Chin. Phys. Lett.    2021, 38 (1): 010301 .   DOI: 10.1088/0256-307X/38/1/010301
Abstract   HTML   PDF (791KB)
Quantum sensing, using quantum properties of sensors, can enhance resolution, precision, and sensitivity of imaging, spectroscopy, and detection. An intriguing question is: Can the quantum nature (quantumness) of sensors and targets be exploited to enable schemes that are not possible for classical probes or classical targets? Here we show that measurement of the quantum correlations of a quantum target indeed allows for sensing schemes that have no classical counterparts. As a concrete example, in the case that the second-order classical correlation of a quantum target could be totally concealed by non-stationary classical noise, the higher-order quantum correlations can single out a quantum target from the classical noise background, regardless of the spectrum, statistics, or intensity of the noise. Hence a classical-noise-free sensing scheme is proposed. This finding suggests that the quantumness of sensors and targets is still to be explored to realize the full potential of quantum sensing. New opportunities include sensitivity beyond classical approaches, non-classical correlations as a new approach to quantum many-body physics, loophole-free tests of the quantum foundation, et cetera.
First-Principles Study of Electronic Structure and Optical Properties of Cubic Perovskite CsCaF3
K. Ephraim Babu, A. Veeraiah, D. Tirupati Swamy, V. Veeraiah
Chin. Phys. Lett.    2012, 29 (11): 117102 .   DOI: 10.1088/0256-307X/29/11/117102
Abstract   PDF (895KB)
Electronic, structural and optical properties of the cubic perovskite CsCaF3 are calculated by using the full potential linearized augmented plane wave (FP-LAPW) plus local orbitals method with generalized gradient approximation (GGA) in the framework of the density functional theory. The calculated lattice constant is in good agreement with the experimental result. The electronic band structure shows that the fundamental band gap is wide and indirect at (ΓR) point. The contribution of the different bands is analyzed from the total and partial density of states curves. The charge density plots show strong ionic bonding in Cs-F, and ionic and weak covalent bonding between Ca and F. Calculations of the optical spectra, viz., the dielectric function, optical reflectivity, absorption coefficient, real part of optical conductivity, refractive index, extinction coefficient and electron energy loss, are performed for the energy range 0–30 eV.
Dynamical Evolution of Highway Traffic Flow: from Microscopic to Macroscopic
WANG Bing-hong, HUI Pak-ming, GU Guo-qing
Chin. Phys. Lett.    1997, 14 (3): 202-205 .  
Abstract   PDF (230KB)
In this paper, a derivation of the macroscopic mean field theory of the cellular automaton (CA) model of highway traffic flow starting from the microscopic dynamical point of view is presented. Starting from an equation describing the time evolution of the Boolean state variable at each site of the basic CA model, and using a two-site approximation for the multi-site correlation functions, a dynamical mapping between the macroscopic average speeds v(t + 1) and v ( t ) at different time can be derived. Mean field results consistent with the simulation data are obtained by considering the attractors of the mapping and their corresponding basins.
Tunable Superconductivity in 2H-NbSe$_{2}$ via $\boldsymbol In~Situ$ Li Intercalation
Kaiyao Zhou, Jun Deng, Liwei Guo, and Jiangang Guo
Chin. Phys. Lett.    2020, 37 (9): 097402 .   DOI: 10.1088/0256-307X/37/9/097402
Abstract   HTML   PDF (1354KB)
Using the newly-developed solid ionic gating technique, we measure the electrical transport property of a thin-flake NbSe$_{2}$ superconductor ($T_{\rm c} = 6.67$ K) under continuous Li intercalation and electron doping. It is found that the charge-density-wave transition is suppressed, while at the same time a carrier density, decreasing from $7\times 10^{14}$ cm$^{-2}$ to $2\times 10^{14}$ cm$^{-2}$ also occurs. This tunable capability in relation to carrier density is 70%, which is 5 times larger than that found using the liquid ionic gating method [Phys. Rev. Lett. 117 (2016) 106801]. Meanwhile, we find that the scattering type of conduction electrons transits to the $s$–$d$ process, which may be caused by the change of the occupied states of 4$d$-electrons in Nb under the condition of Li intercalation. Simultaneously, we observe a certain decrement of electron-phonon coupling (EPC), based on the electron-phonon scattering model, in the high temperature range. Based on data gathered from in situ measurements, we construct a full phase diagram of carrier density, EPC and $T_{\rm c}$ in the intercalated NbSe$_{2}$ sample, and qualitatively explain the variation of $T_{\rm c}$ within the BCS framework. It is our opinion that the in situ solid ionic gating method provides a direct route to describing the relationship between carrier density and superconductivity, which is helpful in promoting a clearer understanding of electronic phase competition in transition metal dichalcogenides.
A Search for Solar Axions and Anomalous Neutrino Magnetic Moment with the Complete PandaX-II Data
Xiaopeng Zhou, Xinning Zeng, Xuyang Ning, Abdusalam Abdukerim, Wei Chen, Xun Chen, Yunhua Chen, Chen Cheng, Xiangyi Cui, Yingjie Fan, Deqing Fang, Changbo Fu, Mengting Fu, Lisheng Geng, Karl Giboni, Linhui Gu, Xuyuan Guo, Ke Han, Changda He, Di Huang, Yan Huang, Yanlin Huang, Zhou Huang, Xiangdong Ji, Yonglin Ju, Shuaijie Li, Huaxuan Liu, Jianglai Liu, Xiaoying Lu, Wenbo Ma, Yugang Ma, Yajun Mao, Yue Meng, Kaixiang Ni, Jinhua Ning, Xiangxiang Ren, Changsong Shang, Guofang Shen, Lin Si, Andi Tan, Anqing Wang, Hongwei Wang, Meng Wang, Qiuhong Wang, Siguang Wang, Wei Wang, Xiuli Wang, Zhou Wang, Mengmeng Wu, Shiyong Wu, Weihao Wu, Jingkai Xia, Mengjiao Xiao, Pengwei Xie, Binbin Yan, Jijun Yang, Yong Yang, Chunxu Yu, Jumin Yuan, Ying Yuan, Dan Zhang, Tao Zhang, Li Zhao, Qibin Zheng, Jifang Zhou, and Ning Zhou (PandaX-II Collaboration)
Chin. Phys. Lett.    2021, 38 (1): 011301 .   DOI: 10.1088/0256-307X/38/1/011301
Abstract   HTML   PDF (764KB)
We report a search for new physics signals using the low energy electron recoil events in the complete data set from PandaX-II, in light of the recent event excess reported by XENON1T. The data correspond to a total exposure of 100.7 ton$\cdot$day with liquid xenon. With robust estimates of the dominant background spectra, we perform sensitive searches on solar axions and neutrinos with enhanced magnetic moment. It is found that the axion-electron coupling $g_{\rm Ae} < 4.6\times 10^{-12}$ for an axion mass less than 0.1 keV/$c^2$ and the neutrino magnetic moment $\mu_{\nu} < 4.9\times 10^{-11}\mu_{\rm B}$ at 90% confidence level. The observed excess from XENON1T is within our experimental constraints.
Isotropic Thermal Cloaks with Thermal Manipulation Function
Quan-Wen Hou, Jia-Chi Li , and Xiao-Peng Zhao 
Chin. Phys. Lett.    2021, 38 (1): 010503 .   DOI: 10.1088/0256-307X/38/1/010503
Abstract   HTML   PDF (1123KB)
By extending the conventional scattering canceling theory, we propose a new design method for thermal cloaks based on isotropic materials. When the objects are covered by the designed cloaks, they will not disturb the temperature profile in the background zone. In addition, if different inhomogeneity coefficients are selected in the thermal cloak design process, these cloaks can manipulate the temperature gradient of the objects, i.e., make the temperature gradients higher, lower, or equal to the thermal gradient in the background zone. Therefore, thermal transparency, heat concentration or heat shield effects can be realized under a unified framework.
Phase-Gradient Metasurfaces Based on Local Fabry–Pérot Resonances
Yanyan Cao, Bocheng Yu, Yangyang Fu, Lei Gao, and Yadong Xu
Chin. Phys. Lett.    2020, 37 (9): 097801 .   DOI: 10.1088/0256-307X/37/9/097801
Abstract   HTML   PDF (965KB)
In this work, we present a new mechanism for designing phase-gradient metasurfaces (PGMs) to control an electromagnetic wavefront with high efficiency. Specifically, we design a transmission-type PGM, formed by a periodic subwavelength metallic slit array filled with identical dielectrics of different heights. It is found that when Fabry–Pérot (FP) resonances occur locally inside the dielectric regions, in addition to the common phenomenon of complete transmission, the transmitted phase differences between two adjacent slits are exactly the same, being a nonzero constant. These local FP resonances ensure total phase shift across a supercell, fully covering a range of 0 to $2\pi$, satisfying the design requirements of PGMs. Further research reveals that, due to local FP resonances, there is a one-to-one correspondence between the phase difference and the permittivity of the filled dielectric. A similar approach can be extended to the reflection-type case and other wavefront transformations, creating new opportunities for wave manipulation.
Effective Dielectric Properties of Au-ZnS and Au-ZnO Plasmonics Nanocomposites in the Terahertz Regime
A. Zolanvar, H. Sadeghi, A. Ranjgar
Chin. Phys. Lett.    2014, 31 (10): 106201 .   DOI: 10.1088/0256-307X/31/10/106201
Abstract   PDF (492KB)
Composite materials based on plasmonic nanoparticles allow building metamaterials with very large effective permittivity (positive or negative). Moreover, if clustered or combined with other nanoparticles, it is also possible to generate effective magnetic permeability (positive or negative), and an ad-hoc design would result in the generation of double negative materials, and therefore backward wave propagation. In this work, the optical properties such as the effective permittivity, permeability and refractive index of Au-ZnS and Au-ZnO nanocomposites in a broad frequency range are studied. The enhancement is attributed to energy transfer from ZnS or ZnO to Au followed by a large local electromagnetic field on or near the surface of the Au nanoparticles. Local surface plasmon resonance could be the key reason for this enhancement. The surface plasmon, in response to changes in the refractive index of the local environment, also depends on the type of metal through the bulk plasma wavelength and the nano-particle compositions and geometry.
Strain Tunable Berry Curvature Dipole, Orbital Magnetization and Nonlinear Hall Effect in WSe$_{2}$ Monolayer
Mao-Sen Qin , Peng-Fei Zhu , Xing-Guo Ye , Wen-Zheng Xu , Zhen-Hao Song , Jing Liang , Kaihui Liu , and Zhi-Min Liao
Chin. Phys. Lett.    2021, 38 (1): 017301 .   DOI: 10.1088/0256-307X/38/1/017301
Abstract   HTML   PDF (2896KB)
The electronic topology is generally related to the Berry curvature, which can induce the anomalous Hall effect in time-reversal symmetry breaking systems. Intrinsic monolayer transition metal dichalcogenides possesses two nonequivalent $K$ and $K'$ valleys, having Berry curvatures with opposite signs, and thus vanishing anomalous Hall effect in this system. Here we report the experimental realization of asymmetrical distribution of Berry curvature in a single valley in monolayer WSe$_2$ via applying uniaxial strain to break $C_{3v}$ symmetry. As a result, although the Berry curvature itself is still opposite in $K$ and $K'$ valleys, the two valleys would contribute equally to nonzero Berry curvature dipole. Upon applying electric field ${\boldsymbol E}$, the emergent Berry curvature dipole ${\boldsymbol D}$ would lead to an out-of-plane orbital magnetization $M \propto {\boldsymbol D} \cdot {\boldsymbol E}$, which further induces an anomalous Hall effect with a linear response to $E^2$, known as nonlinear Hall effect. We show the strain modulated transport properties of nonlinear Hall effect in monolayer WSe$_2$ with moderate hole-doping by gating. The second-harmonic Hall signals show quadratic dependence on electric field, and the corresponding orbital magnetization per current density $M/J$ can reach as large as 60. In contrast to the conventional Rashba–Edelstein effect with in-plane spin polarization, such current-induced orbital magnetization is along the out-of-plane direction, thus promising for high-efficient electrical switching of perpendicular magnetization.
Accurate Evaluation on the Interactions of SARS-CoV-2 with Its Receptor ACE2 and Antibodies CR3022/CB6
Hong-ming Ding, Yue-wen Yin, Song-di Ni, Yan-jing Sheng, and Yu-qiang Ma
Chin. Phys. Lett.    2021, 38 (1): 018701 .   DOI: 10.1088/0256-307X/38/1/018701
Abstract   HTML   PDF (1775KB)
The spread of the coronavirus disease 2019 (COVID-19) caused by severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) has become a global health crisis. The binding affinity of SARS-CoV-2 (in particular the receptor binding domain, RBD) to its receptor angiotensin converting enzyme 2 (ACE2) and the antibodies is of great importance in understanding the infectivity of COVID-19 and evaluating the candidate therapeutic for COVID-19. We propose a new method based on molecular mechanics/Poisson–Boltzmann surface area (MM/PBSA) to accurately calculate the free energy of SARS-CoV-2 RBD binding to ACE2 and antibodies. The calculated binding free energy of SARS-CoV-2 RBD to ACE2 is $-13.3$ kcal/mol, and that of SARS-CoV RBD to ACE2 is $-11.4$ kcal/mol, which agree well with the experimental results of $-11.3$ kcal/mol and $-10.1$ kcal/mol, respectively. Moreover, we take two recently reported antibodies as examples, and calculate the free energy of antibodies binding to SARS-CoV-2 RBD, which is also consistent with the experimental findings. Further, within the framework of the modified MM/PBSA, we determine the key residues and the main driving forces for the SARS-CoV-2 RBD/CB6 interaction by the computational alanine scanning method. The present study offers a computationally efficient and numerically reliable method to evaluate the free energy of SARS-CoV-2 binding to other proteins, which may stimulate the development of the therapeutics against the COVID-19 disease in real applications.
Superconductivity of Lanthanum Superhydride Investigated Using the Standard Four-Probe Configuration under High Pressures
Fang Hong, Liuxiang Yang, Pengfei Shan, Pengtao Yang, Ziyi Liu, Jianping Sun, Yunyu Yin, Xiaohui Yu, Jinguang Cheng, and Zhongxian Zhao
Chin. Phys. Lett.    2020, 37 (10): 107401 .   DOI: 10.1088/0256-307X/37/10/107401
Abstract   HTML   PDF (688KB)
Recently, the theoretically predicted lanthanum superhydride, LaH$_{10 \pm \delta}$, with a clathrate-like structure was successfully synthesized and found to exhibit a record high superconducting transition temperature $T_{\rm c} \approx 250$ K at $\sim $170 GPa, opening a new route for room-temperature superconductivity. However, since in situ experiments at megabar pressures are very challenging, few groups have reported the $\sim $250 K superconducting transition in LaH$_{10 \pm \delta}$. Here, we establish a simpler sample-loading procedure that allows a relatively large sample size for synthesis and a standard four-probe configuration for resistance measurements. Following this procedure, we successfully synthesized LaH$_{10 \pm \delta}$ with dimensions up to $10 \times 20$ μm$^{2}$ by laser heating a thin La flake and ammonia borane at $\sim $1700 K in a symmetric diamond anvil cell under the pressure of 165 GPa. The superconducting transition at $T_{\rm c} \approx 250$ K was confirmed through resistance measurements under various magnetic fields. Our method will facilitate explorations of near-room-temperature superconductors among metal superhydrides.
Enhanced Ferromagnetism of CrI$_{3}$ Bilayer by Self-Intercalation
Yu Guo , Nanshu Liu , Yanyan Zhao , Xue Jiang , Si Zhou, and Jijun Zhao 
Chin. Phys. Lett.    2020, 37 (10): 107506 .   DOI: 10.1088/0256-307X/37/10/107506
Abstract   HTML   PDF (2562KB)
Two-dimensional (2D) ferromagnets with high Curie temperature have long been the pursuit for electronic and spintronic applications. CrI$_{3}$ is a rising star of intrinsic 2D ferromagnets, however, it suffers from weak exchange coupling. Here we propose a general strategy of self-intercalation to achieve enhanced ferromagnetism in bilayer CrI$_{3}$. We show that filling either Cr or I atoms into the van der Waals gap of stacked and twisted CrI$_{3}$ bilayers can induce the double exchange effect and significantly strengthen the interlayer ferromagnetic coupling. According to our first-principles calculations, the intercalated native atoms act as covalent bridge between two CrI$_{3}$ layers and lead to discrepant oxidation states for the Cr atoms. These theoretical results offer a facile route to achieve high-Curie-temperature 2D magnets for device implementation.
Making Axion Dynamical in Non-Centrosymmetric Magnetic Topological Insulators
Chaoxing Liu
Chin. Phys. Lett.    2021, 38 (1): 010101 .   DOI: 10.1088/0256-307X/38/1/010101
Abstract   HTML   PDF (276KB)
Investigation of Oxygen Vacancy and Interstitial Oxygen Defects in ZnO Films by Photoluminescence and X-Ray Photoelectron Spectroscopy
FAN Hai-Bo, YANG Shao-Yan, ZHANG Pan-Feng, WEI Hong-Yuan, LIU Xiang-Lin, JIAO Chun-Mei, ZHU Qin-Sheng, CHEN Yong-Hai, WANG Zhan-Guo
Chin. Phys. Lett.    2007, 24 (7): 2108-2111 .  
Abstract   PDF (341KB)
ZnO films prepared at different temperatures and annealed at 900°C in
oxygen are studied by photoluminescence (PL) and x-ray photoelectron
spectroscopy (XPS). It is observed that in the PL of the as-grown films the green luminescence (GL) and the yellow luminescence (YL) are related, and after annealing the GL is restrained and the YL is enhanced. The O 1s XPS results also show the coexistence of oxygen vacancy (VO) and interstitial oxygen (Oi) before annealing and the quenching of the VO after annealing. By combining the two results it is deduced that the GL and YL are related to the VO and Oi defects, respectively.
How Does van der Waals Confinement Enhance Phonon Transport?
Xiaoxiang Yu, Dengke Ma, Chengcheng Deng, Xiao Wan, Meng An, Han Meng, Xiaobo Li, Xiaoming Huang, and Nuo Yang
Chin. Phys. Lett.    2021, 38 (1): 014401 .   DOI: 10.1088/0256-307X/38/1/014401
Abstract   HTML   PDF (2162KB)
We study the mechanism of van der Waals (vdW) interactions on phonon transport in atomic scale, which would boost developments in heat management and energy conversion. Commonly, the vdW interactions are regarded as a hindrance in phonon transport. Here we propose that the vdW confinement can enhance phonon transport. Through molecular dynamics simulations, it is realized that the vdW confinement is able to make more than two-fold enhancement on thermal conductivity of both polyethylene single chain and graphene nanoribbon. The quantitative analyses of morphology, local vdW potential energy and dynamical properties are carried out to reveal the underlying physical mechanism. It is found that the confined vdW potential barriers reduce the atomic thermal displacement magnitudes, leading to less phonon scattering and facilitating thermal transport. Our study offers a new strategy to modulate the phonon transport.
Uncooperative Effect of Hydrogen Bond on Water Dimer
Danhui Li, Zhiyuan Zhang, Wanrun Jiang, Yu Zhu, Yi Gao, and Zhigang Wang
Chin. Phys. Lett.    2021, 38 (1): 013101 .   DOI: 10.1088/0256-307X/38/1/013101
Abstract   HTML   PDF (2226KB)
The water dimer demonstrates a completely different protype in water systems, it prefers not forming larger clusters instead existing in vapor phase stably, which contracts the viewpoint of the cooperative effect of hydrogen bond (O–H$\cdots$O). It is well accepted that the cooperative effect is beneficial to forming more hydrogen bonds (O–H$\cdots$O), leading to stronger H-bond (H$\cdots$O) and increase in the O–H bond length with contraction of intermolecular distance. Herein, the high-precision ab initio methods of calculations applied on water dimer shows that the O–H bond length decreases and H-bond (H$\cdots$O) becomes weaker with decreasing H-bond length and O$\cdots$O distance, which can be considered as the uncooperative effect of hydrogen bond (O–H$\cdots$O). It is ascribed to the exchange repulsion of electrons, which results in decrease of the O–H bond length and prevents the decrease in the O$\cdots$O distance connected with the increasing scale of water clusters. Our findings highlight the uncooperative effect of hydrogen bond attributed to exchange repulsion of electrons as the mechanism for stabilizing water dimer in vapor phase, and open a new perspective for studies of hydrogen-bonded systems.
Dynamic Crossover in Metallic Glass Nanoparticles
Shan Zhang, Weihua Wang, and Pengfei Guan
Chin. Phys. Lett.    2021, 38 (1): 016802 .   DOI: 10.1088/0256-307X/38/1/016802
Abstract   HTML   PDF (2203KB)
We report the dynamic crossover behavior in metallic glass nanoparticles (MGNs) with the size reduction based on the extensive molecular dynamics (MD) simulations combined with the activation-relaxation technique (ART). The fragile-to-strong transition of dynamics can be achieved by just modulating the characteristic size of MGNs. It can be attributed to the abnormal fast surface dynamics enhanced by the surface curvature. By determining the potential energy surface, we reveal the hierarchy-to-flat transition of potential energy landscape (PEL) in MGNs, and demonstrate the intrinsic flat potential landscape feature of the MGN with size smaller than a critical size. Our results provide an important piece of the puzzle about the size-modulated potential energy landscape and shed some lights on the unique properties of MGs in nanoscale.
Exciton Vortices in Two-Dimensional Hybrid Perovskite Monolayers
Yingda Chen, Dong Zhang, and Kai Chang
Chin. Phys. Lett.    2020, 37 (11): 117102 .   DOI: 10.1088/0256-307X/37/11/117102
Abstract   HTML   PDF (1191KB)
We study theoretically the exciton Bose–Einstein condensation and exciton vortices in a two-dimensional (2D) perovskite (PEA)${_2}$PbI${_4}$ monolayer. Combining the first-principles calculations and the Keldysh model, the exciton binding energy of in a (PEA)${_2}$PbI${_4}$ monolayer can approach hundreds of meV, which make it possible to observe the excitonic effect at room temperature. Due to the large exciton binding energy, and hence the high density of excitons, we find that the critical temperature of the exciton condensation could approach the liquid nitrogen regime. In the presence of perpendicular electric fields, the dipole-dipole interaction between excitons is found to drive the condensed excitons confined in (PEA)${_2}$PbI${_4}$ monolayer flakes into patterned vortices, as the evolution time of vortex patterns is comparable to the exciton lifetime.
Exact Solutions of the Dirac Equation for an Electron in a Magnetic Field with Shape Invariant Method
M. R. Setare, O. Hatami
Chin. Phys. Lett.    2008, 25 (11): 3848-3851 .  
Abstract   PDF (169KB)
Based on the shape invariance property we obtain exact solutions of the Dirac equation for an electron moving in the presence of a certain varying magnetic field, then we also show its non-relativistic limit.
Giant Spin Transfer Torque in Atomically Thin Magnetic Bilayers
Weihao Cao, Matisse Wei-Yuan Tu, Jiang Xiao, and Wang Yao
Chin. Phys. Lett.    2020, 37 (10): 107201 .   DOI: 10.1088/0256-307X/37/10/107201
Abstract   HTML   PDF (1297KB)
In cavity quantum electrodynamics, the multiple reflections of a photon between two mirrors defining a cavity is exploited to enhance the light-coupling of an intra-cavity atom. We show that this paradigm for enhancing the interaction of a flying particle with a localized object can be generalized to spintronics based on van der Waals 2D magnets. Upon tunneling through a magnetic bilayer, we find that the spin transfer torques per electron incidence can become orders of magnitude larger than $\hbar /2$, made possible by electron's multi-reflection path through the ferromagnetic monolayers as an intermediate of their angular momentum transfer. Over a broad energy range around the tunneling resonances, the damping-like spin transfer torque per electron tunneling features a universal value of $(\hbar/2)\tan (\theta /2)$, depending only on the angle $\theta$ between the magnetizations. These findings expand the scope of magnetization manipulations for high-performance and high-density storage based on van der Waals magnets.
X-Ray Diffraction Pattern of Graphite Oxide
MU Shi-Jia, SU Yu-Chang, XIAO Li-Hua, LIU Si-Dong, HU Te, TANG Hong-Bo
Chin. Phys. Lett.    2013, 30 (9): 096101 .   DOI: 10.1088/0256-307X/30/9/096101
Abstract   PDF (633KB)
X-ray diffraction patterns of graphite oxide (GO) are theoretically simulated as a function of the displacements of carbon atoms using the Debye–Waller factor in terms of the Warren–Bodenstein equation. The results demonstrate that GO has the turbostratically stacked structure. The high order (00l) peaks gradually disappear with the increase in atomic thermal vibrations along c-axis while the (hk0) ones weaken for the vibrations along a-axis. When the displacement deviation ua=0.015 nm and uc=0.100 nm the computed result is consistent with the experimental measurements.
BaCuS$_{2}$: A Superconductor with Moderate Electron-Electron Correlation
Yuhao Gu, Xianxin Wu, Kun Jiang, and Jiangping Hu
Chin. Phys. Lett.    2021, 38 (1): 017501 .   DOI: 10.1088/0256-307X/38/1/017501
Abstract   HTML   PDF (4790KB)
We show that the layered-structure BaCuS$_{2}$ is a moderately correlated electron system in which the electronic structure of the CuS layer bears a resemblance to those in both cuprates and iron-based superconductors. Theoretical calculations reveal that the in-plane $d$–$p$ $\sigma^*$-bonding bands are isolated near the Fermi level. As the energy separation between the $d$ and $p$ orbitals are much smaller than those in cuprates and iron-based superconductors, BaCuS$_{2}$ is expected to be moderately correlated. We suggest that this material is an ideal system to study the competitive/collaborative nature between two distinct superconducting pairing mechanisms, namely the conventional BCS electron-phonon interaction and the electron-electron correlation, which may be helpful to establish the elusive mechanism of unconventional high-temperature superconductivity.
Room-Temperature Inductively Coupled Plasma Etching of InP Using Cl2N2 and Cl2/CH4/H2
LEE Chee-Wei, CHIN Mee-Koy
Chin. Phys. Lett.    2006, 23 (4): 903-906 .  
Abstract   PDF (638KB)
We optimize the room-temperature etching of InP using Cl2N2 and Cl2/CH4/H2 inductively coupled plasma reactive ions. A design of experiment is used in the optimization. The results, in terms of etch rate, surface roughness and etched profile, are presented. These Cl2-based recipes do not require substrate heating and thus can be more cost effectively and widely applied. The Cl2/CH4/H2 process is able to give a higher etch rate (about 850nm/min) and cleaner surface with less polymer formation compared to the conventional CH4/H2 process. The Cl2/N2 process produces even higher etch rate (as high as 2μm/min), but rougher surface with slight sidewall undercut. The Cl2/N2 process also has no polymer formation due to the absence of methane gas. Both the processes give very good selectivity to the silicon dioxide (SiO2) etch mask. The selectivity of InP to the oxide mask (up to 55:1) for the Cl2/N2 process is one of the highest reported so far. The etched structures possess reasonably good sidewall verticality and surface quality comparable to that obtained under elevated temperature condition (>200°C).
Characterization and Magnetic Properties of Nickel Ferrite Nanoparticles Prepared by Ball Milling Technique
G. Nabiyouni**, M. Jafari Fesharaki, M. Mozafari, J. Amighian
Chin. Phys. Lett.    2010, 27 (12): 126401 .   DOI: 10.1088/0256-307X/27/12/126401
Abstract   PDF (676KB)
Nickel ferrite nanoparicles with various grain sizes are synthesized using annealing treatment followed by ball milling of its bulk component materials. Commercially available nickel and iron oxide powders are first mixed, and then annealed at 1100°C in an oxygen environment furnace and for 3 h. The samples are then milled for different times in an SPEX mill. X-ray diffraction pattern indicates that in this stage the sample is single phase. The average grain size is estimated by scanning electron microscopy (SEM) and x-ray diffraction techniques. Magnetic behavior of the sample at room temperature is studied using a superconducting quantum interference device (SQUID). The Curie temperature of the powders is measured by an LCR–meter unit. The x-ray diffraction patterns clearly indicate that increasing the milling time leads to a decrease in the grain size and consequently leads to a decrease in the saturation magnetization as well as the Curie temperatures. This result is attributed to the spin-glass-like surface layer on the nanocrystalline nickel ferrite with a ferrimagnetically aligned core.
Electro-Optically Switchable Optical True Delay Lines of Meter-Scale Lengths Fabricated on Lithium Niobate on Insulator Using Photolithography Assisted Chemo-Mechanical Etching
Jun-xia Zhou, Ren-hong Gao, Jintian Lin, Min Wang, Wei Chu, Wen-bo Li, Di-feng Yin, Li Deng, Zhi-wei Fang, Jian-hao Zhang, Rong-bo Wuand Ya Cheng
Chin. Phys. Lett.    2020, 37 (8): 084201 .   DOI: 10.1088/0256-307X/37/8/084201
Abstract   HTML   PDF (1221KB)
Optical true delay lines (OTDLs) of low propagation losses, small footprints and high tuning speeds and efficiencies are of critical importance for various photonic applications. Here, we report fabrication of electro-optically switchable OTDLs on lithium niobate on insulator using photolithography assisted chemo-mechanical etching. Our device consists of several low-loss optical waveguides of different lengths which are consecutively connected by electro-optical switches to generate different amounts of time delay. The fabricated OTLDs show an ultra-low propagation loss of $\sim 0.03$ dB/cm for waveguide lengths well above 100 cm.
Electrical Conductivity and Current--Voltage Characteristics of Individual Conducting Polymer PEDOT Nanowires
LONG Yun-Ze, DUVAIL Jean-Luc, CHEN Zhao-Jia, JIN Ai-Zi, GU Chang-Zhi
Chin. Phys. Lett.    2008, 25 (9): 3474-3477 .  
Abstract   PDF (1280KB)
We report the current--voltage (I-V) characteristics and electrical conductivity of individual template-synthesized poly(3,4-ethylenedioxythiophene) (PEDOT)
nanowires (190±6 nm in diameter and σRT= 11.2±2,Ω-1cm-1) over a wide temperature range from 300 to 10K. With lowering temperature, the I-V characteristics become nonlinear around 50K, and a clear Coulomb gap-like structure appears in the differential conductance (dI/dV) spectra. The
temperature dependence of the resistance below 70K follows ln R T1/2, which can be interpreted as Efros--Shklovskii hopping conduction in the presence of a Coulomb gap. In addition, the influences of measurement methods such as the applied bias voltage magnitude, the two-probe and four-probe techniques used in the resistance measurements are also reported and discussed.
Bistability in a Hybrid Optomechanical System under the Effect of a Nonlinear Medium
A. Asghari Nejad, H. R. Askari, H. R. Baghshahi
Chin. Phys. Lett.    2017, 34 (8): 084205 .   DOI: 10.1088/0256-307X/34/8/084205
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We investigate a hybrid optomechanical system consisting of two coupled cavities, one of them is composed of two-end fixed mirrors (called the traditional cavity), and the other has a one-end oscillating mirror (named as the optomechanical cavity). A Kerr medium is inside the traditional cavity to enhance the nonlinearity due to the fact that it can cause observing of bistable behavior in intracavity intensity for the optomechanical cavity. The Hamiltonian of the system is written in a rotating frame and its dynamics is described by quantum Langevin equations of motion. Our proposed system exhibits unconventional plots for the mean photon number of the optomechanical cavity which are not observed in previous works. The present results show a deep effect of the Kerr medium on optical bistability of intracavity intensity for the optomechanical cavity. Also, coupling strength of the cavities can effectively change the stability of the system.
Experimental Realization of an Intrinsic Magnetic Topological Insulator
Yan Gong, Jingwen Guo, Jiaheng Li, Kejing Zhu, Menghan Liao, Xiaozhi Liu, Qinghua Zhang, Lin Gu, Lin Tang, Xiao Feng, Ding Zhang, Wei Li, Canli Song, Lili Wang, Pu Yu, Xi Chen, Yayu Wang, Hong Yao, Wenhui Duan, Yong Xu, Shou-Cheng Zhang, Xucun Ma, Qi-Kun Xue, Ke He
Chin. Phys. Lett.    2019, 36 (7): 076801 .   DOI: 10.1088/0256-307X/36/7/076801
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An intrinsic magnetic topological insulator (TI) is a stoichiometric magnetic compound possessing both inherent magnetic order and topological electronic states. Such a material can provide a shortcut to various novel topological quantum effects but remained elusive experimentally for a long time. Here we report the experimental realization of thin films of an intrinsic magnetic TI, MnBi$_{2}$Te$_{4}$, by alternate growth of a Bi$_{2}$Te$_{3}$ quintuple layer and a MnTe bilayer with molecular beam epitaxy. The material shows the archetypical Dirac surface states in angle-resolved photoemission spectroscopy and is demonstrated to be an antiferromagnetic topological insulator with ferromagnetic surfaces by magnetic and transport measurements as well as first-principles calculations. The unique magnetic and topological electronic structures and their interplays enable the material to embody rich quantum phases such as quantum anomalous Hall insulators and axion insulators at higher temperature and in a well-controlled way.
Accuracy of Machine Learning Potential for Predictions of Multiple-Target Physical Properties
Yulou Ouyang, Zhongwei Zhang, Cuiqian Yu, Jia He, Gang Yan, and Jie Chen
Chin. Phys. Lett.    2020, 37 (12): 126301 .   DOI: 10.1088/0256-307X/37/12/126301
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The accurate and rapid prediction of materials' physical properties, such as thermal transport and mechanical properties, are of particular importance for potential applications of featuring novel materials. We demonstrate, using graphene as an example, how machine learning potential, combined with the Boltzmann transport equation and molecular dynamics simulations, can simultaneously provide an accurate prediction of multiple-target physical properties, with an accuracy comparable to that of density functional theory calculation and/or experimental measurements. Benchmarked quantities include the Grüneisen parameter, the thermal expansion coefficient, Young's modulus, Poisson's ratio, and thermal conductivity. Moreover, the transferability of commonly used empirical potential in predicting multiple-target physical properties is also examined. Our study suggests that atomic simulation, in conjunction with machine learning potential, represents a promising method of exploring the various physical properties of novel materials.