Volume 38 Issue 4

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Continuous-Variable Measurement-Device-Independent Quantum Key Distribution with One-Time Shot-Noise Unit Calibration
Luyu Huang , Yichen Zhang, and Song Yu 
Chin. Phys. Lett. 2021, 38 (4): 040301 .   DOI: 10.1088/0256-307X/38/4/040301
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Imperfections in practical detectors, including limited detection efficiency, and inherent electronic noise, can seriously decrease the transmission distance of continuous-variable measurement-device-independent quantum key distribution systems. Owing to the difficulties inherent in realizing a high-efficiency fiber homodyne detector, challenges still exist in continuous-variable measurement-device-independent quantum key distribution system implementation. We offer an alternative approach in an attempt to solve these difficulties and improve the potential for system implementation. Here, a novel practical detector modeling method is utilized, which is combined with a one-time shot-noise-unit calibration method for the purpose of system realization. The new modeling method benefits greatly from taking advantage of one-time shot-noise-unit calibration methods, such as measuring electronic noise and shot noise directly to a novel shot-noise unit, so as to eliminate the statistical fluctuations found in previous methods; this makes the implementation of such systems simpler, and the calibration progress more accurate. We provide a simulation of the secret key rate versus distance with different parameters. In addition, the minimal detection efficiency required at each distance, as well as the contrast between the two methods, are also shown, so as to provide a reference in terms of system realization.
Phase-Modulated 2D Topological Physics in a One-Dimensional Ultracold System
Gang-Feng Guo, Xi-Xi Bao, Lei Tan, and Huai-Qiang Gu
Chin. Phys. Lett. 2021, 38 (4): 040302 .   DOI: 10.1088/0256-307X/38/4/040302
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We propose a one-dimensional optical lattice model to simulate and explore two-dimensional topological phases with ultracold atoms, considering the phases of the hopping strengths as an extra dimension. It is shown that the model exhibits nontrivial phases, and corresponding two chiral-edge states. Moreover, we demonstrate the connections between changes in the topological invariants and the Dirac points. Furthermore, the topological order detected by the particle pumping approach in cold atoms is also investigated. The results obtained here provide a feasible and flexible method of simulating and exploring high-dimensional topological phases in low-dimension systems via the controllable phase of the hopping strength.
Bidirectional Information Flow Quantum State Tomography
Huikang Huang, Haozhen Situ, and Shenggen Zheng
Chin. Phys. Lett. 2021, 38 (4): 040303 .   DOI: 10.1088/0256-307X/38/4/040303
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The exact reconstruction of many-body quantum systems is one of the major challenges in modern physics, because it is impractical to overcome the exponential complexity problem brought by high-dimensional quantum many-body systems. Recently, machine learning techniques are well used to promote quantum information research and quantum state tomography has also been developed by neural network generative models. We propose a quantum state tomography method, which is based on a bidirectional gated recurrent unit neural network, to learn and reconstruct both easy quantum states and hard quantum states in this study. We are able to use fewer measurement samples in our method to reconstruct these quantum states and to obtain high fidelity.
Excess Diffusion of a Driven Colloidal Particle in a Convection Array
Qingqing Yin, Yunyun Li, Fabio Marchesoni, Debajyoti Debnath, and Pulak K. Ghosh
Chin. Phys. Lett. 2021, 38 (4): 040501 .   DOI: 10.1088/0256-307X/38/4/040501
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We numerically investigate the transport of a passive colloidal particle in a periodic array of planar counter-rotating convection rolls, at high Péclet numbers. It is shown that an external bias, oriented parallel to the array, produces a huge excess diffusion peak, in cases where bias and advection drag become comparable. This effect is not restricted to one-dimensional convection geometries, and occurs independently of the array's boundary conditions.
Exploring $J/\psi$ Production Mechanism at the Future Electron-Ion Collider
Jian-Wei Qiu, Xiang-Peng Wang, and Hongxi Xing
Chin. Phys. Lett. 2021, 38 (4): 041201 .   DOI: 10.1088/0256-307X/38/4/041201
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We propose to use transverse momentum $p_{\rm T}$ distribution of $J/\psi$ production at the future Electron Ion Collider (EIC) to explore the production mechanism of heavy quarkonia in high energy collisions. We apply QCD and QED collinear factorization to the production of a $c\bar{c}$ pair at high $p_{\rm T}$, and non-relativistic QCD factorization to the hadronization of the pair to a $J/\psi$. We evaluate $J/\psi$ $p_{\rm T}$-distribution at both leading and next-to-leading order in strong coupling, and show that production rates for various color-spin channels of a $c\bar{c}$ pair in electron-hadron collisions are very different from that in hadron-hadron collisions, which provides a strong discriminative power to determine various transition rates for the pair to become a $J/\psi$. We predict that the $J/\psi$ produced in electron-hadron collisions is likely unpolarized, and the production is an ideal probe for gluon distribution of colliding hadron (or nucleus). We find that the $J/\psi$ production is dominated by the color-octet channel, providing an excellent probe to explore the gluon medium in large nuclei at the EIC.
Constraining Isovector Nuclear Interactions with Giant Dipole Resonance and Neutron Skin in $^{208}$Pb from a Bayesian Approach
Jun Xu
Chin. Phys. Lett. 2021, 38 (4): 042101 .   DOI: 10.1088/0256-307X/38/4/042101
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The remaining uncertainties in relation to isovector nuclear interactions call for reliable experimental measurements of isovector probes in finite nuclei. Based on the Bayesian analysis, although neutron-skin thickness data or isovector giant dipole resonance data in $^{208}$Pb can constrain only one isovector interaction parameter, correlations among other parameters can also be built. Using combined data for both the neutron-skin thickness and the isovector giant dipole resonance helps to significantly constrain all isovector interaction parameters; as such, it serves as a useful methodology for future research.
Higher-Twist Effect in Pion Parton Distribution
Lihong Wan and Jianhong Ruan
Chin. Phys. Lett. 2021, 38 (4): 042501 .   DOI: 10.1088/0256-307X/38/4/042501
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A higher-twist modified parton evolution equation is used to evolve the initial valence quark distributions in pions, which are derived based on light-front quantization via BLFQ collaboration. The results are consistent with the valence quark distributions of the E615 experiment, and the pion structure function of the H1 experiment. The structure function data highlight the necessity for a higher-twist modification in the small $x$ region. Comparisons with some other models are also given.
Configuration Interaction of Electronic Structure and Spectroscopy of AlH and Its Cation
Shu-Tao Zhao, Jun Li, Rui Li, Shuang Yin, and Hui-Jie Guo
Chin. Phys. Lett. 2021, 38 (4): 043101 .   DOI: 10.1088/0256-307X/38/4/043101
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We carry out a detailed study of the low-lying states of AlH and AlH$^{+}$, using a multireference configuration interaction method. Based on the computed potential energy curves, the spectroscopic constants of bound $\varLambda$–$S$ states are fitted; these agree with the results for the measurements. The values of the permanent dipole moment of the $\varLambda$–$S$ states are calculated, and the charge transfer mechanism is discussed. Based on the calculated transition dipole moments and vibrational levels, the radiative lifetimes of bound states are determined. Finally, tunneling lifetimes, and $\nu' = 0$–2 vibrational levels of 4$^{2}\!\varSigma^{+}$ and 3$^{2}\!\varPi$ states with a potential barrier are investigated.
Strong-Field-Induced N$_{2}^{+}$ Air Lasing in Nitrogen Glow Discharge Plasma
Nana Dong, Yan Zhou, Shanbiao Pang, Xiaodong Huang, Ke Liu, Lunhua Deng, and Huailiang Xu
Chin. Phys. Lett. 2021, 38 (4): 043301 .   DOI: 10.1088/0256-307X/38/4/043301
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We investigate N$_{2}^{+}$ air lasing at 391 nm, induced by strong laser fields in a nitrogen glow discharge plasma. We generate forward N$_{2}^{+}$ air lasing on the $B^{2}\!\varSigma_{\rm u}^{+}(v'=0)$–$X^{2}\!\varSigma_{\rm g}^{+} (v'' =0)$ transition at 391 nm by irradiating an intense 35-fs, 800-nm laser in a pure nitrogen gas, finding that the 391-nm lasing quenches when the nitrogen gas is electrically discharged. In contrast, the 391-nm fluorescence measured from the side of the laser beam is strongly enhanced, demonstrating that this discharge promotes the population in the $B^{2}\!\varSigma_{\rm u}^{+}(v'=0)$ state. By comparing the lasing and fluorescence spectra of the nitrogen gas obtained in the discharged and laser-induced plasma, we show that the quenching of N$_{2}^{+}$ lasing is caused by the efficient suppression of population inversion between the $B^{2}\!\varSigma_{\rm u}^{+}$ and $X^{2}\!\varSigma_{\rm g}^{+}$ states of N$_{2}^{+}$, in which a much higher population occurs in the $X^{2}\!\varSigma_{\rm g}^{+}$ state in the discharge plasma. Our results clarify the important role of population inversion in generating N$_{2}^{+}$ air lasing, and also indicate the potential for the enhancement of N$_{2}^{+}$ lasing via further manipulation of the population in the $X^{2}\!\varSigma_{\rm g}^{+}$ state in the discharged medium.
A Noise-Robust Pulse for Excitation Transfer in a Multi-Mode Quantum Memory
Bo Gong , Tao Tu, Ao-Lin Guo , Le-Tian Zhu , and Chuan-Feng Li
Chin. Phys. Lett. 2021, 38 (4): 044201 .   DOI: 10.1088/0256-307X/38/4/044201
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Multi-mode quantum memory is a basic element required for long-distance quantum communication, as well as scalable quantum computation. For on-demand readout and long storage times, control pulses are crucial in order to transfer atomic excitations back and forth into spin excitations. Here, we introduce noise-robust composite pulse sequences for high-fidelity excitation transfer in multi-mode quantum memory. These pulses are robust to the deviations in amplitude and the detuning parameters of realistic conditions. We show the efficiency of these composite pulses with a typical rare-earth ion-doped system. This approach could be applied to a variety of quantum memory schemes.
Approach to Phonon Relaxation Time and Mean Free Path in Nonlinear Lattices
Yue Liu  and Dahai He
Chin. Phys. Lett. 2021, 38 (4): 044401 .   DOI: 10.1088/0256-307X/38/4/044401
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Based on the self-consistent phonon theory, the spectral energy density is calculated by the canonical transformation and the Fourier transformation. Through fitting the spectral energy density by the Lorentzian profile, the phonon frequency as well as the phonon relaxation time is obtained in one-dimensional nonlinear lattices, which is validated in the Fermi–Pasta–Ulam-$\beta$ (FPU-$\beta$) and $\phi^{4}$ lattices at different temperatures. The phonon mean free path is then evaluated in terms of the phonon relaxation time and phonon group velocity. The results show that, in the FPU-$\beta$ lattice, the phonon mean free path as well as the phonon relaxation time displays divergent power-law behavior. The divergent exponent coincides well with that derived from the Peierls–Boltzmann theory at weak anharmonic nonlinearity. The value of the divergent exponent expects a power-law divergent heat conductivity with system size, which violates Fourier's law. For the $\phi^{4}$ lattice, both the phonon relaxation time and mean free path are finite, which ensures normal heat conduction.
Generation of a Plasma Waveguide with Slow-Wave Structure
Xiao-Bo Zhang , Xin Qiao , Ai-Xia Zhang , and Ju-Kui Xue
Chin. Phys. Lett. 2021, 38 (4): 045201 .   DOI: 10.1088/0256-307X/38/4/045201
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Using the particle-in-cell simulations, we report an efficient scheme to generate a slow wave structure in the electron density of a plasma waveguide, based on the array laser–plasma interaction. The spatial distribution of the electron density of the plasma waveguide is modulated via effective control of the super-Gaussian index and array pattern code of the lasers. A complete overview of the holding time, and the bearable laser's intensity of the electron density structure of the plasma waveguide, is obtained. In addition, the holding time of the slow wave structure of the plasma waveguide is also controlled by adjusting the frequency of the array laser beam. Finally, effects due to ion motion are discussed in detail.
Nonlinear Simulations of the Bump-on-Tail Instabilities in Tokamak Plasmas
Yumei Hou, Wei Chen, Liming Yu , Yunpeng Zou , Min Xu , and Xuru Duan 
Chin. Phys. Lett. 2021, 38 (4): 045202 .   DOI: 10.1088/0256-307X/38/4/045202
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We reproduce nonlinear behaviors, including frequency chirping and mode splitting, referred to as bump-on-tail instabilities. As has been reported in previous works, the generation and motion of phase-space hole-clump pairs in a kinetically driven, dissipative system can result in frequency chirping. We provide examples of frequency chirping, both with and without pure diffusion, in order to illustrate the role of the diffusion effect, which can suppress holes and clumps; Asymmetric frequency chirpings are produced with drag effect, which is essential to enhance holes, and suppress clumps. Although both diffusion and drag effect suppress the clumps, downward sweepings are observed, caused by a complicated interaction of diffusion and drag. In addition, we examine the discrepancies in frequency chirping between marginally unstable, and far from marginally unstable cases, which we elucidate by means of a dissipative system. In addition, mode splitting is also produced via BOT code for a marginal case with large diffusion.
Energetic Particle Transport Prediction for CFETR Steady State Scenario Based on Critical Gradient Model
Yunpeng Zou, V. S. Chan, Wei Chen, Yongqin Wang, Yumei Hou, and Yiren Zhu
Chin. Phys. Lett. 2021, 38 (4): 045203 .   DOI: 10.1088/0256-307X/38/4/045203
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The critical gradient mode (CGM) is employed to predict the energetic particle (EP) transport induced by the Alfvén eigenmode (AE). To improve the model, the normalized critical density gradient is set as an inverse proportional function of energetic particle density; consequently, the threshold evolves during EP transport. Moreover, in order to consider the EP orbit loss mechanism in CGM, ORBIT code is employed to calculate the EP loss cone in phase space. With these improvements, the AE enhances EPs radial transport, pushing the particles into the loss cone. The combination of the two mechanisms raises the lost fraction to 6.6%, which is higher than the linear superposition of the two mechanisms. However, the loss is still far lower than that observed in current experiments. Avoiding significant overlap between the AE unstable region and the loss cone is a key factor in minimizing EP loss.
Coriolis Force Effect on Suppression of Neo-Classical Tearing Mode Triggered Explosive Burst in Reversed Magnetic Shear Tokamak Plasmas
Tong Liu , Lai Wei , Feng Wang, and Zheng-Xiong Wang 
Chin. Phys. Lett. 2021, 38 (4): 045204 .   DOI: 10.1088/0256-307X/38/4/045204
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We numerically investigate the Coriolis force effect on the suppression of an explosive burst, triggered by the neo-classical tearing mode, in reversed magnetic shear configuration tokamak plasmas, using a reduced magnetohydrodynamic model, including bootstrap current. Previous works have shown that applying differential poloidal rotation, with rotation shear located near the outer rational surface, is an effective way to suppress an explosive burst. In comparison with cases where there is no Coriolis force, the amplitude of differential poloidal rotation required to effectively suppress the explosive burst is clearly reduced once the effect of Coriolis force is taken into consideration. Moreover, the effective radial region of the rotation shear location is broadened in cases where the Coriolis force effect is present. Applying rotation with shear located between the radial positions of $q_{\rm min}$ and the outer rational surface always serves to effectively suppress explosive bursts, which we anticipate will reduce operational difficulties in controlling explosive bursts, and will consequently prevent plasma disruption in tokamak experiments.
Superior Mechanical Properties of GaAs Driven by Lattice Nanotwinning
Zhenjiang Han, Han Liu, Quan Li, Dan Zhou, and Jian Lv
Chin. Phys. Lett. 2021, 38 (4): 046201 .   DOI: 10.1088/0256-307X/38/4/046201
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Gallium arsenide (GaAs), a typical covalent semiconductor, is widely used in the electronic industry, owing to its superior electron transport properties. However, its brittle nature is a drawback that has so far significantly limited its application. An exploration of the structural deformation modes of GaAs under large strain at the atomic level, and the formulation of strategies to enhance its mechanical properties is highly desirable. The stress-strain relations and deformation modes of single-crystal and nanotwinned GaAs under various loading conditions are systematically investigated, using first-principles calculations. Our results show that the ideal strengths of nanotwinned GaAs are 14% and 15% higher than that of single-crystal GaAs under pure and indentation shear strains, respectively, without producing a significantly negative effect in terms of its electronic performance. The enhancement in strength stems from the rearrangement of directional covalent bonds at the twin boundary. Our results offer a fundamental understanding of the mechanical properties of single crystal GaAs, and provide insights into the strengthening mechanism of nanotwinned GaAs, which could prove highly beneficial in terms of developing reliable electronic devices.
Zintl Phase BaAgSb: Low Thermal Conductivity and High Performance Thermoelectric Material in Ab Initio Calculation
Shao-Fei Wang, Zhi-Gang Zhang, Bao-Tian Wang, Jun-Rong Zhang, and Fang-Wei Wang
Chin. Phys. Lett. 2021, 38 (4): 046301 .   DOI: 10.1088/0256-307X/38/4/046301
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Thermoelectric materials are critical parts in thermal electric devices. Here, Zintl phase BaAgSb in space group of P6$_3$/mmc is reported as a promising thermoelectric material in density function theory. The anisotropic lattice thermal conductivity and phonon transport properties are investigated in theory. The strong phonon-phonon scattering in BaAgSb exhibits ultra-low lattice thermal conductivity of 0.59 W$\cdot$m$^{-1}$$\cdot$K$^{-1}$ along $c$-axis at 800 K, and high thermoelectric performance ZT = 0.94 at 400 K. The mix of covalent and ionic bond supports high carrier mobility and low thermal conductivity. The unusual features make BaAgSb a potential thermoelectric material.
Non-Monotonic Evolution of Carrier Density and Mobility under Thermal Cycling Treatments in Dirac Semimetal Cd$_{3}$As$_{2}$ Microbelts
Zheng Chen, Min Wu, Yequn Liu, Wenshuai Gao, Yuyan Han, Jianhui Zhou, Wei Ning, and Mingliang Tian
Chin. Phys. Lett. 2021, 38 (4): 047201 .   DOI: 10.1088/0256-307X/38/4/047201
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Tunable carrier density plays a key role in the investigation of novel transport properties in three-dimensional topological semimetals. We demonstrate that the carrier density, as well as the mobility, of Dirac semimetal Cd$_{3}$As$_{2}$ nanoplates can be effectively tuned via in situ thermal treatment at 350 K for one hour, resulting in non-monotonic evolution by virtue of the thermal cycling treatments. The upward shift of Fermi level relative to the Dirac nodes blurs the surface Fermi-arc states, accompanied by an anomalous phase shift in the oscillations of bulk states, due to a change in the topology of the electrons. Meanwhile, the oscillation peaks of bulk longitudinal magnetoresistivity shift at high fields, due to their coupling to the oscillations of the surface Fermi-arc states. Our work provides a thermal control mechanism for the manipulation of quantum states in Dirac semimetal Cd$_{3}$As$_{2}$ at high temperatures, via their carrier density.
Emergence of Chern Insulating States in Non-Magic Angle Twisted Bilayer Graphene
Cheng Shen, Jianghua Ying, Le Liu, Jianpeng Liu, Na Li, Shuopei Wang, Jian Tang, Yanchong Zhao, Yanbang Chu, Kenji Watanabe, Takashi Taniguchi, Rong Yang, Dongxia Shi, Fanming Qu, Li Lu, Wei Yang, and Guangyu Zhang
Chin. Phys. Lett. 2021, 38 (4): 047301 .   DOI: 10.1088/0256-307X/38/4/047301
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Twisting two layers into a magic angle (MA) of $\sim$$1.1^{\circ}$ is found essential to create low energy flat bands and the resulting correlated insulating, superconducting, and magnetic phases in twisted bilayer graphene (TBG). While most of previous works focus on revealing these emergent states in MA-TBG, a study of the twist angle dependence, which helps to map an evolution of these phases, is yet less explored. Here, we report a magneto-transport study on one non-magic angle TBG device, whose twist angle $\theta$ changes from 1.25$^{\circ}$ at one end to 1.43$^{\circ}$ at the other. For $\theta =1.25^{\circ}$ we observe an emergence of topological insulating states at hole side with a sequence of Chern number $\left| C \right|=4-\left| v \right|$, where $v$ is the number of electrons (holes) in moiré unite cell. When $\theta >1.25^{\circ}$, the Chern insulator from flat band disappears and evolves into fractal Hofstadter butterfly quantum Hall insulator where magnetic flux in one moiré unite cell matters. Our observations will stimulate further theoretical and experimental investigations on the relationship between electron interactions and non-trivial band topology.
Physical Properties Revealed by Transport Measurements for Superconducting Nd$_{0.8}$Sr$_{0.2}$NiO$_{2}$ Thin Films
Ying Xiang, Qing Li, Yueying Li, Huan Yang, Yuefeng Nie, and Hai-Hu Wen
Chin. Phys. Lett. 2021, 38 (4): 047401 .   DOI: 10.1088/0256-307X/38/4/047401
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The newly discovered superconductivity in infinite-layer nickelate superconducting films has attracted much attention, largely because their crystalline and electronic structures are similar to those of high-$T_{\rm c}$ cuprate superconductors. The upper critical field can provide a great deal of information on the subject of superconductivity, but detailed experimental data are still lacking for these films. We present the temperature- and angle-dependence of resistivity, measured under different magnetic fields $H$ in Nd$_{0.8}$Sr$_{0.2}$NiO$_{2}$ thin films. The onset superconducting transition occurs at about 16.2 K at 0 T. Temperature-dependent upper critical fields, determined using a criterion very close to the onset transition, show a clear negative curvature near the critical transition temperature, which can be explained as a consequence of the paramagnetically limited effect on superconductivity. The temperature-dependent anisotropy of the upper critical field is obtained from resistivity data, which yields a value decreasing from 3 to 1.2 with a reduction in temperature. This can be explained in terms of the variable contribution from the orbital limit effect on the upper critical field. The angle-dependence of resistivity at a fixed temperature, and at different magnetic fields, cannot be scaled to a curve, which deviates from the prediction of the anisotropic Ginzburg–Landau theory. However, at low temperatures, the resistance difference can be scaled via the parameter $H^\beta |\cos\theta|$ ($\beta=6$–1), with $\theta$ being the angle enclosed between the $c$-axis and the applied magnetic field. As the first detailed study of the upper critical field of nickelate thin films, our results clearly indicate a small anisotropy, and a paramagnetically limited effect, in terms of superconductivity, in nickelate superconductors.
Combined Study of Structural, Magnetic and Transport Properties of Eu$_{0.5}$$Ln$$_{0.5}$BiS$_{2}$F Superconductor
Hui-Fei Zhai, Bo Lin, Pan Zhang, Hao Jiang, Yu-Ke Li, and Guang-Han Cao
Chin. Phys. Lett. 2021, 38 (4): 047402 .   DOI: 10.1088/0256-307X/38/4/047402
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Superconductivity below 0.3 K and a charge-density-wave-like (CDW-like) anomaly at 280 K were observed in EuBiS$_{2}$F recently. Here we report a systematic study of structural and transport properties in Eu$_{0.5}Ln_{0.5}$BiS$_{2}$F ($Ln$ = La, Ce, Pr, Nd, Sm) by electrical resistivity, magnetization, and specific heat measurements. The lattice constants have a significant change upon rare earth substitution for Eu, suggesting an effective doping. As $Ln$ is changed from Sm to La, the superconducting transition temperature $T_{\rm c}$ increases from 1.55 K to 2.8 K. In contrast to the metallic parent compound, the temperature dependence of electrical resistivity displays semiconducting-like behavior for all the Eu$_{0.5}Ln_{0.5}$BiS$_{2}$F samples. Meanwhile, the CDW-like anomaly observed in EuBiS$_{2}$F is completely suppressed. Unlike the mixed valence state in the undoped compound, Eu ions in these rare-earth-doped samples are mainly divalent. A specific anomaly at 1.3 K resembling that in EuBiS$_{2}$F suggests the coexistence of superconductivity and spin glass state for Eu$_{0.5}$La$_{0.5}$BiS$_{2}$F. Coexistence of ferromagnetic order and superconductivity is found below 2.2 K in Eu$_{0.5}$Ce$_{0.5}$BiS$_{2}$F samples. Our results supplies a rich diagram showing that many interesting properties can be induced in BiS$_{2}$-based compounds.
Spin-Wave Dynamics in an Artificial Kagome Spin Ice
Qiuyang Li, Suqin Xiong, Lina Chen, Kaiyuan Zhou, Rongxin Xiang, Haotian Li, Zhenyu Gao, Ronghua Liu, and Youwei Du
Chin. Phys. Lett. 2021, 38 (4): 047501 .   DOI: 10.1088/0256-307X/38/4/047501
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Artificial spin ice (ASI) structures have significant technological potential as reconfigurable metamaterials and magnetic storage media. We investigate the field/frequency-dependent magnetic dynamics of a kagome ASI made of 25-nm-thick permalloy nanomagnet elements, combining magnetoresistance (MR) and microscale ferromagnetic resonance (FMR) techniques. Our FMR spectra show a broadband absorption spectrum from 0.2 GHz to 3 GHz at $H$ below 0.3 kOe, where the magnetic configuration of the kagome ASI is in the multidomain state, because the external magnetic field is below the obtained coercive field $H_{\rm c} \sim 0.3$ kOe, based on both the low-field range MR loops and simulations, suggesting that the low-field magnetization dynamics of kagome ASI is dominated by a multimode resonance regime. However, the FMR spectra exhibit five distinctive resonance modes at the high-field quasi-uniform magnetization state. Furthermore, our micromagnetic simulations provide additional spatial resolution of these resonance modes, identifying the presence of two high-frequency primary modes, localized in the horizontal and vertical bars of the ASI, respectively; three other low-frequency modes are mutually exclusive and separately pinned at the corners of the kagome ASI by an edge-induced dipolar field. Our results suggest that an ASI structural design can be adopted as an efficient approach for the development of low-power filters and magnonic devices.
Rare-Earth Chalcohalides: A Family of van der Waals Layered Kitaev Spin Liquid Candidates
Jianting Ji, Mengjie Sun, Yanzhen Cai, Yimeng Wang, Yingqi Sun, Wei Ren, Zheng Zhang, Feng Jin, and Qingming Zhang
Chin. Phys. Lett. 2021, 38 (4): 047502 .   DOI: 10.1088/0256-307X/38/4/047502
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The Kitaev spin liquid (KSL) system has attracted tremendous attention in recent years because of its fundamental significance in condensed matter physics and promising applications in fault-tolerant topological quantum computation. Material realization of such a system remains a major challenge in the field due to the unusual configuration of anisotropic spin interactions, though great effort has been made before. Here we reveal that rare-earth chalcohalides REChX (RE = rare earth; Ch = O, S, Se, Te; X = F, Cl, Br, I) can serve as a family of KSL candidates. Most family members have the typical SmSI-type structure with a high symmetry of $R\bar{3}m$, and rare-earth magnetic ions form an undistorted honeycomb lattice. The strong spin-orbit coupling of $4f$ electrons intrinsically offers anisotropic spin interactions as required by the Kitaev model. We have grown the crystals of YbOCl and synthesized the polycrystals of SmSI, ErOF, HoOF and DyOF, and made careful structural characterizations. We carry out magnetic and heat capacity measurements down to 1.8 K and find no obvious magnetic transition in all the samples but DyOF. The van der Waals interlayer coupling highlights the true two-dimensionality of the family which is vital for the exact realization of Abelian/non-Abelian anyons, and the graphene-like feature will be a prominent advantage for developing miniaturized devices. The family is expected to act as an inspiring material platform for the exploration of KSL physics.
Polarization Mechanism in Filled Tungsten Bronze Ba$_{4}$Eu$_{2}$Ti$_{4}$Nb$_{6}$O$_{30}$ with Pinched $P$–$E$ Hysteresis Loops
Lang Zhu , Xiao-Li Zhu, Xiao-Qiang Liu , and Xiang-Ming Chen 
Chin. Phys. Lett. 2021, 38 (4): 047701 .   DOI: 10.1088/0256-307X/38/4/047701
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Pinched $P$–$E$ hysteresis loops have been observed in filled tungsten bronze Ba$_{4}$Eu$_{2}$Ti$_{4}$Nb$_{6}$O$_{30}$, indicating the presence of novel polarization mechanisms. We investigate the evolution of polar order in filled tungsten bronze Ba$_{4}$Eu$_{2}$Ti$_{4}$Nb$_{6}$O$_{30}$, together with its dielectric properties over a wide temperature range, from 50 K to 773 K. The temperature dependences of the dielectric properties exhibit two low-temperature dielectric relaxations, at around 300 K (P1), and 100 K (P2), and a high temperature peak at 588 K with no frequency dispersion, indicating the ferroelectric transition temperature $T_{\rm c}$. Pinched $P$–$E$ loops are observed in the temperature range between the low temperature relaxation at P1, and the ferroelectric transition. On cooling, the pinched $P$–$E$ hysteresis loops open gradually, with increasing remnant polarization ($P_{\rm r}$). Two pairs of reversal electric fields indicate two types of polar reversal mechanisms, with an activated energy of 1.41 eV ($E_{1}$), and 0.94 eV ($E_{2}$), respectively. One corresponds to the field-induced transition from a nonpolar to a polar state, which dominates at a high temperature close to $T_{\rm c}$, while the other relates to the reversal of ferroelectric domains which stabilize gradually on cooling. At temperatures below 300 K, the polarization exhibits an evident decrease, probably related to the disruption of the polar order due to the dielectric relaxation at P1.
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