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Localization Dynamics at the Exceptional Point of Non-Hermitian Creutz Ladder
S. M. Zhang, T. Y. He, and L. Jin
Chin. Phys. Lett.    2024, 41 (2): 027201 .   DOI: 10.1088/0256-307X/41/2/027201
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We propose a quasi-one-dimensional non-Hermitian Creutz ladder with an entirely flat spectrum by introducing alternating gain and loss components while maintaining inversion symmetry. Destructive interference generates a flat spectrum at the exceptional point, where the Creutz ladder maintains coalesced and degenerate eigenvalues with compact localized states distributed in a single plaquette. All excitations are completely confined within the localization area, unaffected by gain and loss. Single-site excitations exhibit nonunitary dynamics with intensities increasing due to level coalescence, while multiple-site excitations may display oscillating or constant intensities at the exceptional point. These results provide insights into the fascinating dynamics of non-Hermitian localization, where level coalescence and degeneracy coexist at the exceptional point.
Ultrathin Limit on the Anisotropic Superconductivity of Single-Layered Cuprate Films
Feng Ran, Pan Chen, Dingyi Li, Peiyu Xiong, Zixin Fan, Haoming Ling, Yan Liang, and Jiandi Zhang
Chin. Phys. Lett.    2024, 41 (2): 027401 .   DOI: 10.1088/0256-307X/41/2/027401
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Exploring dimensionality effects on cuprates is important for understanding the nature of high-temperature superconductivity. By atomically layer-by-layer growth with oxide molecular beam epitaxy, we demonstrate that La$_{2- x}$Sr$_{x}$CuO$_{4}$ ($x = 0.15$) thin films remain superconducting down to 2 unit cells of thickness but quickly reach the maximum superconducting transition temperature at and above 4 unit cells. By fitting the critical magnetic field (${\mu_{0}H}_{\rm c2}$), we show that the anisotropy of the film's superconductivity increases with decreasing film thickness, indicating that the superconductivity of the film gradually evolves from weak three- to two-dimensional character. These results are helpful to gain more insight into the nature of high-temperature superconductivity with dimensionality.
Strong Anisotropic Order Parameters at All-Nitride Ferromagnet/Superconductor Interfaces
Qiao Jin, Meng Yang, Guozhu Song, Nan Zhao, Shengru Chen, Haitao Hong, Ting Cui, Dongke Rong, Qianying Wang, Yiyan Fan, Chen Ge, Can Wang, Jiachang Bi, Yanwei Cao, Liusuo Wu, Shanmin Wang, Kui-Juan Jin, Zhi-Gang Cheng, and Er-Jia Guo
Chin. Phys. Lett.    2024, 41 (2): 027402 .   DOI: 10.1088/0256-307X/41/2/027402
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Proximity effects between superconductors and ferromagnets (SC/FM) hold paramount importance in comprehending the spin competition transpiring at their interfaces. This competition arises from the interplay between Cooper pairs and ferromagnetic exchange interactions. The proximity effects between transition metal nitrides (TMNs) are scarcely investigated due to the formidable challenges of fabricating high-quality SC/FM interfaces. We fabricated heterostructures comprising SC titanium nitride (TiN) and FM iron nitride (Fe$_{3}$N) with precise chemical compositions and atomically well-defined interfaces. The magnetoresistance of Fe$_{3}$N/TiN heterostructures shows a distinct magnetic anisotropy and strongly depends on the external perturbations. Moreover, the superconducting transition temperature $T_{\scriptscriptstyle{\rm C}}$ and critical field of TiN experience notable suppression when proximity to Fe$_{3}$N. We observe the intriguing competition of interfacial spin orientations near $T_{\scriptscriptstyle{\rm C}}$ ($\sim$ $1.25$ K). These findings not only add a new materials system for investigating the interplay between superconductor and ferromagnets, but also potentially provide a building block for future research endeavors and applications in the realms of superconducting spintronic devices.
Synthesis Methods and Property Control of Two-Dimensional Magnetic Materials
Ming-Shuang Li, Hui-Min Li, and Song Liu
Chin. Phys. Lett.    2024, 41 (2): 027501 .   DOI: 10.1088/0256-307X/41/2/027501
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Two-dimensional (2D) magnetic materials have been demonstrated to have excellent chemical, optical, electrical, and magnetic properties, particularly in the development of multifunctional electronic and spin electronic devices, showcasing tremendous potential. Therefore, corresponding synthesis techniques for 2D magnetic materials that offer high quality, high yield, low cost, time-saving, and simplicity are highly desired. This review provides a comprehensive overview of recent research advances in preparation of magnetic 2D materials, with a particular focus on the preparation methods employed. Moreover, the characteristics and applications of these magnetic materials are also discussed. Finally, the challenges and prospects of synthesis methods for magnetic 2D materials are briefly addressed. This review serves as a guiding reference for the controlled synthesis of 2D magnetic materials.
Magnetic Topological Dirac Semimetal Transition Driven by SOC in EuMg$_2$Bi$_2$
J. M. Wang, H. J. Qian, Q. Jiang, S. Qiao, and M. Ye
Chin. Phys. Lett.    2024, 41 (1): 017101 .   DOI: 10.1088/0256-307X/41/1/017101
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Magnetic topological semimetals have been at the forefront of condensed matter physics due to their ability to exhibit exotic transport phenomena. Investigating the interplay between magnetic and topological orders in systems with broken time-reversal symmetry is crucial for realizing non-trivial quantum effects. We delve into the electronic structure of the rare-earth-based antiferromagnetic Dirac semimetal EuMg$_2$Bi$_2$ using first-principles calculations and angle-resolved photoemission spectroscopy. Our calculations reveal that the spin–orbit coupling (SOC) in EuMg$_2$Bi$_2$ prompts an insulator to topological semimetal transition, with the Dirac bands protected by crystal symmetries. The linearly dispersive states near the Fermi level, primarily originating from Bi 6$p$ orbitals, are observed on both the (001) and (100) surfaces, confirming that EuMg$_2$Bi$_2$ is a three-dimensional topological Dirac semimetal. This research offers pivotal insights into the interplay between magnetism, SOC and topological phase transitions in spintronics applications.
Predicted Critical State Based on Invariance of the Lyapunov Exponent in Dual Spaces
Tong Liu and Xu Xia
Chin. Phys. Lett.    2024, 41 (1): 017102 .   DOI: 10.1088/0256-307X/41/1/017102
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Critical states in disordered systems, fascinating and subtle eigenstates, have attracted a lot of research interests. However, the nature of critical states is difficult to describe quantitatively, and in general, it cannot predict a system that hosts the critical state. We propose an explicit criterion whereby the Lyapunov exponent of the critical state should be 0 simultaneously in dual spaces, namely the Lyapunov exponent remains invariant under the Fourier transform. With this criterion, we can exactly predict a one-dimensional quasiperiodic model which is not of self-duality, but hosts a large number of critical states. Then, we perform numerical verification of the theoretical prediction and display the self-similarity of the critical state. Due to computational complexity, calculations are not performed for higher dimensional models. However, since the description of extended and localized states by the Lyapunov exponent is universal and dimensionless, utilizing the Lyapunov exponent of dual spaces to describe critical states should also be universal. Finally, we conjecture that some kind of connection exists between the invariance of the Lyapunov exponent and conformal invariance, which can promote the research of critical phenomena.
Signature of Superconductivity in Pressurized La$_{4}$Ni$_{3}$O$_{10}$
Qing Li, Ying-Jie Zhang, Zhe-Ning Xiang, Yuhang Zhang, Xiyu Zhu, and Hai-Hu Wen
Chin. Phys. Lett.    2024, 41 (1): 017401 .   DOI: 10.1088/0256-307X/41/1/017401
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The discovery of high-temperature superconductivity near 80 K in bilayer nickelate La$_{3}$Ni$_{2}$O$_{7}$ under high pressures has renewed the exploration of superconducting nickelate in bulk materials. The extension of superconductivity in other nickelates in a broader family is also essential. Here, we report the experimental observation of superconducting signature in trilayer nickelate La$_{4}$Ni$_{3}$O$_{10}$ under high pressures. By using a modified sol-gel method and post-annealing treatment under high oxygen pressure, we successfully obtained polycrystalline La$_{4}$Ni$_{3}$O$_{10}$ samples with different transport behaviors at ambient pressure. Then we performed high-pressure electrical resistance measurements on these samples in a diamond-anvil-cell apparatus. Surprisingly, the signature of possible superconducting transition with a maximum transition temperature ($T_{\rm c}$) of about 20 K under high pressures is observed, as evidenced by a clear drop of resistance and the suppression of resistance drops under magnetic fields. Although the resistance drop is sample-dependent and relatively small, it appears in all of our measured samples. We argue that the observed superconducting signal is most likely to originate from the main phase of La$_{4}$Ni$_{3}$O$_{10}$. Our findings will motivate the exploration of superconductivity in a broader family of nickelates and shed light on the understanding of the underlying mechanisms of high-$T_{\rm c}$ superconductivity in nickelates.
High-Temperature Superconductivity in La$_3$Ni$_2$O$_7$
Kun Jiang, Ziqiang Wang, and Fu-Chun Zhang
Chin. Phys. Lett.    2024, 41 (1): 017402 .   DOI: 10.1088/0256-307X/41/1/017402
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Motivated by the recent discovery of high-temperature superconductivity in bilayer La$_3$Ni$_2$O$_7$ under pressure, we study its electronic properties and superconductivity due to strong electron correlation. Using the inversion symmetry, we decouple the low-energy electronic structure into block-diagonal symmetric and antisymmetric sectors. It is found that the antisymmetric sector can be reduced to a one-band system near half filling, while the symmetric bands occupied by about two electrons are heavily overdoped individually. Using the strong coupling mean field theory, we obtain strong superconducting pairing with $B_{\rm 1g}$ symmetry in the antisymmetric sector. We propose that due to the spin-orbital exchange coupling between the two sectors, $B_{\rm 1g}$ pairing is induced in the symmetric bands, which in turn boosts the pairing gap in the antisymmetric band and enhances the high-temperature superconductivity with a congruent d-wave symmetry in pressurized La$_3$Ni$_2$O$_7$.
A Composite Ansatz for Calculation of Dynamical Structure Factor
Yupei Zhang, Chongjie Mo, Ping Zhang, and Wei Kang
Chin. Phys. Lett.    2024, 41 (1): 017801 .   DOI: 10.1088/0256-307X/41/1/017801
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We propose an ansatz without adjustable parameters for the calculation of a dynamical structure factor. The ansatz combines the quasi-particle Green's function, especially the contribution from the renormalization factor, and the exchange-correlation kernel from time-dependent density functional theory together, verified for typical metals and semiconductors from a plasmon excitation regime to the Compton scattering regime. It has the capability to reconcile both small-angle and large-angle inelastic x-ray scattering (IXS) signals with much-improved accuracy, which can be used as the theoretical base model, in inversely inferring electronic structures of condensed matter from IXS experimental signals directly. It may also be used to diagnose thermal parameters, such as temperature and density, of dense plasmas in x-ray Thomson scattering experiments.
VASP2KP: $k\!\cdot\! p$ Models and Landé $g$-Factors from ab initio Calculations
Sheng Zhang, Haohao Sheng, Zhi-Da Song, Chenhao Liang, Yi Jiang, Song Sun, Quansheng Wu, Hongming Weng, Zhong Fang, Xi Dai, and Zhijun Wang
Chin. Phys. Lett.    2023, 40 (12): 127101 .   DOI: 10.1088/0256-307X/40/12/127101
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The $k\!\cdot\! p$ method is significant in condensed matter physics for the compact and analytical Hamiltonian. In the presence of magnetic field, it is described by the effective Zeeman's coupling Hamiltonian with Landé $g$-factors. Here, we develop an open-source package VASP2KP (including two parts: vasp2mat and mat2kp) to compute $k\!\cdot\! p$ parameters and Landé $g$-factors directly from the wavefunctions provided by the density functional theory (DFT) as implemented in Vienna ab initio Simulation Package (VASP). First, we develop a VASP patch vasp2mat to compute matrix representations of the generalized momentum operator $\hat{\boldsymbol \pi}=\hat{\boldsymbol p}+\frac{1}{2mc^2}[\hat{{\boldsymbol s}}\times\nabla V({\boldsymbol r})]$, spin operator $\hat{\boldsymbol s}$, time reversal operator $\hat{T}$, and crystalline symmetry operators $\hat{R}$ on the DFT wavefunctions. Second, we develop a python code mat2kp to obtain the unitary transformation $U$ that rotates the degenerate DFT basis towards the standard basis, and then automatically compute the $k\!\cdot\! p$ parameters and $g$-factors. The theory and the methodology behind VASP2KP are described in detail. The matrix elements of the operators are derived comprehensively and computed correctly within the projector augmented wave method. We apply this package to some materials, e.g., Bi$_2$Se$_3$, Na$_3$Bi, Te, InAs and 1H-TMD monolayers. The obtained effective model's dispersions are in good agreement with the DFT data around the specific wave vector, and the $g$-factors are consistent with experimental data. The VASP2KP package is available at https://github.com/zjwang11/VASP2KP.
Unconventional Nonreciprocal Voltage Transition in Ag$_{2}$Te Nanobelts
Peng-Liang Leng, Xiang-Yu Cao, Qiang Ma, Lin-Feng Ai, Yu-Da Zhang, Jing-Lei Zhang, and Fa-Xian Xiu
Chin. Phys. Lett.    2023, 40 (12): 127201 .   DOI: 10.1088/0256-307X/40/12/127201
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Nonreciprocal effects are consistently observed in noncentrosymmetric materials due to the intrinsic symmetry breaking and in high-conductivity systems due to the extrinsic thermoelectric effect. Meanwhile, nonreciprocal charge transport is widely utilized as an effective experimental technique for detecting intrinsic unidirectional electrical contributions. Here, we show an unconventional nonreciprocal voltage transition in topological insulator Ag$_{2}$Te nanobelts. The nonreciprocal voltage develops from nearly zero to giant values under the applied current $I_{\rm ac}$ and external magnetic fields, while remaining unchanged under various current $I_{\rm dc}$. This unidirectional electrical contribution is further evidenced by the differential resistance ($dV/dI$) measurements. Furthermore, the transition possesses two-dimensional properties under a tilted magnetic field and occurs when the voltage between two electrodes exceeds a certain value. We propose a possible mechanism based on the development of edge channels in Ag$_{2}$Te nanobelts to interpret the phenomenon. Our results not only introduce a peculiar nonreciprocal voltage transition in topological materials but also enrich the understanding of the intrinsic mechanism that strongly affects nonreciprocal charge transport.
Unleashing the Power of Moiré Materials in Neuromorphic Computing
John Paul Strachan
Chin. Phys. Lett.    2023, 40 (12): 127202 .   DOI: 10.1088/0256-307X/40/12/127202
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Realizations, Characterizations, and Manipulations of Two-Dimensional Electron Systems Floating above Superfluid Helium Surfaces
Haoran Wei, Mengmeng Wu, Renfei Wang, Mingcheng He, Hiroki Ikegami, Yang Liu, and Zhi Gang Cheng
Chin. Phys. Lett.    2023, 40 (12): 127301 .   DOI: 10.1088/0256-307X/40/12/127301
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Electron systems in low dimensions are enriched with many superior properties for both fundamental research and technical developments. Wide tunability of electron density, high mobility of motion, and feasible controllability in microscales are the most prominent advantages that researchers strive for. Nevertheless, it is always difficult to fulfill all in one solid-state system. Two-dimensional electron systems (2DESs) floating above the superfluid helium surfaces are thought to meet these three requirements simultaneously, ensured by the atomic smoothness of surfaces and the electric neutrality of helium. Here we report our recent work in preparing, characterizing, and manipulating 2DESs on superfluid helium. We realized a tunability of electron density over one order of magnitude and tuned their transport properties by varying electron distribution and measurement frequency. The work we engage in is crucial for advancing research in many-body physics and for development of single-electron quantum devices rooted in these electron systems.
Effective Bi-Layer Model Hamiltonian and Density-Matrix Renormalization Group Study for the High-$T_{\rm c}$ Superconductivity in La$_{3}$Ni$_{2}$O$_{7}$ under High Pressure
Yang Shen, Mingpu Qin, and Guang-Ming Zhang
Chin. Phys. Lett.    2023, 40 (12): 127401 .   DOI: 10.1088/0256-307X/40/12/127401
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High-$T_{\rm c}$ superconductivity with possible $T_{\rm c}\approx 80$ K has been reported in the single crystal of ${\rm La}_{3}{\rm Ni}_{2}{\rm O}_{7}$ under high pressure. Based on the electronic structure given by the density functional theory calculations, we propose an effective bi-layer model Hamiltonian including both $3d_{z^{2}}$ and $3d_{x^{2}-y^{2}}$ orbital electrons of the nickel cations. The main feature of the model is that the $3d_{z^{2}}$ electrons form inter-layer $\sigma$-bonding and anti-bonding bands via the apical oxygen anions between the two layers, while the $3d_{x^{2}-y^{2}}$ electrons hybridize with the $3d_{z^{2}}$ electrons within each NiO$_2$ plane. The chemical potential difference of these two orbital electrons ensures that the $3d_{z^{2}}$ orbitals are close to half-filling and the $3d_{x^{2}-y^{2}}$ orbitals are near quarter-filling. The strong on-site Hubbard repulsion of the $3d_{z^{2}}$ orbital electrons gives rise to an effective inter-layer antiferromagnetic spin super-exchange $J$. Applying pressure can self dope holes on the $3d_{z^{2}}$ orbitals with the same amount of electrons doped on the $3d_{x^{2}-y^{2}}$ orbitals. By performing numerical density-matrix renormalization group calculations on a minimum setup and focusing on the limit of large $J$ and small doping of $3d_{z^{2}}$ orbitals, we find the superconducting instability on both the $3d_{z^{2}}$ and $3d_{x^{2}-y^{2}}$ orbitals by calculating the equal-time spin singlet pair–pair correlation function. Our numerical results may provide useful insights in the high-$T_{\rm c}$ superconductivity in single crystal La$_3$Ni$_2$O$_7$ under high pressure.
Ground State and Its Topological Properties of Three-Dimensional Spin-Orbit Coupled Degenerate Fermi Gases
Long Xiong, Ming Gong, Zhao-Xiang Fang, and Rui Sun
Chin. Phys. Lett.    2023, 40 (12): 127402 .   DOI: 10.1088/0256-307X/40/12/127402
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Three-dimensional (3D) degenerate Fermi gases in the presence of spin-orbit coupling, inducing various kinds of physical findings and phenomena, have attracted tremendous attention in these years. We investigate the 3D spin-orbit coupled degenerate Fermi gases in theory and first present the analytic expression of their ground state. Our study provides an innovative perspective into understanding of the topological properties of 3D unconventional superconductors, and describes the topological phase transitions in trivial and topological phase areas. Further, such a system is provided with a richer set of Cooper pairings than traditional superconductors. The dual Cooper pairs with same spin directions emerge and exhibit peculiar behaviors, leading to topological phase transitions. Our study and discussion can be generalized to some other types of unconventional superconductors and areas of optical lattices.
Highly Anisotropic Magnetism and Nearly Isotropic Magnetocaloric Effect in Mn$_{3}$Sn$_{2}$ Single Crystals
Jianli Bai, Qingxin Dong, Libo Zhang, Qiaoyu Liu, Jingwen Cheng, Pinyu Liu, Cundong Li, Yingrui Sun, Yu Huang, Zhian Ren, and Genfu Chen
Chin. Phys. Lett.    2023, 40 (12): 127501 .   DOI: 10.1088/0256-307X/40/12/127501
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Mn$_{3}$Sn$_{2}$ has been proposed as an ideal material for magnetic refrigeration. It undergoes two successive ferromagnetic transitions ($T_{\rm C1} = 262$ K and $T_{\rm C2} = 227$ K) and one antiferromagnetic transition ($T_{\rm N} = 192$ K). Herein we report, for the first time, the preparation of single crystals of Mn$_{3}$Sn$_{2}$ from Bi flux. The resultant anisotropic magnetic properties and magnetocaloric effect are investigated along the three principal crystallographic directions of the crystal. Significant anisotropy of magnetic susceptibility and multiple field-induced metamagnetic transitions were found at low fields, whereas the magnetocaloric effect was found to be almost isotropic and larger than that of the polycrystalline one. The maximum magnetic entropy change amounts to $-\Delta S_{\rm M} = 4.01$ J$\cdot$kg$^{-1}\cdot$K$^{-1}$ near $T_{\rm C1}$ under a magnetic field change of $\mu_{0}\Delta H = 5$ T along the $c$-axis, with the corresponding refrigerant capacity of 1750 mJ$\cdot$cm$^{-3}$. Combined with a much wider cooling temperature span ($\sim$ $80$ K), our results demonstrate Mn$_{3}$Sn$_{2}$ single crystal to be an attractive candidate working material for active magnetic refrigeration at low temperatures.
Observation of Enhanced Faraday Effect in Eu-Doped Ce:YIG Thin Films
Han-Xu Zhang, Sen-Yin Zhu, Jin Zhan, Xian-Jie Wang, Yi Wang, Tai Yao, N. I. Mezin, and Bo Song
Chin. Phys. Lett.    2023, 40 (12): 127801 .   DOI: 10.1088/0256-307X/40/12/127801
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Ce:YIG thin films are taken as an ideal candidate for magneto-optical devices with giant Faraday effect in the near-infrared range, but it is hindered by a limited Ce$^{3+}$/Ce$^{4+}$ ratio and a high saturation driving field. To address this issue, Eu doping can increase the Faraday rotation angle by $\sim$ 40% to $1.315\times 10^{4}$ deg/cm and decrease the saturation driving field by $\sim$ 38% to 1.17 kOe in Eu$_{0.75}$Ce$_{1}$Y$_{1.25}$Fe$_{5}$O$_{12}$ compared to Ce$_{1}$Y$_{2}$Fe$_{5}$O$_{12}$ pristine. The mechanism is attributed to the conversion of Ce$^{4+}$ to Ce$^{3+}$ and the weakening of ferrimagnetism by Eu doping. This work not only provides strategies for improving Ce$^{3+}$/Ce$^{4+}$ ratio in Ce:YIG, but also develops (Eu,Ce):YIG with a promising Faraday rotation angle for magneto-optical devices.
Energy Landscape and Phase Competition of CsV$_{3}$Sb$_{5}$, CsV$_{6}$Sb$_{6}$ and TbMn$_{6}$Sn$_{6}$-Type Kagome Materials
Guanghui Cai, Yutao Jiang, Hui Zhou, Ze Yu, Kun Jiang, Youguo Shi, Sheng Meng, and Miao Liu
Chin. Phys. Lett.    2023, 40 (11): 117101 .   DOI: 10.1088/0256-307X/40/11/117101
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Finding viable Kagome lattices is vital for materializing novel phenomena in quantum materials. In this study, we performed element substitutions on CsV$_{3}$Sb$_{5}$ with space group $P6/mmm$, TbMn$_{6}$Sn$_{6}$ with space group $P6/mmm$, and CsV$_{6}$Sb$_{6}$ with space group $R\bar{3}m$, as the parent compounds. Totally 4158 materials were obtained through element substitutions, and these materials were then calculated via density functional theory in high-throughput mode. Afterwards, 48 materials were identified with high thermodynamic stability ($E_{\rm{hull}} < 5$ meV/atom). Furthermore, we compared the thermodynamic stability of three different phases with the same elemental composition and predicted some competing phases that may arise during material synthesis. Finally, by calculating the electronic structures of these materials, we attempted to identify patterns in the electronic structure variations as the elements change. This study provides guidance for discovering promising AM$_{3}$X$_{5}$/AM$_{6}$X$_{6}$ Kagome materials from a vast phase space.
Ferroelectricity and Large Rashba Splitting in Two-Dimensional Tellurium
Yao Wang, Zhenzhen Lei, Jinsen Zhang, Xinyong Tao, Chenqiang Hua, and Yunhao Lu
Chin. Phys. Lett.    2023, 40 (11): 117102 .   DOI: 10.1088/0256-307X/40/11/117102
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Two-dimensional (2D) ferroelectric (FE) systems are promising candidates for non-volatile nanodevices. Previous studies mainly focused on 2D compounds. Though counter-intuitive, here we propose several new phases of tellurium with (anti)ferroelectricity. Two-dimensional films can be viewed as a collection of one-dimensional chains, and lone-pair instability is responsible for the (anti)ferroelectricity. The total polarization is determined to be $0.34 \times 10^{-10}$ C/m for the FE ground state. Due to the local polarization field in the FE film, we show a large Rashba splitting ($\alpha_{\scriptscriptstyle{\rm R}} \sim 2$ eV$\cdot$Å) with nonzero spin Hall conductivity for experimental detection. Furthermore, a dipole-like distribution of Berry curvature is verified, which may facilitate a nonlinear Hall effect. Because Rashba-splitting/Berry-curvature distributions are fully coupled with a polarization field, they can be reversed through FE phase transition. Our results not only broaden the elemental FE materials, but also shed light on their intriguing transport phenomena.
Electron-Correlation-Induced Charge Density Wave in FeGe
Lin Wu, Yating Hu, Dongze Fan, Di Wang, and Xiangang Wan
Chin. Phys. Lett.    2023, 40 (11): 117103 .   DOI: 10.1088/0256-307X/40/11/117103
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As the first magnetic kagome material to exhibit the charge density wave (CDW) order, FeGe has attracted much attention in recent research. Similar to $A$V$_{3}$Sb$_{5}$ ($A$ = K, Cs, Rb), FeGe exhibits the CDW pattern with an in-plane 2$\times$2 structure and the existence of van Hove singularities near the Fermi level. However, sharply different from $A$V$_{3}$Sb$_{5}$ which has phonon instability at $M$ point, all the theoretically calculated phonon frequencies in FeGe remain positive. Based on first-principles calculations, we surprisingly find that the maximum of nesting function is at $K$ point instead of $M$ point. Two Fermi pockets with Fe-$d_{xz}$ and Fe-$d_{x^{2}-y^{2}}$/$d_{xy}$ orbital characters have large contribution to the Fermi nesting, which evolve significantly with $k_{z}$, indicating the highly three-dimensional (3D) feature of FeGe in contrast to $A$V$_{3}$Sb$_{5}$. Considering the effect of local Coulomb interaction, we reveal that the instability at $K$ point is significantly suppressed due to the sublattice interference mechanism. Meanwhile, the wave functions nested by vector $M$ have many ingredients located at the same Fe site, thus the instability at $M$ point is enhanced. This indicates that the electron correlation, rather than electron-phonon interaction, plays a key role in the CDW transition at $M$ point.
Moiré Synaptic Transistor for Homogeneous-Architecture Reservoir Computing
Pengfei Wang, Moyu Chen, Yongqin Xie, Chen Pan, Kenji Watanabe, Takashi Taniguchi, Bin Cheng, Shi-Jun Liang, and Feng Miao
Chin. Phys. Lett.    2023, 40 (11): 117201 .   DOI: 10.1088/0256-307X/40/11/117201
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Reservoir computing has been considered as a promising intelligent computing paradigm for effectively processing complex temporal information. Exploiting tunable and reproducible dynamics in the single electronic device have been desired to implement the “reservoir” and the “readout” layer of reservoir computing system. Two-dimensional moiré materials, with an artificial lattice constant many times larger than the atomic length scale, are one type of most studied artificial quantum materials in community of material science and condensed-matter physics over the past years. These materials are featured with gate-tunable periodic potential and electronic correlation, thus varying the electric field allows the electrons in the moiré potential per unit cell to exhibit distinct and reproducible dynamics, showing great promise in robust reservoir computing. Here, we report that a moiré synaptic transistor can be used to implement the reservoir computing system with a homogeneous reservoir-readout architecture. The synaptic transistor is fabricated based on an h-BN/bilayer graphene/h-BN moiré heterostructure, exhibiting ferroelectricity-like hysteretic gate voltage dependence of resistance. Varying the magnitude of the gate voltage enables the moiré transistor to switch between long-term memory and short-term memory with nonlinear dynamics. By employing the short- and long-term memories as the reservoir nodes and weights of the readout layer, respectively, we construct a full-moiré physical neural network and demonstrate that the classification accuracy of 90.8% can be achieved for the MNIST (Modified National Institute of Standards and Technology) handwritten digits database. Our work would pave the way towards the development of neuromorphic computing based on moiré materials.
Cooling by Coulomb Heat Drag Based on Three Coupled Quantum Dots
Jin-Zhu Gao, Xing Liu, Jian-Hui Wang, and Ji-Zhou He
Chin. Phys. Lett.    2023, 40 (11): 117301 .   DOI: 10.1088/0256-307X/40/11/117301
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We establish a model for a four-terminal thermoelectric system, based on three coupled quantum dots, which consists of a left/right electron reservoir (the source and the drain), two thermal reservoirs and three coupled quantum dots. Based on the master equation theory, we derive the expressions of the electron current and heat flow among the three quantum dots and the corresponding reservoir. We show that the source can be cooled by passing a thermal current between the two thermal reservoirs, with no net heat exchange between the thermal reservoirs and the electron reservoirs. This effect is called the Coulomb heat drag effect. Then, we define the coefficient of performance (COP) and the cooling power. The influence of the main system parameters, such as charging energy, energy level, and temperature, on the performance of the four-terminal thermoelectric system is analyzed in detail. By choosing appropriate parameters one can obtain the maximum cooling power and the corresponding COP. Finally, we also show that the Maxwell demon effect can be realized by using nonequilibrium thermal reservoirs in our four-terminal thermoelectric system.
Emergence of High-Temperature Superconducting Phase in Pressurized La$_{3}$Ni$_{2}$O$_7$ Crystals
Jun Hou, Peng-Tao Yang, Zi-Yi Liu, Jing-Yuan Li, Peng-Fei Shan, Liang Ma, Gang Wang, Ning-Ning Wang, Hai-Zhong Guo, Jian-Ping Sun, Yoshiya Uwatoko, Meng Wang, Guang-Ming Zhang, Bo-Sen Wang, and Jin-Guang Cheng
Chin. Phys. Lett.    2023, 40 (11): 117302 .   DOI: 10.1088/0256-307X/40/11/117302
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The recent report of pressure-induced structural transition and signature of superconductivity with $T_{\rm c}\approx80$ K above 14 GPa in La$_{3}$Ni$_{2}$O$_{7}$ crystals has garnered considerable attention. To further elaborate this discovery, we carried out comprehensive resistance measurements on La$_{3}$Ni$_{2}$O$_{7}$ crystals grown in an optical-image floating zone furnace under oxygen pressure (15 bar) using a diamond anvil cell (DAC) and cubic anvil cell (CAC), which employ a solid (KBr) and liquid (glycerol) pressure-transmitting medium, respectively. Sample 1 measured in the DAC exhibits a semiconducting-like behavior with large resistance at low pressures and gradually becomes metallic upon compression. At pressures $P \geqslant13.7$ GPa we observed the appearance of a resistance drop of as much as $\sim$ 50% around 70 K, which evolves into a kink-like anomaly at pressures above 40 GPa and shifts to lower temperatures gradually with increasing magnetic field. These observations are consistent with the recent report mentioned above. On the other hand, sample 2 measured in the CAC retains metallic behavior in the investigated pressure range up to 15 GPa. The hump-like anomaly in resistance around $\sim$ 130 K at ambient pressure disappears at $P\geqslant2$ GPa. In the pressure range of 11–15 GPa we observed the gradual development of a shoulder-like anomaly in resistance at low temperatures, which evolves into a pronounced drop of resistance of 98% below 62 K at 15 GPa, reaching a temperature-independent resistance of 20 $µ \Omega$ below 20 K. Similarly, this resistance anomaly can be progressively shifted to lower temperatures by applying external magnetic fields, resembling a typical superconducting transition. Measurements on sample 3 in the CAC reproduce the resistance drop at pressures above 10 GPa and realize zero resistance below 10 K at 15 GPa even though an unusual semiconducting-like behavior is retained in the normal state. Based on these results, we constructed a dome-shaped superconducting phase diagram and discuss some issues regarding the sample-dependent behaviors on pressure-induced high-temperature superconductivity in the La$_{3}$Ni$_{2}$O$_{7}$ crystals.
Physical Origin of Color Changes in Lutetium Hydride under Pressure
Run Lv, Wenqian Tu, Dingfu Shao, Yuping Sun, and Wenjian Lu
Chin. Phys. Lett.    2023, 40 (11): 117401 .   DOI: 10.1088/0256-307X/40/11/117401
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Recently, near-ambient superconductivity was claimed in nitrogen-doped lutetium hydride (LuH$_{3-\delta}$N$_{\rm{\varepsilon}})$. Unfortunately, all follow-up research still cannot find superconductivity signs in successfully synthesized lutetium dihydride (LuH$_{2}$) and N-doped LuH$_{2\pm x}$N$_{y}$. However, a similar intriguing observation was the pressure-induced color changes (from blue to pink and subsequent red). The physical understanding of its origin and the correlation between the color, crystal structure, and chemical composition of Lu–H–N is still lacking. In this work, we systematically investigated the optical properties of LuH$_{2}$ and LuH$_{3}$, and the effects of hydrogen vacancies and nitrogen doping using the first-principles calculations by considering both interband and intraband contributions. Our results demonstrate that the evolution of reflectivity peaks near blue and red light, which is driven by changes in the band gap and Fermi velocity of free electrons, resulting in the blue-to-red color change under pressure. In contrast, LuH$_{3}$ exhibits gray and no color change up to 50 GPa. Furthermore, we investigated the effects of hydrogen vacancies and nitrogen doping on its optical properties. Hydrogen vacancies can significantly decrease the pressure of blue-to-red color change in LuH$_{2}$ but do not have a noticeable effect on the color of LuH$_{3}$. The N-doped LuH$_{2}$ with the substitution of a hydrogen atom at the tetrahedral position maintains the color change when the N-doping concentration is low. As the doping level increases, this trend becomes less obvious, while other N-doped structures do not show a blue-to-red color change. Our results can clarify the origin of the experimental observed blue-to-red color change in lutetium hydride and also provide a further understanding of the potential N-doped lutetium dihydride.
Spin Hall Magnetoresistance in Pt/BiFeO$_{3}$ Bilayer
Anpeng He, Yu Lu, Jun Du, Yufei Li, Zhong Shi, Di Wu, and Qingyu Xu
Chin. Phys. Lett.    2023, 40 (11): 117402 .   DOI: 10.1088/0256-307X/40/11/117402
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Multiferroic materials are general antiferromagnets with negligibly small net magnetization, which strongly limits their magnetoelectric applications in spintronics. Spin Hall magnetoresistance (SMR) is sensitive to the orientation of the Néel vector, which can be applied for the detection of antiferromagnetic states. Here, we apply SMR on the unique room-temperature antiferromagnetic multiferroic material BiFeO$_{3}$ (BFO). The angular dependence of SMR in a bilayer of epitaxial BFO (001) and heavy metal Pt is studied. By rotating the sample under a magnetic field of 80 kOe in the film plane, the resistance shows the maximum when the field is perpendicular to the current while it shows the minimum when the field is along the current. This can be well explained by the SMR in the bilayer of heavy metal/antiferromagnet with the relative orientation between the Néel vector and current direction. In contrast, the angular dependence of the resistance of Pt directly deposited on a SrTiO$_{3}$ (001) substrate shows a 90$^{\circ}$ shift with the magnetic field rotating in the film plane, which originates from the Hanle magnetoresistance of Pt. The obtained spin mixing conductance at the Pt/BFO interface clearly confirms the efficient spin transmission. Our results provide a possible solution for applications of antiferromagnetic multiferroic materials in spintronics.
Giant 2D Skyrmion Topological Hall Effect with Ultrawide Temperature Window and Low-Current Manipulation in 2D Room-Temperature Ferromagnetic Crystals
Gaojie Zhang, Qingyuan Luo, Xiaokun Wen, Hao Wu, Li Yang, Wen Jin, Luji Li, Jia Zhang, Wenfeng Zhang, Haibo Shu, and Haixin Chang
Chin. Phys. Lett.    2023, 40 (11): 117501 .   DOI: 10.1088/0256-307X/40/11/117501
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The discovery and manipulation of topological Hall effect (THE), an abnormal magnetoelectric response mostly related to the Dzyaloshinskii–Moriya interaction (DMI), are promising for next-generation spintronic devices based on topological spin textures such as magnetic skyrmions. However, most skyrmions and THE are stabilized in a narrow temperature window either below or over room temperature with high critical current manipulation. It is still elusive and challenging to achieve large THE with both wide temperature window till room temperature and low critical current manipulation. Here, using controllable, naturally oxidized sub-20 and sub-10 nm 2D van der Waals room-temperature ferromagnetic Fe$_{3}$GaTe$_{2-x}$ crystals, we report robust 2D skyrmion THE with ultrawide temperature window ranging in three orders of magnitude from 2 to 300 K, in combination with giant THE of $\sim$ 5.4 $µ \Omega\cdot$cm at 10 K and $\sim$ 0.15 $µ \Omega\cdot$cm at 300 K, which is 1–3 orders of magnitude larger than that of all known room-temperature 2D skyrmion systems. Moreover, room-temperature current-controlled THE is also realized with a low critical current density of $\sim$ $6.2\times10^{5}$ A$\cdot$cm$^{-2}$. First-principles calculations unveil natural oxidation-induced highly enhanced 2D interfacial DMI reasonable for robust giant THE. This work paves the way to room-temperature electrically controlled 2D THE-based practical spintronic devices.
Ultrafast Condensed Matter Physics at Attoseconds
Shi-Qi Hu and Sheng Meng
Chin. Phys. Lett.    2023, 40 (11): 117801 .   DOI: 10.1088/0256-307X/40/11/117801
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Our understanding of how photons couple to different degrees of freedom in solids forms the bedrock of ultrafast physics and materials sciences. In this review, the emergent ultrafast dynamics in condensed matter at the attosecond timescale have been intensively discussed. In particular, the focus is put on recent developments of attosecond dynamics of charge, exciton, and magnetism. New concepts and indispensable role of interactions among multiple degrees of freedom in solids are highlighted. Applications of attosecond electronic metrology and future prospects toward attosecond dynamics in condensed matter are further discussed. These pioneering studies promise future development of advanced attosecond science and technology such as attosecond lasers, laser medical engineering, and ultrafast electronic devices.
Contrasting Transport Performance of Electron- and Hole-Doped Epitaxial Graphene for Quantum Resistance Metrology
Xinyi Wan, Xiaodong Fan, Changwei Zhai, Zhenyu Yang, Lilong Hao, Lin Li, Yunfeng Lu, and Changgan Zeng
Chin. Phys. Lett.    2023, 40 (10): 107201 .   DOI: 10.1088/0256-307X/40/10/107201
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Epitaxial graphene grown on silicon carbide (SiC/graphene) is a promising solution for achieving a high-precision quantum Hall resistance standard. Previous research mainly focused on the quantum resistance metrology of n-type SiC/graphene, while a comprehensive understanding of the quantum resistance metrology behavior of graphene with different doping types is lacking. Here, we fabricated both n- and p-type SiC/graphene devices via polymer-assisted molecular adsorption and conducted systematic magneto-transport measurements in a wide parameter space of carrier density and temperature. It is demonstrated that n-type devices show greater potential for development of quantum resistance metrology compared with p-type devices, as evidenced by their higher carrier mobility, lower critical magnetic field for entering quantized Hall plateaus, and higher robustness of the quantum Hall effect against thermal degeneration. These discrepancies can be reasonably attributed to the weaker scattering from molecular dopants for n-type devices, which is further supported by the analyses on the quantum interference effect in multiple devices. These results enrich our understanding of the charged impurity on electronic transport performance of graphene and, more importantly, provide a useful reference for future development of graphene-based quantum resistance metrology.
Coexistence of Zero-Dimensional Electride State and Superconductivity in AlH$_{2}$ Monolayer
Qiuping Yang, Xue Jiang, and Jijun Zhao
Chin. Phys. Lett.    2023, 40 (10): 107401 .   DOI: 10.1088/0256-307X/40/10/107401
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Electrides, which confine “excess anionic electrons” in subnanometer-sized cavities of a lattice, are exotic ionic crystals. We propose a non-stoichiometric strategy to realize intrinsic two-dimensional (2D) superconducting electride. AlH$_{2}$ monolayer, which is structurally identical to 1H-MoS$_{2}$, possesses zero-dimensionally confined anionic electrons in the interstitial sites of Al triangles, corresponding to a chemical formula of [AlH$_{2}$]$^{+}e^{-}$. The interaction between interstitial anionic electrons (IAEs) and host cation lattice mainly accounts for stabilization of 1H-AlH$_{2}$ electride. Impressively, 1H-AlH$_{2}$ monolayer is an intrinsic Bardeen–Cooper–Schrieffer superconductor with $T_{\rm c}=38$ K, which is the direct consequence of strong coupling of the H-dominated high electronic states with Al acoustic branch vibrations and mid-frequency H-derived phonon softening modes caused by Kohn anomalies. Under tensile strain, IAEs transform into itinerant electrons, favoring the formation of stable Cooper pairs. Therefore, $T_{\rm c}$ reaches up to 53 K at a biaxial fracture strain of 5%. Our findings provide valuable insights into the correlation between non-stoichiometric electrides and superconducting microscopic mechanisms at the 2D limit.
Pressure-Induced Superconductivity in the Charge-Density-Wave Compound LaTe$_{2- x}$Sb$_{x}$ ($x = 0$ and 0.4)
Xu Chen, Pei-han Sun, Zhenkai Xie, Fanqi Meng, Cuiying Pei, Yanpeng Qi, Tianping Ying, Kai Liu, Jian-gang Guo, and Xiaolong Chen
Chin. Phys. Lett.    2023, 40 (10): 107402 .   DOI: 10.1088/0256-307X/40/10/107402
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Magnetic CeTe$_{2}$ achieving superconductivity under external pressure has received considerable attention. The intermingling of 4$f$ and 5$d$ electrons from Ce raised the speculation of an unconventional pairing mechanism arising from magnetic fluctuations. Here, we address this speculation using a nonmagnetic 4$f$-electron-free LaTe$_{2}$ as an example. No structural phase transition can be observed up to 35 GPa in the in situ synchrotron diffraction patterns. Subsequent high-pressure electrical measurements show that LaTe$_{2}$ exhibits superconductivity at 20 Gpa with its $T_{\rm c}$ (4.5 K) being two times higher than its Ce-counterpart. Detailed theoretical calculations reveal that charge transfer from the 4$p$ orbitals of the planar square Te–Te network to the 5$d$ orbitals of La is responsible for the emergence of superconductivity in LaTe$_{2}$, as confirmed by Hall experiments. Furthermore, we study the modulation of $q_{\scriptscriptstyle{\rm CDW}}$ by Sb substitution and find a record high $T_{\rm c}^{\rm onset} \sim 6.5$ K in LaTe$_{1.6}$Sb$_{0.4}$. Our work provides an informative clue to comprehend the role of $5d$–$4p$ hybridization in the relationship between charge density wave (CDW) and superconductivity in these RETe$_{2}$ (RE = rare-earth elements) compounds.
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