Showing 1–50 of 168 results for author: Xie, H

Topological Valley Transport in Bilayer Graphene Induced by Interlayer Sliding

Authors: Jie Pan, Huanhuan Wang, Lin Zou, Xiaoyu Wang, Lihao Zhang, Xueyan Dong, Haibo Xie, Yi Ding, Yuze Zhang, Takashi Taniguchi, Kenji Watanabe, Shuxi Wang, Zhe Wang

Abstract: Interlayer sliding, together with twist angle, is a crucial parameter that defines the atomic registry and thus determines the properties of two-dimensional (2D) material homobilayers. Here, we theoretically demonstrate that controlled interlayer sliding in bilayer graphene induces Berry curvature reversals, leading to topological states confined within a one-dimensional moiré channel. We experime… ▽ More Interlayer sliding, together with twist angle, is a crucial parameter that defines the atomic registry and thus determines the properties of two-dimensional (2D) material homobilayers. Here, we theoretically demonstrate that controlled interlayer sliding in bilayer graphene induces Berry curvature reversals, leading to topological states confined within a one-dimensional moiré channel. We experimentally realize interlayer sliding by bending the bilayer graphene geometry across a nanoridge. Systematic electronic transport measurements reveal topological valley transport when the Fermi energy resides within the band gap, consistent with theoretical predictions of eight topological channels. Our findings establish interlayer sliding as a powerful tool for tuning the electronic properties of bilayer graphene and underscore its potential for broad application across 2D material systems. △ Less

Submitted 15 November, 2025; originally announced November 2025.

Comments: 19 pages

Journal ref: Phys. Rev. Lett. 135, 126603(2025)

A finite-element Delta-Sternheimer approach for accurate all-electron RPA correlation energies of arbitrary molecules

Authors: Hao Peng, Haochen Liu, Chuhao Li, Hehu Xie, Xinguo Ren

Abstract: The incompleteness of single-particle basis sets has long cast a shadow over correlated electronic-structure methods, making it highly challenging to obtain numerically converged results. In this work, we compute the RPA correlation energies of general molecules using the finite element method, while ingeniously combining atomic orbital basis sets to accelerate the convergence of total energies. W… ▽ More The incompleteness of single-particle basis sets has long cast a shadow over correlated electronic-structure methods, making it highly challenging to obtain numerically converged results. In this work, we compute the RPA correlation energies of general molecules using the finite element method, while ingeniously combining atomic orbital basis sets to accelerate the convergence of total energies. We report atomization energies for 50 molecules within the RPA framework, achieving accuracies on the order of meV per atom. The computational strategy that integrates real-space discretization techniques with atomic orbitals is expected to inspire the entire correlated electronic-structure community. △ Less

Submitted 17 October, 2025; originally announced October 2025.

Anomalous inverse Faraday effect for graphene quantum dots in optical vortices

Authors: Zi-Yang Xu, Wei E. I. Sha, Hang Xie

Abstract: Chiral photon interactions with two-dimensional (2D) materials enable unprecedented control of quantum phenomena. In this paper, we report anomalous inverse Faraday effects (IFE) in graphene quantum dots (GQDs) under linearly polarized optical vortex illumination, where transferred orbital angular momentum (OAM) generates light-induced magnetic moments. Employing our recently developed time-depend… ▽ More Chiral photon interactions with two-dimensional (2D) materials enable unprecedented control of quantum phenomena. In this paper, we report anomalous inverse Faraday effects (IFE) in graphene quantum dots (GQDs) under linearly polarized optical vortex illumination, where transferred orbital angular momentum (OAM) generates light-induced magnetic moments. Employing our recently developed time-dependent quantum perturbation framework [Phys. Rev. B 110, 085425 (2024)], we demonstrate a counterintuitive observation: some reversed magnetic moments at off-axis positions occur-manifested as counter-rotating currents to the vortex helical wavefront. Phase-difference analysis and eigenmode decomposition resolve this anomaly, revealing that the OAM transfer efficiency is orders of magnitude weaker than its spin counterpart. This work establishes a new paradigm for optical OAM-to-magnetization conversion in quantum-engineered 2D systems. △ Less

Submitted 16 September, 2025; originally announced September 2025.

Comments: 8 pages, 4 figure

Transitional patterns on a spherical surface: from scars to domain defects of mixed lattices

Authors: Wenyu Liu, Han Xie, Yu Du, Baohui Li, Jeff Z. Y. Chen, Yao Li

Abstract: The system of mixed hexagonal and square lattices on a spherical surface is examined, with an emphasis on the exploration of the disclination patterns that form in the square-rich regime. To demonstrate the possible outcomes, the Hertzian potential energy is used as a model for pairwise molecular interactions, which is known to support coexistent hexagonal and square lattices. Through molecular dy… ▽ More The system of mixed hexagonal and square lattices on a spherical surface is examined, with an emphasis on the exploration of the disclination patterns that form in the square-rich regime. To demonstrate the possible outcomes, the Hertzian potential energy is used as a model for pairwise molecular interactions, which is known to support coexistent hexagonal and square lattices. Through molecular dynamics simulations, we show that at least four different disclination morphologies arise in a square-rich background: triangular defect domains composed of hexagonal lattices arranged in a cubic formation, bridged cubic state, linear scar disclinations with no hexagon content, and open scar disclinations containing a significant amount of hexagonal lattice in the open regions. Order parameters are also introduced to highlight the significance of the bridged and open-scar disclinations, both being the new morphologies reported in this study. The fact that the bridged state is an energetically preferred one is further demonstrated by a separate elastic energy model, which confirms its prevalence over the unbridged cubic state. △ Less

Submitted 8 September, 2025; originally announced September 2025.

Comments: 10 pages, 7 figures, accepted by Soft Matter

Optical selection rules of topological excitons in flat bands

Authors: Mara Lozano, Hong-Yi Xie, Bruno Uchoa

Abstract: Topological excitons are superpositions of electron-hole pair states, characterized by an envelope function with finite vorticity in momentum space. This vorticity is determined by the underlying topology of the electronic bands. We derive the optical selection rules for topological excitons in flat bands, considering different topological two-band models: a family of Hamiltonians with skyrmion ps… ▽ More Topological excitons are superpositions of electron-hole pair states, characterized by an envelope function with finite vorticity in momentum space. This vorticity is determined by the underlying topology of the electronic bands. We derive the optical selection rules for topological excitons in flat bands, considering different topological two-band models: a family of Hamiltonians with skyrmion pseudo-spin textures, the flattened BHZ model for a single spin and the flattened Haldane model. We derive the selection rules for these three models accounting for short-range interactions. We also consider the non-hydrogenic spectrum of excitons in the single-spin flattened BHZ model with Coulomb interactions. We show that for the case of two flat bands with skyrmion pseudo-spin textures, all excitons are bright, and the handedness of the light that couples to them is fixed by the vorticity of the pseudo-spin texture. For the single-spin flattened BHZ model, we show that bright excitons couple to circularly polarized light, regardless of the range of the interactions. In the flattened Haldane model, topological excitons couple to elliptically polarized light. We obtain the phase diagram for the polarization of light in this model as a function of microscopic parameters of the Hamiltonian. Our results demonstrate how band topology affects exciton properties, offering a framework for predicting light-matter interactions in topological materials with flat bands. △ Less

Submitted 9 September, 2025; v1 submitted 3 September, 2025; originally announced September 2025.

Comments: 12 pages, 5 figures

Quantum Geometric Renormalization of the Hall Coefficient and Unconventional Hall Resistivity in ZrTe5

Authors: Huimin Xie, Bo Fu, Huan-Wen Wang, Wenyu Shan, Shun-Qing Shen

Abstract: The anomalous Hall effect (AHE), conventionally associated with time-reversal symmetry breaking in ferromagnetic materials, has recently been observed in nonmagnetic topological materials, raising questions about its origin. We unravel the unconventional Hall response in the nonmagnetic Dirac material ZrTe5, known for its massive Dirac bands and unique electronic and transport properties. Using th… ▽ More The anomalous Hall effect (AHE), conventionally associated with time-reversal symmetry breaking in ferromagnetic materials, has recently been observed in nonmagnetic topological materials, raising questions about its origin. We unravel the unconventional Hall response in the nonmagnetic Dirac material ZrTe5, known for its massive Dirac bands and unique electronic and transport properties. Using the Kubo-Streda formula within the Landau level framework, we explore the interplay of quantum effects induced by the magnetic field (B) and disorder across the semiclassical and quantum regimes. In the semiclassical regime, the Hall resistivity remains linear in the magnetic field, but the Hall coefficient will be renormalized by the quantum geometric effects and electron-hole coherence, especially at low carrier densities where the disorder scattering dominates. In quantum limit, the Hall conductivity exhibits an unsaturating 1/B scaling. As a result, the transverse conductivity dominates transport in the ultra-quantum limit, and the Hall resistivity crosses over from B to B^{-1} dependence as the system transitions from the semiclassical regime to the quantum limit. This work elucidates the mechanisms underlying the unconventional Hall effect in ZrTe5 and provides insights into the AHE in other nonmagnetic Dirac materials as well. △ Less

Submitted 21 August, 2025; originally announced August 2025.

Comments: 23 pages, 4 figures. Comments are welcomed

Deep Variational Free Energy Calculation of Hydrogen Hugoniot

Authors: Zihang Li, Hao Xie, Xinyang Dong, Lei Wang

Abstract: We develop a deep variational free energy framework to compute the equation of state of hydrogen in the warm dense matter region. This method parameterizes the variational density matrix of hydrogen nuclei and electrons at finite temperature using three deep generative models: a normalizing flow model for the Boltzmann distribution of the classical nuclei, an autoregressive transformer for the dis… ▽ More We develop a deep variational free energy framework to compute the equation of state of hydrogen in the warm dense matter region. This method parameterizes the variational density matrix of hydrogen nuclei and electrons at finite temperature using three deep generative models: a normalizing flow model for the Boltzmann distribution of the classical nuclei, an autoregressive transformer for the distribution of electrons in excited states, and a permutational equivariant flow model for the unitary backflow transformation of electron coordinates in Hartree-Fock states. By jointly optimizing the three neural networks to minimize the variational free energy, we obtain the equation of state and related thermodynamic properties of dense hydrogen for the temperature range where electrons occupy excited states. We compare our results with other theoretical and experimental results on the deuterium Hugoniot curve, aiming to resolve existing discrepancies. Our results bridge the gap between the results obtained by path-integral Monte Carlo calculations at high temperature and ground-state electronic methods at low temperature, thus providing a valuable benchmark for hydrogen in the warm dense matter region. △ Less

Submitted 22 December, 2025; v1 submitted 24 July, 2025; originally announced July 2025.

Comments: 8+18 pages, 4+12 figures, for source code and raw data, see https://github.com/fermiflow/Hugoniot, https://github.com/ZihangL/hqc, https://huggingface.co/datasets/Kelvin2025q/hugoniot

MXene triggers high toughness, high strength and low hysteresis hydrogels for printed artificial tissue

Authors: Chendong Zhao, Yaxing Li, Qinglong He, Shangpeng Qin, Huiqi Xie, Chuanfang Zhang

Abstract: Substituting load-bearing tissues requires hydrogels with rapid processability, excellent mechanical strength and fatigue resistance. Conventional homogeneously polymerized hydrogels with short-chains/excessive branching exhibit low strength/toughness, being inadequate for artificial tissues. Here we introduce the heterogeneous polymerization-accelerated reaction kinetics on the Ti3C2Tx MXene micr… ▽ More Substituting load-bearing tissues requires hydrogels with rapid processability, excellent mechanical strength and fatigue resistance. Conventional homogeneously polymerized hydrogels with short-chains/excessive branching exhibit low strength/toughness, being inadequate for artificial tissues. Here we introduce the heterogeneous polymerization-accelerated reaction kinetics on the Ti3C2Tx MXene microreactor and sluggish kinetics beyond-to rapidly produce hydrogels within minutes. This allows the hyperbranched domains embedded within a highly entangled matrix, leading to excellent strength (2.4 MPa)/toughness (75.2 kJ m-2) and low hysteresis (2.9%) in hydrogels superior to the rest ones. The rapid liquid-to-solid transition triggered by MXene suggests the great possibility of 3D printed robust hydrogels toward artificial tissue. Importantly, these printed hydrogels-based artificial ligaments have demonstrated impressive load-bearing capacity, wear resistance, and suturability compared to commercial analogs. △ Less

Submitted 15 June, 2025; originally announced June 2025.

Discovery of a Robust Non-Janus Hybrid MoSH Monolayer as a Two-Gap Superconductor via High-Throughput Computational Screening

Authors: Zhijing Huang, Hongmei Xie, Zhibin Gao, Longyuzhi Xu, Lin Zhang, Li Yang, Zonglin Gu, Shuming Zeng

Abstract: The atomic-scale determination of hydrogen positions in MoSH monolayers remains experimentally challenging, and existing studies are confined to Janus-type configurations. Here, we combine high-throughput structural screening with first-principles calculations to predict a novel non-Janus Hybrid 1T$^{'}$-MoSH monolayer, which energetically surpasses all previously reported MoSH phases with a bindi… ▽ More The atomic-scale determination of hydrogen positions in MoSH monolayers remains experimentally challenging, and existing studies are confined to Janus-type configurations. Here, we combine high-throughput structural screening with first-principles calculations to predict a novel non-Janus Hybrid 1T$^{'}$-MoSH monolayer, which energetically surpasses all previously reported MoSH phases with a binding energy of -3.02 eV. This structure emerges as a hybrid of MoS$_2$ and MoH$_2$, featuring alternating S and H atoms on both sides of the Mo layer. Comprehensive stability analyses confirm its robustness in energy, mechanics, dynamics, and thermodynamics (stable up to 1600 K). Remarkably, anisotropic Migdal-Eliashberg theory predicts Hybrid 1T$^{'}$-MoSH as a two-gap superconductor with a critical temperature T$_c$ of 16.34 K, driven by strong electron-phonon coupling ($λ$$=$1.39). Substituting Mo with Hf, Ta, or Ti drastically suppresses T$_c$ $\sim$ (0.53-2.42 K), highlighting Mo$^{'}$s unique role in enhancing superconductivity. Our work not only expands the family of 2D transition metal chalcogenides but also proposes a promising candidate for quantum technologies, bridging theoretical design to functional material discovery. △ Less

Submitted 12 April, 2025; originally announced April 2025.

Synthesis of intrinsic magnetic topological insulator MnBi2nTe3n+1 family by chemical vapor transport method with feedback regulation

Authors: Heng Zhang, Yiying Zhang, Yong Zhang, Bo Chen, Jingwen Guo, Yu Du, Jiajun Li, Hangkai Xie, Zhixin Zhang, Fuwei Zhou, Tianqi Wang, Wuyi Qi, Xuefeng Wang, Fucong Fei, Fengqi Song

Abstract: MnBi2nTe3n+1 (MBT) is a representative family of intrinsic magnetic topological insulators, in which numerous exotic phenomena such as the quantum anomalous Hall effect are expected. The high-quality crystal growth and magnetism manipulation are the most essential processes. Here we develop a modified chemical vapor transport method using a feedback-regulated strategy, which provides the closed-lo… ▽ More MnBi2nTe3n+1 (MBT) is a representative family of intrinsic magnetic topological insulators, in which numerous exotic phenomena such as the quantum anomalous Hall effect are expected. The high-quality crystal growth and magnetism manipulation are the most essential processes. Here we develop a modified chemical vapor transport method using a feedback-regulated strategy, which provides the closed-loop control of growth temperature within +/- 0.1 degree Celsius. Single crystals of MnBi2Te4, MnBi4Te7, and MnBi6Te10 are obtained under different temperature intervals respectively, and show variable tunability on magnetism by finely tuning the growth temperatures. Specifically, the cold-end temperatures not only vary the strength of antiferromagnetic coupling in MnBi2Te4, but also induce magnetic ground state transitions from antiferromagnetism to ferromagnetism in MnBi4Te7 and MnBi6Te10. In MnBi2Te4 with optimized magnetism, quantized transport with Chern insulator state is also realized at the low field of 3.7 T. Our results provide a systematic picture for the crystal growth and the rich magnetic tunability of MBT family, providing richer platforms for the related researches combining magnetism and topological physics. △ Less

Submitted 19 May, 2025; v1 submitted 11 April, 2025; originally announced April 2025.

Journal ref: Adv. Mater. 2025, 2405686

Carbon-Nanotube/$β$-Ga$_2$O$_3$ Heterojunction PIN Diodes

Authors: Hunter D. Ellis, Botong Li, Haoyu Xie, Jichao Fan, Imteaz Rahaman, Weilu Gao, Kai Fu

Abstract: $β$-Ga$_2$O$_3… ▽ More $β$-Ga$_2$O$_3$ is gaining attention as a promising semiconductor for next-generation high-power, high-efficiency, and high-temperature electronic devices, thanks to its exceptional material properties. However, challenges such as the lack of viable p-type doping have hindered its full potential, particularly in the development of ambipolar devices. This work introduces a novel heterojunction diode (HD) that combines p-type carbon nanotubes (CNTs) with i/n-type $β$-Ga$_2$O$_3$ to overcome these limitations. For the first time, a CNT/$β$-Ga$_2$O$_3$ hetero-p-n-junction diode is fabricated. Compared to a traditional Schottky barrier diode (SBD) with the same $β$-Ga$_2$O$_3$ epilayer, the CNT/$β$-Ga$_2$O$_3$ HD demonstrates significant improvements, including a higher rectifying ratio ($1.2 \times 10^{11}$), a larger turn-on voltage (1.96 V), a drastically reduced leakage current at temperatures up to 300 °C, and a 26.7% increase in breakdown voltage. Notably, the CNT/$β$-Ga$_2$O$_3$ HD exhibits a low ideality factor of 1.02, signifying an ideal interface between the materials. These results underline the potential of CNT/$β$-Ga$_2$O$_3$ heterojunctions for electronic applications, offering a promising solution to current limitations in $β$-Ga$_2$O$_3$-based devices. △ Less

Submitted 27 March, 2025; originally announced March 2025.

Comments: 17 pages, 5 figures

Thermal and thermoelectric transport in flat bands with non-trivial quantum geometry

Authors: Kevin Wen, Hong-Yi Xie, Assa Auerbach, Bruno Uchoa

Abstract: Although quasiparticles in flat bands have zero group velocity, they can display an anomalous velocity due to the quantum geometry. We address the thermal and thermoelectric transport in flat bands in the clean limit with a small amount of broadening due to inelastic scattering. We derive general Kubo formulas for flat bands in the DC limit up to linear order in the broadening and extract expressi… ▽ More Although quasiparticles in flat bands have zero group velocity, they can display an anomalous velocity due to the quantum geometry. We address the thermal and thermoelectric transport in flat bands in the clean limit with a small amount of broadening due to inelastic scattering. We derive general Kubo formulas for flat bands in the DC limit up to linear order in the broadening and extract expressions for the thermal conductivity, the Seebeck and Nernst coefficients. We show that the Seebeck coefficient for flat Chern bands is topological up to second order corrections in the broadening. We identify thermal and thermoelectric transport signatures for two generic flat Chern bands and also for the generalized flattened Lieb model, which describes a family of three equally spaced flat Chern bands where the middle one is topologically trivial. Finally, we address the saturation of the quantum metric lower bound for a general family of Hamiltonians with an arbitrary number of flat Chern bands corresponding to SU(2) coherent states. We find that only the extremal bands in this class of Hamiltonians saturate the bound, provided that the momentum dependence of their Hamiltonians is described by a meromorphic function. △ Less

Submitted 14 February, 2025; originally announced February 2025.

Journal ref: Phys. Rev. B 111, 205140 (2025)

An ab initio dataset of size-dependent effective thermal conductivity for advanced technology transistors

Authors: Han Xie, Ru Jia, Yonglin Xia, Lei Li, Yue Hu, Jiaxuan Xu, Yufei Sheng, Yuanyuan Wang, Hua Bao

Abstract: As the size of transistors shrinks and power density increases, thermal simulation has become an indispensable part of the device design procedure. However, existing works for advanced technology transistors use simplified empirical models to calculate effective thermal conductivity in the simulations. In this work, we present a dataset of size-dependent effective thermal conductivity with electro… ▽ More As the size of transistors shrinks and power density increases, thermal simulation has become an indispensable part of the device design procedure. However, existing works for advanced technology transistors use simplified empirical models to calculate effective thermal conductivity in the simulations. In this work, we present a dataset of size-dependent effective thermal conductivity with electron and phonon properties extracted from ab initio computations. Absolute in-plane and cross-plane thermal conductivity data of eight semiconducting materials (Si, Ge, GaN, AlN, 4H-SiC, GaAs, InAs, BAs) and four metallic materials (Al, W, TiN, Ti) with the characteristic length ranging from 5 to 50 nanometers have been provided. Besides the absolute value, normalized effective thermal conductivity is also given, in case it needs to be used with updated bulk thermal conductivity in the future. The dataset presented in this paper are openly available at https://doi.org/10.57760/sciencedb.j00113.00154. △ Less

Submitted 26 January, 2025; originally announced January 2025.

A Denser Hydrogen Inferred from First-Principles Simulations Challenges Jupiter's Interior Models

Authors: Cesare Cozza, Kousuke Nakano, Saburo Howard, Hao Xie, Ravit Helled, Guglielmo Mazzola

Abstract: First-principle modeling of dense hydrogen is crucial in materials and planetary sciences. Despite its apparent simplicity, predicting the ionic and electronic structure of hydrogen is a formidable challenge, and it is connected with the insulator-to-metal transition, a century-old problem in condensed matter. Accurate simulations of liquid hydrogen are also essential for modeling gas giant planet… ▽ More First-principle modeling of dense hydrogen is crucial in materials and planetary sciences. Despite its apparent simplicity, predicting the ionic and electronic structure of hydrogen is a formidable challenge, and it is connected with the insulator-to-metal transition, a century-old problem in condensed matter. Accurate simulations of liquid hydrogen are also essential for modeling gas giant planets. Here we perform an exhaustive study of the equation of state of hydrogen using Density Functional Theory and quantum Monte Carlo simulations. We find that the pressure predicted by Density Functional Theory may vary qualitatively when using different functionals. The predictive power of first-principle simulations is restored by validating each functional against higher-level wavefunction theories, represented by computationally intensive variational and diffusion Monte Carlo calculations. Our simulations provide evidence that hydrogen is denser at planetary conditions, compared to currently used equations of state. For Jupiter, this implies a lower bulk metallicity (i.e., a smaller mass of heavy elements). Our results further amplify the inconsistency between Jupiter's atmospheric metallicity measured by the Galileo probe and the envelope metallicity inferred from interior models. △ Less

Submitted 28 July, 2025; v1 submitted 22 January, 2025; originally announced January 2025.

Comments: We add Huguniot calculation with SCAN+vv10, benchmarks with more functionals, and provide additional tests

Accurate and thermodynamically consistent hydrogen equation of state for planetary modeling with flow matching

Authors: Hao Xie, Saburo Howard, Guglielmo Mazzola

Abstract: Accurate determination of the equation of state of dense hydrogen is essential for understanding gas giants. Currently, there is still no consensus on methods for calculating its entropy, which play a fundamental role and can result in qualitatively different predictions for Jupiter's interior. Here, we investigate various aspects of entropy calculation for dense hydrogen based on ab initio molecu… ▽ More Accurate determination of the equation of state of dense hydrogen is essential for understanding gas giants. Currently, there is still no consensus on methods for calculating its entropy, which play a fundamental role and can result in qualitatively different predictions for Jupiter's interior. Here, we investigate various aspects of entropy calculation for dense hydrogen based on ab initio molecular dynamics simulations. Specifically, we employ the recently developed flow matching method to validate the accuracy of the traditional thermodynamic integration approach. We then clearly identify pitfalls in previous attempts and propose a reliable framework for constructing the hydrogen equation of state, which is accurate and thermodynamically consistent across a wide range of temperature and pressure conditions. This allows us to conclusively address the long-standing discrepancies in Jupiter's adiabat among earlier studies, demonstrating the potential of our approach for providing reliable equations of state of diverse materials. △ Less

Submitted 9 August, 2025; v1 submitted 17 January, 2025; originally announced January 2025.

Comments: 7+7 pages, 4+8 figures

Journal ref: Phys. Rev. Research 7, L032028 (2025)

Competing Hexagonal and Square Lattices on a Spherical Surface

Authors: Han Xie, Wenyu Liu, Zhenyue Lu, Jeff Z. Y. Chen, Yao Li

Abstract: The structural properties of packed soft-core particles provide a platform to understand the cross-pollinated physical concepts in solid-state- and soft-matter physics. Confined on spherical surface, the traditional differential geometry also dictates the overall defect properties in otherwise regular crystal lattices. Using molecular dynamics simulation of the Hertzian model as a tool, we report… ▽ More The structural properties of packed soft-core particles provide a platform to understand the cross-pollinated physical concepts in solid-state- and soft-matter physics. Confined on spherical surface, the traditional differential geometry also dictates the overall defect properties in otherwise regular crystal lattices. Using molecular dynamics simulation of the Hertzian model as a tool, we report here the emergence of new types of disclination patterns: domain and counter-domain defects, when hexagonal and square patterns coexist. A new angle is presented to understand the incompatibility between tiling lattice shapes and the available spherical areal shapes, which is common in nature -- from molecular systems in biology to backbone construction in architectures. △ Less

Submitted 2 January, 2025; originally announced January 2025.

Comments: Authors' version of the article submitted to Nano Letters and accepted

Inducing Berry Curvature Dipole in Multilayer Graphene through Inhomogeneous Interlayer Sliding

Authors: Jie Pan, Huanhuan Wang, Lin Zou, Haibo Xie, Yi Ding, Yuze Zhang, Aiping Fang, Zhe Wang

Abstract: Breaking lattice symmetry is crucial for generating a nonzero Berry curvature. While manipulating twisting angles between adjacent layers has successfully broken lattice symmetry through strain field and generated nonzero Berry curvature, interlayer sliding in principle offers a promising alternative route. However, realizing uniform interlayer sliding faces experimental challenges due to its ener… ▽ More Breaking lattice symmetry is crucial for generating a nonzero Berry curvature. While manipulating twisting angles between adjacent layers has successfully broken lattice symmetry through strain field and generated nonzero Berry curvature, interlayer sliding in principle offers a promising alternative route. However, realizing uniform interlayer sliding faces experimental challenges due to its energetic instability. In this work, we introduce an experimentally feasible method, using a corrugated substrate to induce an inhomogeneous but energetically more stable interlayer sliding in multilayer graphene. Our simulations demonstrate that inhomogeneous interlayer sliding produces a sizable Berry curvature dipole, which can be further tuned by varying the interlayer sliding distances and potential differences. The resulting Berry curvature dipole magnitude is remarkably up to 100 times greater than the maximum displacement involved in the inhomogeneous sliding. Our results highlight inhomogeneous interlayer sliding as a viable and effective method to induce a significant Berry curvature dipole in graphene systems and propose the experimentally feasible way to realize it. △ Less

Submitted 17 December, 2024; originally announced December 2024.

Comments: 9 pages, 7 figures

Journal ref: Physical Review B, 110, 235418 (2024)

Bias Voltage Driven Tunneling Magnetoresistance Polarity Reversal in 2D Stripy Antiferromagnet CrOCl

Authors: Lihao Zhang, Xiaoyu Wang, Qi Li, Haibo Xie, Liangliang Zhang, Lei Zhang, Jie Pan, Yingchun Cheng, Zhe Wang

Abstract: Atomically thin materials with coupled magnetic and electric polarization are critical for developing energy-efficient and high-density spintronic devices, yet they remain scarce due to often conflicting requirements of stabilizing both magnetic and electric orders. The recent discovery of the magnetoelectric effect in the 2D stripy antiferromagnet CrOCl highlights this semiconductor as a promisin… ▽ More Atomically thin materials with coupled magnetic and electric polarization are critical for developing energy-efficient and high-density spintronic devices, yet they remain scarce due to often conflicting requirements of stabilizing both magnetic and electric orders. The recent discovery of the magnetoelectric effect in the 2D stripy antiferromagnet CrOCl highlights this semiconductor as a promising platform to explore electric field effects on magnetoresistance. In this study, we systematically investigate the magnetoresistance in tunneling junctions of bilayer and monolayer CrOCl. We observe that the transition from antiferromagnetic to ferrimagnetic phases in both cases induces a positive magnetoresistance at low bias voltages, which reverses to a negative value at higher bias voltages. This polarity reversal is attributed to the additional electric dipoles present in the antiferromagnetic state, as supported by our theoretical calculations. These findings suggest a pathway for the electric control of spintronic devices and underscore the potential of 2D magnets like CrOCl in advancing energy-efficient spintronic applications. △ Less

Submitted 6 December, 2024; originally announced December 2024.

Journal ref: Applied Physics Letters 125, 222403(2024)

Significant Impact of Quantum and Anharmonic Effects on the Structural Stability and Superconductivity of NbH3 at High Pressures

Authors: Pugeng Hou, Yao Ma, Hui Xie, Mingqi Li, Yongmao Cai, Yuhua Shen, Xuewu Wang, Mi Pang

Abstract: First-principles calculations combined with the stochastic self-consistent harmonic approximation reveal significant effects of the quantum ionic fluctuations and lattice anharmonicity on the dynamical stability of NbH3 under high pressures. Previous theoretical predictions, which ignored ionic fluctuations and relied on the harmonic approximation, suggested that the I43d phase is the most thermod… ▽ More First-principles calculations combined with the stochastic self-consistent harmonic approximation reveal significant effects of the quantum ionic fluctuations and lattice anharmonicity on the dynamical stability of NbH3 under high pressures. Previous theoretical predictions, which ignored ionic fluctuations and relied on the harmonic approximation, suggested that the I43d phase is the most thermodynamically favorable structure between 33 and 400 GPa, with the Fm3m phase considered thermodynamically metastable. However, recent experiments at 187 GPa identified the Fm3m phase, conflicting with the prediction. In contrast, the present study indicates that the Fm3m phase remains dynamically stable down to at least 145 GPa, approximately 145 GPa lower than harmonic estimates, while the I43d phase is dynamically unstable at 187 GPa, consistent with the experimental findings. Furthermore, systematic calculations are performed on the structural, vibrational and superconducting properties of Fm3m NbH3 under pressures ranging from 100 to 300 GPa, revealing dramatic modifications due to the quantum and anharmonic effects. The calculated superconducting critical temperature (Tc) from the McMillan equation for Fm3m NbH3 at 187 GPa is 44 K, with mu set at 0.15, close to the measured value. These findings highlight the crucial role of quantum anharmonic effects in stabilizing the Fm3m phase. △ Less

Submitted 27 October, 2024; v1 submitted 11 October, 2024; originally announced October 2024.

Optical modeling, solver, and design of wafer-scale single-enantiomer carbon nanotube film and reconfigurable chiral photonic device

Authors: Jichao Fan, Benjamin Hillam, Cheng Guo, Hiroyuki Fujinami, Shiba Koki, Haoyu Xie, Ruiyang Chen, Kazuhiro Yanagi, Weilu Gao

Abstract: The interaction of circularly polarized light with chiral matter and functional devices enables novel phenomena and applications. Recently, wafer-scale solid-state single-enantiomer carbon nanotube (CNT) films have become feasible and are emerging as a chiral photonic material platform thanks to their quantum-confinement-induced optical properties and facile scalable assembly. However, optical mod… ▽ More The interaction of circularly polarized light with chiral matter and functional devices enables novel phenomena and applications. Recently, wafer-scale solid-state single-enantiomer carbon nanotube (CNT) films have become feasible and are emerging as a chiral photonic material platform thanks to their quantum-confinement-induced optical properties and facile scalable assembly. However, optical modeling, solver, and device design tools for such materials are non-existent. Here, we prepare wafer-scale single-enantiomer (6,5) and (11,-5) randomly oriented CNT films and create an optical material model based on measured experimental optical spectra. We also implement a highly-parallel graphic-processing-unit accelerated transfer matrix solver for general bi-anisotropic materials and layered structures. Further, we demonstrate reconfigurable chiral photonic devices in a heterostructure with phase change materials through machine learning-enabled efficient gradient-based inverse design and optimization. Our developed full stack of a chiral photonic material and device hardware platform and a corresponding high-performance differential-programming-enabled solver opens the door for future chiral photonic devices and applications based on single-enantiomer CNT films. △ Less

Submitted 11 October, 2024; originally announced October 2024.

On the Nonlinear Excitation of Phononic Frequency Combs in Molecules

Authors: Hongbin Lei, Qian Zhang, Hongqiang Xie, Congsen Meng, Zhaoyang Peng, Jinlei Liu, Guangru Bai, Adarsh Ganesan, Zengxiu Zhao

Abstract: The mechanical analog of optical frequency combs, phononic frequency combs (PFCs), has recently been demonstrated in mechanical resonators via nonlinear coupling among multiple phonon modes. However, for exciting phononic combs in molecules, the requisite strong nonlinear couplings need not be readily present. To overcome this limitation, this paper introduces an alternative route for the generati… ▽ More The mechanical analog of optical frequency combs, phononic frequency combs (PFCs), has recently been demonstrated in mechanical resonators via nonlinear coupling among multiple phonon modes. However, for exciting phononic combs in molecules, the requisite strong nonlinear couplings need not be readily present. To overcome this limitation, this paper introduces an alternative route for the generation of phononic combs in polar molecules. Theoretically, we investigated the radiation and phononic spectra generated from CO molecule possessing relatively large permanent dipole moment with density matrix formalism. By considering rovibronic excitation of the ground-state CO molecule while avoiding the electronic excitation, the contribution of the permanent dipole moment and electric dipole polarizability to the creation of PFCs is demonstrated and distinguished. The finding could motivate the possible extension of combs to molecular systems to offer new avenues in molecular sciences. △ Less

Submitted 29 September, 2024; originally announced September 2024.

Comments: 10 pages, 7 figures

Artificially built Kondo chains with organic radicals on metallic surfaces: new model system of heavy fermion quantum criticality

Authors: En Li, Bimla Danu, Yufeng Liu, Huilin Xie, Jacky Wing Yip Lam, Ben Zhong Tang, Shiyong Wang, Fakher F. Assaad, Nian Lin

Abstract: Heavy fermion quantum criticality is an extremely rich domain of research which represents a framework to understand strange metals as a consequence of a Kondo breakdown transition. Here we provide an experimental realization of such systems in terms of organic radicals on a metallic surface. The ground state of organic radicals is a Kramer's doublet that can be modeled by a spin 1/2 degree of fre… ▽ More Heavy fermion quantum criticality is an extremely rich domain of research which represents a framework to understand strange metals as a consequence of a Kondo breakdown transition. Here we provide an experimental realization of such systems in terms of organic radicals on a metallic surface. The ground state of organic radicals is a Kramer's doublet that can be modeled by a spin 1/2 degree of freedom. Using on-surface synthesis and scanning tunneling microscopy (STM) tip manipulation, one can controllably engineer and characterize chains of organic radicals on a Au(111) surface. The spatial-resolved differential conductance reveals site-dependent low-energy excitations, which support the picture of emergent many-body Kondo physics. Using quantum Monte Carlo simulations, we show that a Kondo lattice model of spin chains on a metallic surface reproduces accurately the experimental results. This allows us to interpret the experimental results in terms of a heavy fermion metal, below the coherence temperature. We foresee that the tunability of these systems will pave the way to realize quantum simulators of heavy fermion criticality. △ Less

Submitted 2 August, 2024; originally announced August 2024.

Journal ref: J. Am. Chem. Soc. 2025, 147, 50, 46535

A programmable wafer-scale chiroptical heterostructure of twisted aligned carbon nanotubes and phase change materials

Authors: Jichao Fan, Ruiyang Chen, Minhan Lou, Haoyu Xie, Nina Hong, Yingheng Tang, Weilu Gao

Abstract: The ability to design and dynamically control chiroptical responses in solid-state matter at wafer scale enables new opportunities in various areas. Here we present a full stack of computer-aided designs and experimental implementations of a dynamically programmable, unified, scalable chiroptical heterostructure containing twisted aligned one-dimensional (1D) carbon nanotubes (CNTs) and non-volati… ▽ More The ability to design and dynamically control chiroptical responses in solid-state matter at wafer scale enables new opportunities in various areas. Here we present a full stack of computer-aided designs and experimental implementations of a dynamically programmable, unified, scalable chiroptical heterostructure containing twisted aligned one-dimensional (1D) carbon nanotubes (CNTs) and non-volatile phase change materials (PCMs). We develop a software infrastructure based on high-performance machine learning frameworks, including differentiable programming and derivative-free optimization, to efficiently optimize the tunability of both excitonic reciprocal and linear-anisotropy-induced nonreciprocal circular dichroism (CD) responses. We experimentally implement designed heterostructures with wafer-scale self-assembled aligned CNTs and deposited PCMs. We dynamically program reciprocal and nonreciprocal CD responses by inducing phase transitions of PCMs, and nonreciprocal responses display polarity reversal of CD upon sample flipping in broadband spectral ranges. All experimental results agree with simulations. Further, we demonstrate that the vertical dimension of heterostructure is scalable with the number of stacking layers and aligned CNTs play dual roles - the layer to produce CD responses and the Joule heating electrode to electrically program PCMs. This heterostructure platform is versatile and expandable to a library of 1D nanomaterials and electro-optic materials for exploring novel chiral phenomena and photonic and optoelectronic devices. △ Less

Submitted 18 June, 2024; originally announced June 2024.

Diverse Responses in Lattice Thermal Conductivity of $n$-type/$p$-type Semiconductors Driven by Asymmetric Electron-Phonon Interactions

Authors: Jianshi Sun, Shouhang Li, Zhen Tong, Cheng Shao, Han Xie, Meng An, Chuang Zhang, Xiongfei Zhu, Chen Huang, Yucheng Xiong, Xiangjun Liu

Abstract: Accurately assessing the impact of electron-phonon interaction (EPI) on the lattice thermal conductivity of semiconductors is crucial for the thermal management of electronic devices and a unified physical understanding of this issue is highly desired. In this work, we predict the lattice thermal conductivities of typical direct and indirect bandgap semiconductors accounting for EPI based on mode-… ▽ More Accurately assessing the impact of electron-phonon interaction (EPI) on the lattice thermal conductivity of semiconductors is crucial for the thermal management of electronic devices and a unified physical understanding of this issue is highly desired. In this work, we predict the lattice thermal conductivities of typical direct and indirect bandgap semiconductors accounting for EPI based on mode-level first-principles calculations. It is found that EPI has a larger effect on the lattice thermal conductivity of $p$-type doping compared to $n$-type doping in the same semiconductor at high charge carrier concentrations. The stronger EPI in $p$-type doping is attributed to the relatively higher electron density of states caused by the relatively larger $p$-orbital component. Furthermore, EPI has a stronger influence on the lattice thermal conductivity of $n$-type indirect bandgap semiconductors than $n$-type direct bandgap semiconductors. This is attributed to the relatively lower electron density of states in direct bandgap semiconductors stemming from the $s$-orbital component. This work reveals that there exist diverse responses in lattice thermal conductivity of $n$-type/$p$-type semiconductors, which can be attributed to asymmetric EPIs. △ Less

Submitted 17 June, 2024; originally announced June 2024.

Comments: 8 pages,5 figures

Majorana qubit readout by a point-contact detector under finite bias voltages

Authors: Huizi Xie, Sirui Yu, Hong Mao, Jinshuang Jin

Abstract: In this work we revisit the problem of a Majorana box qubit (MBQ) readout by a point-contact (PC) detector. The logic states of the MBQ are associated with the combined fermion parities of the MBQ and its tunnel-coupled quantum dot, which is measured by a PC detector. Beyond the existing studies on limiting bias voltage regimes, we analyze the steady-state current and the current power spectrum ac… ▽ More In this work we revisit the problem of a Majorana box qubit (MBQ) readout by a point-contact (PC) detector. The logic states of the MBQ are associated with the combined fermion parities of the MBQ and its tunnel-coupled quantum dot, which is measured by a PC detector. Beyond the existing studies on limiting bias voltage regimes, we analyze the steady-state current and the current power spectrum across all bias voltages. Our results indicate that the MBQ readout via the parity-dependent detector current is effective only at low bias voltage regime and requires the dot energy level to be off-resonance with the Majorana qubit. In contrast, the current power spectrum allows MBQ readout through the parity-dependent Rabi oscillation peak signals for arbitrary bias voltages, without restrictions on the dot energy level. Particularly, with focus on the MBQ measurement visibility, we analyze the peak-to-pedestal ratio for each characteristic peak (associated with each logic state of the qubit) and the signal-to-noise ratio of the two peaks. By examining these two metrics, we identify the optimal bias voltage window for the PC detector at low temperature limit. △ Less

Submitted 10 February, 2025; v1 submitted 16 June, 2024; originally announced June 2024.

Comments: 7 pages, 4 figures

Journal ref: PhysRevB.111.075412 (2025)

Unveiling the Pockels Coefficient of Ferroelectric Nitride ScAlN

Authors: Guangcanlan Yang, Haochen Wang, Sai Mu, Hao Xie, Tyler Wang, Chengxing He, Mohan Shen, Mengxia Liu, Chris G. Van de Walle, Hong X. Tang

Abstract: Nitride ferroelectrics have recently emerged as promising alternatives to oxide ferroelectrics due to their compatibility with mainstream semiconductor processing. ScAlN, in particular, has exhibited remarkable piezoelectric coupling strength ($K^2$) comparable to that of lithium niobate (LN), making it a valuable choice for RF filters in wireless communications. Recently, ScAlN has sparked intere… ▽ More Nitride ferroelectrics have recently emerged as promising alternatives to oxide ferroelectrics due to their compatibility with mainstream semiconductor processing. ScAlN, in particular, has exhibited remarkable piezoelectric coupling strength ($K^2$) comparable to that of lithium niobate (LN), making it a valuable choice for RF filters in wireless communications. Recently, ScAlN has sparked interest in its use for nanophotonic devices, chiefly due to its large bandgap facilitating operation in blue wavelengths coupled with promises of enhanced nonlinear optical properties such as a large second-order susceptibility ($χ^{(2)}$). It is still an open question whether ScAlN can outperform oxide ferroelectrics concerning the Pockels effect -- an electro-optic coupling extensively utilized in optical communications devices. In this paper, we present a comprehensive theoretical analysis and experimental demonstration of ScAlN's Pockels effect. Our findings reveal that the electro-optic coupling of ScAlN, despite being weak at low Sc concentration, may be significantly enhanced and exceed LiNbO$_3$ at high levels of Sc doping, which points the direction of continued research efforts to unlock the full potential of ScAlN. △ Less

Submitted 18 October, 2024; v1 submitted 13 May, 2024; originally announced May 2024.

Nonlinear Hall effect and scaling law in Sb-doped topological insulator MnBi4Te7

Authors: Shaoyu Wang, Xiubing Li, Heng Zhang, Bo Chen, Hangkai Xie, Congcong Li, Fucong Fei, Shuai Zhang, Fengqi Song

Abstract: Nonlinear Hall effect (NLHE), as a new member of Hall effect family, has been realized in many materials, attracting a great deal of attention. Here, we report the observation of NLHE in magnetic topological insulator Sb-doped MnBi4Te7 flakes. The NLHE generation efficiency can reach up to 0.06 V^-1, which is comparable to that observed in MnBi2Te4. Differently, the NLHE can survive up to 200 K, m… ▽ More Nonlinear Hall effect (NLHE), as a new member of Hall effect family, has been realized in many materials, attracting a great deal of attention. Here, we report the observation of NLHE in magnetic topological insulator Sb-doped MnBi4Te7 flakes. The NLHE generation efficiency can reach up to 0.06 V^-1, which is comparable to that observed in MnBi2Te4. Differently, the NLHE can survive up to 200 K, much larger than the magnetic transition temperature. We further study the scaling behavior of the NLHE with longitudinal conductivity. The linear relationship with opposite slope when temperature is below and above the magnetic transition temperature is uncovered. It reveals that the NLHE originates from skew scattering. Our work provides a platform to search NLHE with larger generation efficiency at higher temperatures. △ Less

Submitted 9 April, 2024; originally announced April 2024.

Journal ref: Appl. Phys. Lett. 124, 153102 (2024)

Electrical Control of Exciton-Polariton Condensate Josephson Junctions via Exciton Stark Effect

Authors: Hua Wang, Hong-Yi Xie, Kieran Mullen

Abstract: We propose harnessing the tools of modern nano-fabrication to provide electrical control of exciton-polariton (EP) condensates. We develop the theory of a device based on the Josephson effect in which electric fields can be used to both switch between and monitor various dynamical modes. In particular, both the bias potential and the Josephson energy can be tuned electrically via the exciton compo… ▽ More We propose harnessing the tools of modern nano-fabrication to provide electrical control of exciton-polariton (EP) condensates. We develop the theory of a device based on the Josephson effect in which electric fields can be used to both switch between and monitor various dynamical modes. In particular, both the bias potential and the Josephson energy can be tuned electrically via the exciton component. We model the device by a Gross-Pitaevskii equation assuming that ideal EP condensates are established with well-balanced pumping and dissipation. We find that the EP condensates can be manipulated through degrees of freedom not easily accessible in other coherent quantum systems, and the dynamics of EP Josephson junctions are richer than that of the conventional superconducting junctions. The ability to control and monitor the condensate by both optical and electrical means allows new ways to study its physics not possible by either, alone. △ Less

Submitted 5 January, 2025; v1 submitted 4 March, 2024; originally announced March 2024.

The valleytronic topological filters in silicene-like inner-edge systems

Authors: Hang Xie, Xiao-Long Lü, Jia-En Yang

Abstract: Inner edge state with spin and valley degrees of freedom is a promising candidate to design a dissipationless device due to the topological protection. The central challenge for the application of inner edge state is to generate and modulate the polarized currents. In this work, we discover a new mechanism to generate fully valley- and spin-valley-polarized current caused by the Bloch wavevector m… ▽ More Inner edge state with spin and valley degrees of freedom is a promising candidate to design a dissipationless device due to the topological protection. The central challenge for the application of inner edge state is to generate and modulate the polarized currents. In this work, we discover a new mechanism to generate fully valley- and spin-valley-polarized current caused by the Bloch wavevector mismatch (BWM). Based on this mechanism, we design some serial-typed inner-edge filters. With once of the BWM, the coincident states could be divided into transmitted and reflected modes, which can serve as a valley or spin-valley filter. In particular, while with twice of the BWM, the incident current is absolutely reflected to support an off state with a specified valley and spin, which is different from the gap effect. These findings give rise to a new platform for designing valleytronics and spin-valleytronics. △ Less

Submitted 22 November, 2023; originally announced November 2023.

Comments: 20 pages, 8 figures

Prominent Josephson tunneling between twisted single copper oxide planes of Bi$_2$Sr$_{2-x}$LaxCuO$_{6+y}$

Authors: Heng Wang, Yuying Zhu, Zhonghua Bai, Zechao Wang, Shuxu Hu, Hong-Yi Xie, Xiaopeng Hu, Jian Cui, Miaoling Huang, Jianhao Chen, Ying Ding, Lin Zhao, Xinyan Li, Qinghua Zhang, Lin Gu, X. J. Zhou, Jing Zhu, Ding Zhang, Qi-Kun Xue

Abstract: Josephson tunneling in twisted cuprate junctions provides a litmus test for the pairing symmetry, which is fundamental for understanding the microscopic mechanism of high temperature superconductivity. This issue is rekindled by experimental advances in van der Waals stacking and the proposal of an emergent d+id-wave. So far, all experiments have been carried out on Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$ (… ▽ More Josephson tunneling in twisted cuprate junctions provides a litmus test for the pairing symmetry, which is fundamental for understanding the microscopic mechanism of high temperature superconductivity. This issue is rekindled by experimental advances in van der Waals stacking and the proposal of an emergent d+id-wave. So far, all experiments have been carried out on Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$ (Bi-2212) with double CuO$_2$ planes but show controversial results. Here, we investigate junctions made of Bi$_2$Sr$_{2-x}$La$_x$CuO$_{6+y}$ (Bi-2201) with single CuO$_2$ planes. Our on-site cold stacking technique ensures uncompromised crystalline quality and stoichiometry at the interface. Junctions with carefully calibrated twist angles around 45° show strong Josephson tunneling and conventional temperature dependence. Furthermore, we observe standard Fraunhofer diffraction patterns and integer Fiske steps in a junction with a twist angle of 45.0$\pm$0.2°. Together, these results pose strong constraints on the d or d+id-wave pairing and suggest an indispensable isotropic pairing component. △ Less

Submitted 20 November, 2023; originally announced November 2023.

Comments: 32 pages, 5 figures

Journal ref: Nature Communications 14, 5201 (2023)

Theory of topological exciton insulators and condensates in flat Chern bands

Authors: Hong-Yi Xie, Pouyan Ghaemi, Matteo Mitrano, Bruno Uchoa

Abstract: Excitons are the neutral quasiparticles that form when Coulomb interactions create bound states between electrons and holes. Due to their bosonic nature, excitons are expected to condense and exhibit superfluidity at sufficiently low temperatures. In interacting Chern insulators, excitons may inherit the nontrivial topology and quantum geometry from the underlying electron wavefunctions. We theore… ▽ More Excitons are the neutral quasiparticles that form when Coulomb interactions create bound states between electrons and holes. Due to their bosonic nature, excitons are expected to condense and exhibit superfluidity at sufficiently low temperatures. In interacting Chern insulators, excitons may inherit the nontrivial topology and quantum geometry from the underlying electron wavefunctions. We theoretically investigate the excitonic bound states and superfluidity in flat-band insulators pumped with light. We find that the exciton wavefunctions exhibit vortex structures in momentum space, with the total vorticity being equal to the difference of Chern numbers between the conduction and valence bands. Moreover, both the exciton binding energy and the exciton superfluid density are proportional to the Brillouin-zone average of the quantum metric and the Coulomb potential energy per unit cell. Spontaneous emission of circularly polarized light from radiative decay is a detectable signature of the vorticity of excitons. We propose that the exciton vorticity can also be experimentally measured by the nonlinear anomalous Hall effect, whereas the exciton superfluidity can be detected by voltage-drop quantization through a combination of the quantum geometry and the Aharonov-Casher effect. Topological excitons and their superfluid phase could be realized in flat bands of twisted Van der Waals heterostructures. △ Less

Submitted 24 January, 2024; v1 submitted 8 November, 2023; originally announced November 2023.

Comments: 25+3 pages, 3 figures

Journal ref: PNAS 121, e2401644121 (2024)

Efficient non-collinear antiferromagnetic state switching induced by orbital Hall effect in chromium

Authors: Hang Xie, Nan Zhang, Yuteng Ma, Xin Chen, Lin Ke, Yihong Wu

Abstract: Recently orbital Hall current has attracted attention as an alternative method to switch the magnetization of ferromagnets. Here we present our findings on electrical switching of antiferromagnetic state in Mn3Sn/Cr, where despite the much smaller spin Hall angle of Cr, the switching current density is comparable to heavy metal based heterostructures. On the other hand, the inverse process, i.e.,… ▽ More Recently orbital Hall current has attracted attention as an alternative method to switch the magnetization of ferromagnets. Here we present our findings on electrical switching of antiferromagnetic state in Mn3Sn/Cr, where despite the much smaller spin Hall angle of Cr, the switching current density is comparable to heavy metal based heterostructures. On the other hand, the inverse process, i.e., spin-to-charge conversion in Cr-based heterostructures is much less efficient than the Pt-based equivalents, as manifested in the almost one order of magnitude smaller terahertz emission intensity and spin current induced magnetoresistance in Cr-based structures. These results in combination with the slow decay of terahertz emission against Cr thickness (diffusion length of ~11 nm) suggest that the observed magnetic switching can be attributed to orbital current generation in Cr, followed by efficient conversion to spin current. Our work demonstrates the potential of light metals like Cr as an efficient orbital/spin current source for antiferromagnetic spintronics. △ Less

Submitted 19 October, 2023; originally announced October 2023.

Comments: 19 pages, 4 figures

A powered full quantum eigensolver for energy band structures

Authors: Bozhi Wang, Jingwei Wen, Jiawei Wu, Haonan Xie, Fan Yang, Shijie Wei, Gui-lu Long

Abstract: There has been an increasing research focus on quantum algorithms for condensed matter systems recently, particularly on calculating energy band structures. Here, we propose a quantum algorithm, the powered full quantum eigensolver(P-FQE), by using the exponentiation of operators of the full quantum eigensolver(FQE). This leads to an exponential increase in the success probability of measuring the… ▽ More There has been an increasing research focus on quantum algorithms for condensed matter systems recently, particularly on calculating energy band structures. Here, we propose a quantum algorithm, the powered full quantum eigensolver(P-FQE), by using the exponentiation of operators of the full quantum eigensolver(FQE). This leads to an exponential increase in the success probability of measuring the target state in certain circumstances where the number of generating elements involved in the exponentiation of operators exhibit a log polynomial dependence on the number of orbitals. Furthermore, we conduct numerical calculations for band structure determination of the twisted double-layer graphene. We experimentally demonstrate the feasibility and robustness of the P-FQE algorithm using superconducting quantum computers for graphene and Weyl semimetal. One significant advantage of our algorithm is its ability to reduce the requirements of extremely high-performance hardware, making it more suitable for energy spectra determination on noisy intermediate-scale quantum (NISQ) devices. △ Less

Submitted 6 August, 2023; originally announced August 2023.

Structural pathway for nucleation and growth of topologically close-packed phase from parent hexagonal crystal

Authors: Junyuan Bai, Hongbo Xie, Xueyong Pang, Min Jiang, Gaowu Qin

Abstract: The solid diffusive phase transformation involving the nucleation and growth of one nucleus is universal and frequently employed but has not yet been fully understood at the atomic level. Here, our first-principles calculations reveal a structural formation pathway of a series of topologically close-packed (TCP) phases within the hexagonally close-packed (hcp) matrix. The results show that the nuc… ▽ More The solid diffusive phase transformation involving the nucleation and growth of one nucleus is universal and frequently employed but has not yet been fully understood at the atomic level. Here, our first-principles calculations reveal a structural formation pathway of a series of topologically close-packed (TCP) phases within the hexagonally close-packed (hcp) matrix. The results show that the nucleation follows a nonclassical nucleation process, and the whole structural transformation is completely accomplished by the shuffle-based displacements, with a specific 3-layer hcp-ordering as the basic structural transformation unit. The thickening of plate-like TCP phases relies on forming these hcp-orderings at their coherent TCP/matrix interface to nucleate ledge, but the ledge lacks the dislocation characteristics considered in the conventional view. Furthermore, the atomic structure of the critical nucleus for the Mg2Ca and MgZn2 Laves phases was predicted in terms of Classical Nucleation Theory (CNT), and the formation of polytypes and off-stoichiometry in TCP precipitates is found to be related to the nonclassical nucleation behavior. Based on the insights gained, we also employed high-throughput screening to explore several common hcp-metallic (including hcp-Mg, Ti, Zr, and Zn) systems that may undergo hcp-to-TCP phase transformations. These insights can deepen our understanding of solid diffusive transformations at the atomic level, and constitute a foundation for exploring other technologically important solid diffusive transformations. △ Less

Submitted 10 September, 2023; v1 submitted 13 July, 2023; originally announced July 2023.

Revealing intrinsic domains and fluctuations of moiré magnetism by a wide-field quantum microscope

Authors: Mengqi Huang, Zeliang Sun, Gerald Yan, Hongchao Xie, Nishkarsh Agarwal, Gaihua Ye, Suk Hyun Sung, Hanyi Lu, Jingcheng Zhou, Shaohua Yan, Shangjie Tian, Hechang Lei, Robert Hovden, Rui He, Hailong Wang, Liuyan Zhao, Chunhui Rita Du

Abstract: Moiré magnetism featured by stacking engineered atomic registry and lattice interactions has recently emerged as an appealing quantum state of matter at the forefront condensed matter physics research. Nanoscale imaging of moiré magnets is highly desirable and serves as a prerequisite to investigate a broad range of intriguing physics underlying the interplay between topology, electronic correlati… ▽ More Moiré magnetism featured by stacking engineered atomic registry and lattice interactions has recently emerged as an appealing quantum state of matter at the forefront condensed matter physics research. Nanoscale imaging of moiré magnets is highly desirable and serves as a prerequisite to investigate a broad range of intriguing physics underlying the interplay between topology, electronic correlations, and unconventional nanomagnetism. Here we report spin defect-based wide-field imaging of magnetic domains and spin fluctuations in twisted double trilayer (tDT) chromium triiodide CrI3. We explicitly show that intrinsic moiré domains of opposite magnetizations appear over arrays of moiré supercells in low-twist-angle tDT CrI3. In contrast, spin fluctuations measured in tDT CrI3 manifest little spatial variations on the same mesoscopic length scale due to the dominant driving force of intralayer exchange interaction. Our results enrich the current understanding of exotic magnetic phases sustained by moiré magnetism and highlight the opportunities provided by quantum spin sensors in probing microscopic spin related phenomena on two-dimensional flatland. △ Less

Submitted 7 July, 2023; originally announced July 2023.

Dyadic Greens function for a topological insulator stratified sphere

Authors: Huai-Yi Xie

Abstract: We construct the dyadic Greens functions (DGFs) for a topological insulator (TI) stratified sphere within the framework of axion electrodynamics. For these DGFs, the additional expansion coefficients are included to account for the axion coupling effect. With the application of these DGFs, we derive the formulation of light scattering from a dipole near a TI stratified sphere. In our numerical stu… ▽ More We construct the dyadic Greens functions (DGFs) for a topological insulator (TI) stratified sphere within the framework of axion electrodynamics. For these DGFs, the additional expansion coefficients are included to account for the axion coupling effect. With the application of these DGFs, we derive the formulation of light scattering from a dipole near a TI stratified sphere. In our numerical studies, we give three types of configurations (a metal-coated TI sphere, a metal-TI-metal-coated TI sphere and an alternating metal-TI stratified sphere) to investigate how the topological magneto-electric (TME) response of the TI sphere (shells) influences on the multipolar plasmonic resonance of the metal shells. For these types, the results show that the TME effect causes some modifications of the decay rate spectrum for an emitting dipole near a TI stratified sphere. For the multipolar resonances of the metal shells, it is observed that the TME-induced red-shifts for the bonding and lower order antibonding modes are found but those for the higher order antibonding modes are insignificant. In addition, for a metal-coated TI sphere, we take into account the effects of losses in the TI core of which the dielectric function is chosen to be the form of the bulk or five quintuple layers (5QL) slab and then the some modifications of the TME-induced decay rate spectrum are obviously suppressed. These phenomenological characteristics provide useful guidance to probing the TME effect via molecular fluorescence experiments. △ Less

Submitted 11 September, 2023; v1 submitted 10 April, 2023; originally announced April 2023.

Journal ref: Phys. Scr. 98 055504 (2023)

Development of 15kA/cm$^2$ Fabrication Process for Superconducting Integrated Digital Circuits

Authors: Liliang Ying, Xue Zhang, Guixiang He, Weifeng Shi, Hui Xie, Linxian Ma, Hui Zhang, Jie Ren, Wei Peng, Zhen Wang

Abstract: A new fabrication process for superconducting integrated digital circuits is reported. We have developed the "SIMIT Nb04" fabrication technique for superconducting integrated circuits with Nb-based Josephson junctions based on the validated "SIMIT Nb03" process and Chemical Mechanical Planarization (CMP) technology. Seven Nb superconducting layers and one Mo resistor layer are included in the "SIM… ▽ More A new fabrication process for superconducting integrated digital circuits is reported. We have developed the "SIMIT Nb04" fabrication technique for superconducting integrated circuits with Nb-based Josephson junctions based on the validated "SIMIT Nb03" process and Chemical Mechanical Planarization (CMP) technology. Seven Nb superconducting layers and one Mo resistor layer are included in the "SIMIT Nb04" process with 19 mask levels. The device structure is composed of active layers including junctions at the bottom, two passive transmission line (PTL) layers in the middle and a DC power layer at the top. The circuit fabrication started with the fabrication of Mo resistors with a target sheet resistance Rsh of 3 $Ω$, followed by the deposition of Nb/Al-AlO$_x$/Nb trilayer Josephson-junction with a target critical current density Jc at 15 kA/cm$^2$. To increase the Al-AlO$_x$ barrier layer etching's repeatability, an additional barrier protection layer was applied. To accomplish high-quality planarization, we created a planarization procedure coupled with dummy filling. To assess the process dependability and controllability, a set of process control monitors (PCMs) for monitoring fabrication and design parameters was designed and monitored. The successful manufacturing and testing of a few small-scale circuits, like our standard library cells, further attests to the viability of our fabrication process for superconducting integrated circuits. △ Less

Submitted 17 August, 2023; v1 submitted 4 April, 2023; originally announced April 2023.

Distilling Accurate Descriptors from Multi-Source Experimental Data for Discovering Highly Active Perovskite OER Catalysts

Authors: Jingzhou Wang, Huachao Xie, Yuanqing Wang, Runhai Ouyang

Abstract: Perovskite oxides are promising catalysts for oxygen evolution reaction (OER), yet the huge chemical space remains largely unexplored due to the lack of effective approaches. Here, we report the distilling of accurate descriptors from multi-source experimental data for accelerated catalysts discovery by using the new method SCMT-SISSO that overcomes the challenge of data inconsistency between diff… ▽ More Perovskite oxides are promising catalysts for oxygen evolution reaction (OER), yet the huge chemical space remains largely unexplored due to the lack of effective approaches. Here, we report the distilling of accurate descriptors from multi-source experimental data for accelerated catalysts discovery by using the new method SCMT-SISSO that overcomes the challenge of data inconsistency between different sources. While many previous descriptors for the catalytic activity were proposed based on respective small datasets, we obtained the new 2D descriptor (d_B, n_B) based on 13 experimental datasets collected from different publications and the SCMT-SISSO. Great universality and predictive accuracy, and the bulk-surface correspondence, of this descriptor have been demonstrated. With this descriptor, hundreds of unreported candidate perovskites with activity greater than the benchmark catalyst Ba0.5Sr0.5Co0.8Fe0.2O3 were identified from a large chemical space. Our experimental validations on five candidates confirmed the three highly active new perovskite catalysts SrCo0.6Ni0.4O3, Rb0.1Sr0.9Co0.7Fe0.3O3, and Cs0.1Sr0.9Co0.4Fe0.6O3. △ Less

Submitted 17 January, 2023; originally announced January 2023.

Journal ref: J. Am. Chem. Soc. 145, 11457-11465 (2023)

Nanoscale Sn off-centering behind low thermal conductivity in SnSe thermoelectric

Authors: E. S. Bozin, H. Xie, A. M. M. Abeykoon, S. M. Everett, M. G. Tucker, M. G. Kanatzidis, S. J. L. Billinge

Abstract: The local atomic structure of SnSe was characterized across its orthorhmbic-to-orthorhombic structural phase transition using x-ray pair distribution function analysis. Substantial Sn off-centering distortions persist in the high symmetry high temperature phase, with symmetry different from that of ordered distortions below the transition. The analysis implies that the transition is neither order-… ▽ More The local atomic structure of SnSe was characterized across its orthorhmbic-to-orthorhombic structural phase transition using x-ray pair distribution function analysis. Substantial Sn off-centering distortions persist in the high symmetry high temperature phase, with symmetry different from that of ordered distortions below the transition. The analysis implies that the transition is neither order-disorder nor displacive, but rather a complex crossover where the character of coupling changes from 3D-like at low temperature to 2D-like at high temperature. Robust ferro-coupled SnSe intra-layer distortions suggest a ferroelectric-like instability as the driving force. Complex local Sn off-centering is integral to the ultra-low lattice thermal conductivity mechanism in SnSe. △ Less

Submitted 13 January, 2023; originally announced January 2023.

Influence of particle geometry on dispersion force

Authors: Yifei Liu, Heping Xie, Cunbao Li, Dong-Sheng Jeng, Bonan Zhang

Abstract: Dispersion forces (van der Waals force and Casimir force) originating from quantum fluctuations are crucial in the cohesion of microscale and nanoscale particles. In reality, these particles have a variety of irregular shapes that differ considerably from any idealized geometry. Previous experiments have demonstrated that dispersion forces strongly depend on the geometry. Because of the nonadditiv… ▽ More Dispersion forces (van der Waals force and Casimir force) originating from quantum fluctuations are crucial in the cohesion of microscale and nanoscale particles. In reality, these particles have a variety of irregular shapes that differ considerably from any idealized geometry. Previous experiments have demonstrated that dispersion forces strongly depend on the geometry. Because of the nonadditivity of these forces, commonly used numerical additive methods, such as the Hamaker and Derjaguin approximations, are not suitable for calculations with complex geometries. Moreover, experimental studies are difficult to identify the contributions of the dispersion force from the many forces that constitute the cohesion. Therefore, no general law about the influence of particle geometry on dispersion forces has been established. Thus, in this paper, the fluctuating surface current (FSC) technique, an exact scattering theory-based nonadditive algorithm, was used to study this influence. To characterize complex geometries, a data-adaptive spatial filtering method was introduced to perform scale decomposition, and descriptors at three observation levels (global, local, and surface) were used. Based on the advanced geometric analyses and accurate numerical calculations, the influence of multiscale surface fluctuations on dispersion forces was determined. Furthermore, a convenient formula for predicting the dispersion forces between particles with complex shapes from the exact Lifshitz solution was established via multistage corrections. △ Less

Submitted 18 October, 2022; originally announced October 2022.

Comments: 32 pages, 10 figures

A new alloy for Al-chalcogen system: AlSe surface alloy on Al (111)

Authors: En-Ze Shao, Kai Liu, Hao Xie, Kaiqi Geng, Keke Bai, Jinglan Qiu, Jing Wang, Wen-Xiao Wang, Juntao Song

Abstract: Metal chalcogenide is a promising material for studying novel underlying physical phenomena and nanoelectronics applications. Here, we systematically investigate the crystal structure and electronic properties of the AlSe surface alloy on Al (111) using scanning tunneling microscopy, angle-resolved photoelectron spectrometer, and first-principle calculations. We reveal that the AlSe surface alloy… ▽ More Metal chalcogenide is a promising material for studying novel underlying physical phenomena and nanoelectronics applications. Here, we systematically investigate the crystal structure and electronic properties of the AlSe surface alloy on Al (111) using scanning tunneling microscopy, angle-resolved photoelectron spectrometer, and first-principle calculations. We reveal that the AlSe surface alloy possesses a hexagonal closed-packed structure. The AlSe surface alloy comprises two atomic sublayers (Se sublayer and Al sublayer) with 1.16 A along the z direction. The dispersion shows two hole-like bands for AlSe surface alloy located at about -2.2 eV, far below the Fermi level, which is sharply different from other metal chalcogenide and binary alloys. These two bands mainly derive from the in-plane orbital of AlSe (px and py). These results provide implications for related Al-chalcogen interface. Meanwhile, AlSe alloy have an advantage of large-scale atomic flatness and a wide band gap near the Fermi level in serving as an interface for two-dimensional materials. △ Less

Submitted 17 October, 2022; originally announced October 2022.

Comments: 15 pages, 5 figures

Magnetization switching in polycrystalline Mn3Sn thin film induced by self-generated spin-polarized current

Authors: Hang Xie, Xin Chen, Qi Zhang, Zhiqiang Mu, Xinhai Zhang, Binghai Yan, Yihong Wu

Abstract: Electrical manipulation of spins is essential to design state-of-the-art spintronic devices and commonly relies on the spin current injected from a second heavy-metal material. The fact that chiral antiferromagnets produce spin current inspires us to explore the magnetization switching of chiral spins using self-generated spin torque. Here, we demonstrate the electric switching of noncollinear ant… ▽ More Electrical manipulation of spins is essential to design state-of-the-art spintronic devices and commonly relies on the spin current injected from a second heavy-metal material. The fact that chiral antiferromagnets produce spin current inspires us to explore the magnetization switching of chiral spins using self-generated spin torque. Here, we demonstrate the electric switching of noncollinear antiferromagnetic state in Mn3Sn by observing a crossover from conventional spin-orbit torque to the self-generated spin torque when increasing the MgO thickness in Ta/MgO/Mn3Sn polycrystalline films. The spin current injection from the Ta layer can be controlled and even blocked by varying the MgO thickness, but the switching sustains even at a large MgO thickness. Furthermore, the switching polarity reverses when the MgO thickness exceeds around 3 nm, which cannot be explained by the spin-orbit torque scenario due to spin current injection from the Ta layer. Evident current-induced switching is also observed in MgO/Mn3Sn and Ti/Mn3Sn bilayers, where external injection of spin Hall current to Mn3Sn is negligible. The inter-grain spin-transfer torque induced by spin-polarized current explains the experimental observations. Our findings provide an alternative pathway for electrical manipulation of non-collinear antiferromagnetic state without resorting to the conventional bilayer structure. △ Less

Submitted 14 September, 2022; originally announced September 2022.

Comments: 29 pages, 6 figures

Deep Variational Free Energy Approach to Dense Hydrogen

Authors: Hao Xie, Zi-Hang Li, Han Wang, Linfeng Zhang, Lei Wang

Abstract: We developed a deep generative model-based variational free energy approach to the equations of state of dense hydrogen. We employ a normalizing flow network to model the proton Boltzmann distribution and a fermionic neural network to model the electron wave function at given proton positions. By jointly optimizing the two neural networks we reached a comparable variational free energy to the prev… ▽ More We developed a deep generative model-based variational free energy approach to the equations of state of dense hydrogen. We employ a normalizing flow network to model the proton Boltzmann distribution and a fermionic neural network to model the electron wave function at given proton positions. By jointly optimizing the two neural networks we reached a comparable variational free energy to the previous coupled electron-ion Monte Carlo calculation. The predicted equation of state of dense hydrogen under planetary conditions is denser than the findings of ab initio molecular dynamics calculation and empirical chemical model. Moreover, direct access to the entropy and free energy of dense hydrogen opens new opportunities in planetary modeling and high-pressure physics research. △ Less

Submitted 25 September, 2023; v1 submitted 13 September, 2022; originally announced September 2022.

Comments: 7+5 pages, 3+4 figures, code: https://github.com/fermiflow/hydrogen

Journal ref: Phys. Rev. Lett. 131, 126501 (2023)

Observation of magnetism induced topological edge state in antiferromagnetic topological insulator MnBi4Te7

Authors: HaoKe Xu, Mingqiang Gu, Fucong Fei, YiSheng Gu, Dang Liu, QiaoYan Yu, ShaSha Xue, XuHui Ning, Bo Chen, Hangkai Xie, Zhen Zhu, Dandan Guan, Shiyong Wang, Yaoyi Li, Canhua Liu, Qihang Liu, Fengqi Song, Hao Zheng, Jinfeng Jia

Abstract: Breaking time reversal symmetry in a topological insulator may lead to quantum anomalous Hall effect and axion insulator phase. MnBi4Te7 is a recently discovered antiferromagnetic topological insulator with TN ~12.5 K, which is constituted of alternatively stacked magnetic layer (MnBi2Te4) and non-magnetic layer (Bi2Te3). By means of scanning tunneling spectroscopy, we clearly observe the electron… ▽ More Breaking time reversal symmetry in a topological insulator may lead to quantum anomalous Hall effect and axion insulator phase. MnBi4Te7 is a recently discovered antiferromagnetic topological insulator with TN ~12.5 K, which is constituted of alternatively stacked magnetic layer (MnBi2Te4) and non-magnetic layer (Bi2Te3). By means of scanning tunneling spectroscopy, we clearly observe the electronic state present at a step edge of a magnetic MnBi2Te4 layer but absent at non-magnetic Bi2Te3 layers at 4.5 K. Furthermore, we find that as the temperature rises above TN, the edge state vanishes, while the point defect induced state persists upon temperature increasing. These results confirm the observation of magnetism induced edge states. Our analysis based on an axion insulator theory reveals that the nontrivial topological nature of the observed edge state. △ Less

Submitted 16 July, 2022; originally announced July 2022.

A vector magnetometer based on a single spin-orbit torque anomalous Hall device

Authors: Xin Chen, Hang Xie, Haoxuan Shen, Yihong Wu

Abstract: In many applications, the ability to measure the vector information of a magnetic field with high spatial resolution and low cost is essential, but it is still a challenge for existing magnetometers composed of multiple sensors. Here, we report a single-device based vector magnetometer, which is enabled by spin-orbit torque, capable of measuring a vector magnetic field using the harmonic Hall resi… ▽ More In many applications, the ability to measure the vector information of a magnetic field with high spatial resolution and low cost is essential, but it is still a challenge for existing magnetometers composed of multiple sensors. Here, we report a single-device based vector magnetometer, which is enabled by spin-orbit torque, capable of measuring a vector magnetic field using the harmonic Hall resistances of a superparamagnetic ferromagnet (FM)/heavy metal (HM) bilayer. Under an ac driving current, the first and second harmonic Hall resistances of the FM/HM bilayer show a linear relationship with the vertical and longitudinal component (along the current direction) of the magnetic field, respectively. By employing a L-shaped Hall device with two orthogonal arms, we can measure all the three field components simultaneously, so as to detect both the amplitude and direction of magnetic field in a three-dimensional space. As proof of concepts, we demonstrate both angular position sensing on the three coordinate planes and vector mapping of magnetic field generated by a permanent magnet, both of which are in good agreement with the simulation results. Crosstalk between vertical and longitudinal field components at large field is discussed using theoretical models. △ Less

Submitted 15 June, 2022; originally announced June 2022.

Comments: 24 pages, 8 figures

Journal ref: Phys. Rev. Applied 18, 024010 (2022)

Evidence of Noncollinear Spin Texture in Magnetic Moiré Superlattices

Authors: Hongchao Xie, Xiangpeng Luo, Zhipeng Ye, Gaihua Ye, Haiwen Ge, Shaohua Yan, Yang Fu, Shangjie Tian, Hechang Lei, Kai Sun, Rui He, Liuyan Zhao

Abstract: Moiré magnetism, parallel with moiré electronics that has led to novel correlated and topological electronic states, emerges as a new venue to design and control exotic magnetic phases in twisted magnetic two-dimensional(2D) crystals. Here, we report direct evidence of noncollinear spin texture in 2D twisted double bilayer (tDB) magnet chromium triiodide (CrI$_3$). Using magneto-optical spectrosco… ▽ More Moiré magnetism, parallel with moiré electronics that has led to novel correlated and topological electronic states, emerges as a new venue to design and control exotic magnetic phases in twisted magnetic two-dimensional(2D) crystals. Here, we report direct evidence of noncollinear spin texture in 2D twisted double bilayer (tDB) magnet chromium triiodide (CrI$_3$). Using magneto-optical spectroscopy in tDB CrI$_3$, we revealed the presence of a net magnetization, unexpected from the composing antiferromagnetic bilayers with compensated magnetizations, and the emergence of noncollinear spins, originated from the moiré exchange coupling-induced spin frustrations. Exploring the twist angle dependence, we demonstrated that both features are present in tDB CrI$_3$ with twist angles from 0.5$^o$ to 5$^o$, but are most prominent in the 1.1$^o$ tDB CrI$_3$. Focusing on the temperature dependence of the 1.1$^o$ tDB CrI$_3$, we resolved the dramatic suppression in the net magnetization onset temperature and the significant softening of noncollinear spins, as a result of the moiré induced frustration. Our results demonstrate the power of moiré superlattices in introducing novel magnetic phenomena that are absent in natural 2D magnets. △ Less

Submitted 4 April, 2022; originally announced April 2022.

A three-stage magnetic phase transition revealed in ultrahigh-quality van der Waals magnet CrSBr

Authors: Wenhao Liu, Xiaoyu Guo, Jonathan Schwartz, Hongchao Xie, Nikhil Dhale, Suk Hyun Sung, Aswin L. N. Kondusamy, Xiqu Wang, Haonan Zhao, Diana Berman, Robert Hovden, Liuyan Zhao, Bing Lv

Abstract: van der Waals (vdW) magnets are receiving ever-growing attention nowadays due to their significance in both fundamental research on low-dimensional magnetism and potential applications in spintronic devices. High crystalline quality of vdW magnets is key for maintaining intrinsic magnetic and electronic properties, especially when exfoliated down to the 2D limit. Here, ultrahigh-quality air-stable… ▽ More van der Waals (vdW) magnets are receiving ever-growing attention nowadays due to their significance in both fundamental research on low-dimensional magnetism and potential applications in spintronic devices. High crystalline quality of vdW magnets is key for maintaining intrinsic magnetic and electronic properties, especially when exfoliated down to the 2D limit. Here, ultrahigh-quality air-stable vdW CrSBr crystals are synthesized using the direct vapor-solid synthesis method. The high single crystallinity and spatial homogeneity have been thoroughly evidenced at length scales from sub-mm to atomic resolution by X-ray diffraction, second harmonic generation, and scanning transmission electron microscopy. More importantly, specific heat measurements of these ultrahigh quality CrSBr crystals show three thermodynamic anomalies at 185K, 156K, and 132K, revealing a stage-by-stage development of the magnetic order upon cooling, which is also corroborated with the magnetization and transport results. Our ultrahigh-quality CrSBr can further be exfoliated down to monolayers and bilayers easily, paving the way to integrate them into heterostructures for spintronic and magneto-optoelectronic applications. △ Less

Submitted 17 March, 2022; originally announced March 2022.

Comments: 13 pages, 5 figures

Torsional Periodic Lattice Distortions and Diffraction of Twisted 2D Materials

Authors: Suk Hyun Sung, Yin Min Goh, Hyobin Yoo, Rebecca Engelke, Hongchao Xie, Kuan Zhang, Zidong Li, Andrew Ye, Parag B. Deotare, Ellad B. Tadmor, Andrew J. Mannix, Jiwoong Park, Liuyan Zhao, Philip Kim, Robert Hovden

Abstract: Twisted 2D materials form complex moiré structures that spontaneously reduce symmetry through picoscale deformation within a mesoscale lattice. We show twisted 2D materials contain a torsional displacement field comprised of three transverse periodic lattice distortions (PLD). The torsional PLD amplitude provides a single order parameter that concisely describes the structural complexity of twiste… ▽ More Twisted 2D materials form complex moiré structures that spontaneously reduce symmetry through picoscale deformation within a mesoscale lattice. We show twisted 2D materials contain a torsional displacement field comprised of three transverse periodic lattice distortions (PLD). The torsional PLD amplitude provides a single order parameter that concisely describes the structural complexity of twisted bilayer moirés. Moreover, the structure and amplitude of a torsional periodic lattice distortion is quantifiable using rudimentary electron diffraction methods sensitive to reciprocal space. In twisted bilayer graphene, the torsional PLD begins to form at angles below 3.89° and the amplitude reaches 8 pm around the magic angle of 1.1°. At extremely low twist angles (e.g. below 0.25°) the amplitude increases and additional PLD harmonics arise to expand Bernal stacked domains separated by well defined solitonic boundaries. The torsional distortion field in twisted bilayer graphene is analytically described and has an upper bound of 22.6 pm. Similar torsional distortions are observed in twisted WS$_2$, CrI$_3$, and WSe$_2$ / MoSe$_2$. △ Less

Submitted 28 December, 2022; v1 submitted 12 March, 2022; originally announced March 2022.

Journal ref: Nature Communications 13, 7826 (2022)

$m^\ast$ of two-dimensional electron gas: a neural canonical transformation study

Authors: Hao Xie, Linfeng Zhang, Lei Wang

Abstract: The quasiparticle effective mass $m^\ast$ of interacting electrons is a fundamental quantity in the Fermi liquid theory. However, the precise value of the effective mass of uniform electron gas is still elusive after decades of research. The newly developed neural canonical transformation approach [Xie et al., J. Mach. Learn. 1, (2022)] offers a principled way to extract the effective mass of elec… ▽ More The quasiparticle effective mass $m^\ast$ of interacting electrons is a fundamental quantity in the Fermi liquid theory. However, the precise value of the effective mass of uniform electron gas is still elusive after decades of research. The newly developed neural canonical transformation approach [Xie et al., J. Mach. Learn. 1, (2022)] offers a principled way to extract the effective mass of electron gas by directly calculating the thermal entropy at low temperature. The approach models a variational many-electron density matrix using two generative neural networks: an autoregressive model for momentum occupation and a normalizing flow for electron coordinates. Our calculation reveals a suppression of effective mass in the two-dimensional spin-polarized electron gas, which is more pronounced than previous reports in the low-density strong-coupling region. This prediction calls for verification in two-dimensional electron gas experiments. △ Less

Submitted 15 June, 2023; v1 submitted 9 January, 2022; originally announced January 2022.

Comments: 21 pages, 6 figures, code: https://github.com/fermiflow/CoulombGas

Journal ref: SciPost Phys. 14, 154 (2023)

Exploring Native Atomic Defects in NiTe2

Authors: Wen-Xiao Wang, Kaihui Li, Xiaoshan Dong, Hao Xie, Jinglan Qiu, Chunqiang Xu, Kai Liu, Juntao Song, Yi-Wen Wei, Ke-Ke Bai, Xiaofeng Xu, Ying Liu

Abstract: Nickel ditelluride (NiTe2), a new discovered type-II Dirac semimetal whose Dirac node lies close to its Fermi level, is expected to exhibit exotic phenomena including Lifshitz transition and superconductivity. As we know, defects are inevitable for transition metal dichalcogenides and have significant impacts on the optical and electronic properties. However, the systematic study of defects in NiT… ▽ More Nickel ditelluride (NiTe2), a new discovered type-II Dirac semimetal whose Dirac node lies close to its Fermi level, is expected to exhibit exotic phenomena including Lifshitz transition and superconductivity. As we know, defects are inevitable for transition metal dichalcogenides and have significant impacts on the optical and electronic properties. However, the systematic study of defects in NiTe2 is still lack. Here, by using high-resolution scanning tunneling microscopy combined with first-principles calculations, the point defects including the vacancy, intercalation and antisite defects in NiTe2 are systematically investigated. We identified five main types native defects and revealed that the growth condition could affect the type of native defects. By controlling the ratio of ingredient during synthesis, the types of point defects are expected to be manipulated, especially antisite defects. Additionally, we find native defects could slightly dope the topological surface states. Our results provide a facile way to manipulate defects for future optimizing the electronic properties of NiTe2 and other related materials. △ Less

Submitted 2 January, 2022; originally announced January 2022.

Comments: 17 pages, 4 figures, 1 table