Showing 1–50 of 85 results for author: Xiao, B

Noise-enhanced temporal boundary states in non-Hermitian systems

Authors: Jielong Zhang, Bozheng Xue, Xianfeng Man, Baizhan Xia

Abstract: Time-periodic modulation introduces a synthetic degree of freedom to manipulate topological phases. This synthetic dimension can trigger a phase transition that localizes a boundary state at the temporal interface. Noise is widely deemed a fundamental threat to topological protection, universally anticipated to weaken or even destroy topological states. Here, we introduce periodically repeated tem… ▽ More Time-periodic modulation introduces a synthetic degree of freedom to manipulate topological phases. This synthetic dimension can trigger a phase transition that localizes a boundary state at the temporal interface. Noise is widely deemed a fundamental threat to topological protection, universally anticipated to weaken or even destroy topological states. Here, we introduce periodically repeated temporal noise and fully random temporal noise into a 2D periodically driven non-Hermitian system. Paradoxically, under ensemble averaging, such temporal noise drives an exponential enhancement of the response intensity at the temporal interface. An averaged superoperator analysis shows that the noiseless band structure is preserved under both types of noise. Yet the noise increases the growth rate of growing modes while suppressing the decay rate of decaying ones. Finally, we experimentally realize the noise-enhanced temporal boundary state in a robotic metamaterial network. This finding establishes temporal noise as a constructive ingredient, enabling the unambiguous emergence of topological states and conferring exceptional robustness upon topological devices. △ Less

Submitted 2 August, 2026; originally announced August 2026.

Layer-dependent quantum transport in KV2Se2O-based altermagnetic tunnel junctions

Authors: Yue Zhao, Bin Xiao, Jiawei Liu, Hui Zeng, Jun Zhao

Abstract: Magnetic tunnel junction (MTJ) is the key component to enable information access and increasing number of MTJs is integrated to develop high-density spintronic devices. However, continuous miniaturization of the conventional MTJs is hindered by stray magnetic fields. Altermagnets, combining the advantages of both ferromagnets and antiferromagnets, provide a promising alternative to fabricate versa… ▽ More Magnetic tunnel junction (MTJ) is the key component to enable information access and increasing number of MTJs is integrated to develop high-density spintronic devices. However, continuous miniaturization of the conventional MTJs is hindered by stray magnetic fields. Altermagnets, combining the advantages of both ferromagnets and antiferromagnets, provide a promising alternative to fabricate versatile MTJs with exotic properties, such as giant spin splitting, high intrinsic frequency, and absence of stray fields. Inspired by the altermagnetic metal candidate KV2Se2O reported recently, we design an altermagnetic tunnel junction (AMTJ) based on KV2Se2O/SrTiO3/KV2Se2O. Using density functional theory combined with non-equilibrium Green's function, we investigate the layer-dependent quantum transport properties and the tunneling magnetoresistance (TMR) of such AMTJ device. Our calculated results reveal that the transmission of the AMTJ device exhibits a pronounced oscillation behavior dependent on the number of layers of the SrTiO3 semiconductor, which is attributed to the interface configuration determined by parity of the layer number. In odd-layer devices, the electron-rich O-Se interface exhibits a smooth effective potential and enables transverse momentum (k||) transport channels, leading to enhanced transmission. In contrast, in even-layer devices, the Ti-Se interface presents a steeper effective potential, impeding quantum transport through transverse momentum (k||) channels. A giant TMR of 4.6*10^7% is predicted to be realized by using a 4-layer SrTiO3. Our findings not only provide physical understanding relevant to the quantum transport in AMTJs, but also unveil that the barrier interface engineering is a strategy to tune the magnetoelectric performance. △ Less

Submitted 16 April, 2026; originally announced April 2026.

Simultaneous measurement of pressure-dependent bulk and interfacial thermal properties in thermal interface materials using square-pulsed source thermoreflectance

Authors: Tao Chen, Bingjia Xiao, Xin Qian, Puqing Jiang

Abstract: Thermal interface materials (TIMs) critically regulate heat dissipation from electronic chips to heat spreaders, yet their thermal conductivity (k), volumetric heat capacity (C), and interfacial thermal resistance (ITR) evolve with mechanical pressure and cannot be determined simultaneously using existing steady-state or transient techniques. As a result, the coupled roles of bulk compaction and i… ▽ More Thermal interface materials (TIMs) critically regulate heat dissipation from electronic chips to heat spreaders, yet their thermal conductivity (k), volumetric heat capacity (C), and interfacial thermal resistance (ITR) evolve with mechanical pressure and cannot be determined simultaneously using existing steady-state or transient techniques. As a result, the coupled roles of bulk compaction and interfacial contact in governing heat transport in TIM assemblies remain poorly resolved. Here, we present a square-pulsed source (SPS) thermoreflectance method that enables simultaneous determination of k, C, and ITR in TIM stacks under controlled mechanical loading. By spanning square-wave modulation frequencies from 1 Hz to 10 MHz, SPS probes a broad range of thermal penetration depths, enabling distinction between heat diffusion in the TIM bulk and interfacial heat transfer at the Al/TIM contact. Measurements on a thermally conductive gel, a thermal pad, and a high-vacuum grease during compression-unloading cycles reveal distinct pressure-dependent thermal transport mechanisms. The gel and pad exhibit increases in k and C, reduced ITR, and pronounced hysteresis, indicating coupled bulk densification and persistent interfacial conformity during loading cycles. In contrast, the grease shows nearly pressure-independent bulk properties but a strong pressure dependence of ITR, consistent with an interface-dominated response. These results resolve the long-standing challenge of simultaneously quantifying bulk and interfacial thermal transport in mechanically loaded TIM assemblies, enabling experimentally constrained thermal management and reliability analysis in electronic packaging. △ Less

Submitted 29 March, 2026; v1 submitted 23 March, 2026; originally announced March 2026.

Stoichiometric FeTe is a Superconductor

Authors: Zi-Jie Yan, Zihao Wang, Bing Xia, Stephen Paolini, Ying-Ting Chan, Nikalabh Dihingia, Hongtao Rong, Pu Xiao, Kalana D. Halanayake, Jiatao Song, Veer Gowda, Danielle Reifsnyder Hickey, Weida Wu, Jiabin Yu, Peter J. Hirschfeld, Cui-Zu Chang

Abstract: Iron-based superconductors are a fascinating family of materials in which multiple electronic bands and strong antiferromagnetic (AFM) correlations are key ingredients for competing ground states, including antiferromagnetism, electronic nematicity, and unconventional superconductivity. FeTe, unlike its superconducting isostructural counterpart FeSe, has long been regarded as an AFM metal sans sup… ▽ More Iron-based superconductors are a fascinating family of materials in which multiple electronic bands and strong antiferromagnetic (AFM) correlations are key ingredients for competing ground states, including antiferromagnetism, electronic nematicity, and unconventional superconductivity. FeTe, unlike its superconducting isostructural counterpart FeSe, has long been regarded as an AFM metal sans superconductivity. In this work, we employ molecular beam epitaxy to grow FeTe films and perform post-growth annealing under a Te flux. By performing spin-polarized scanning tunneling microscopy and spectroscopy, we demonstrate that the AFM order in as-grown FeTe films is induced by interstitial Fe atoms that disrupt the ideal 1:1 stoichiometry. Remarkably, the removal of these interstitial Fe atoms through Te annealing yields stoichiometric FeTe films that show no AFM order and instead exhibit robust superconductivity with a critical temperature of ~13.5K. This superconducting state is further confirmed by the observation of Cooper pair tunneling, zero electrical resistance, and the Meissner effect. Therefore, our results demonstrate that stoichiometric FeTe is inherently a superconductor, overturning a long-held view that it is an AFM metal. This work clarifies the origin of superconductivity in FeTe-based heterostructures and demonstrates the importance of stoichiometry control in understanding the competition between AFM and superconductivity in iron-based superconductors. △ Less

Submitted 17 March, 2026; originally announced March 2026.

Comments: 39 pages, 4 main figures, and 10 extended data figures. Accepted by Nature. Comments are very welcome

Journal ref: Nature 652, 342(2026)

Kinetic obstruction to pairing in the doped Kitaev-Heisenberg ladder

Authors: Bradraj Pandey, Bo Xiao, Satoshi Okamoto, Gonzalo Alvarez, Gábor B. Halász, Elbio Dagotto, Pontus Laurell

Abstract: We investigate the hole-doped Kitaev-Heisenberg ($t$-$J$-$K$) model on a two-leg ladder geometry using the density-matrix renormalization group (DMRG). We first consider the behavior of the antiferromagnetic Kitaev (AFK) spin-liquid phase as a function of hopping strength $t$ and doping level. This reveals intriguing pairing tendencies only for $\frac{t}{K} \lesssim 0.65$, consistent with prior re… ▽ More We investigate the hole-doped Kitaev-Heisenberg ($t$-$J$-$K$) model on a two-leg ladder geometry using the density-matrix renormalization group (DMRG). We first consider the behavior of the antiferromagnetic Kitaev (AFK) spin-liquid phase as a function of hopping strength $t$ and doping level. This reveals intriguing pairing tendencies only for $\frac{t}{K} \lesssim 0.65$, consistent with prior results on three-leg ladders, and firmly supports the emerging picture that the physics of doped Kitaev spin liquids strongly depends on the kinetic energy of the doped holes. Analysis of one- and two-hole doping uncovers close links between the spatial profiles of the plaquette operator and the charge density. We construct a doping-dependent phase diagram for antiferromagnetic Heisenberg interactions and intermediate hopping $t=1$. Upon doping, the rung-singlet region develops dominant superconducting correlations. Charge-density-wave correlations dominate at weak doping near the transition to the stripy phase. Spin-density wave-like behavior is found in the AFK and ferromagnetic Kitaev limits, and in the stripy phase. △ Less

Submitted 12 March, 2026; originally announced March 2026.

Comments: Main text: 11 pages, 6 figures. Supplemental material: 8 pages, 8 figures

Moire Engineering of Cooper-Pair Density Modulation States

Authors: Zihao Wang, Bing Xia, Stephen Paolini, Zi-Jie Yan, Pu Xiao, Jiatao Song, Veer Gowda, Hongtao Rong, Di Xiao, Xiaodong Xu, Weida Wu, Ziqiang Wang, Cui-Zu Chang

Abstract: Cooper-pair density modulation (CPDM) states are superconducting phases in which the order parameter varies periodically in real space without breaking translational symmetry. Recently, moire superlattices in layered materials have emerged as powerful platforms for engineering charge density with tunable lattice symmetry, offering a new route to creating and controlling CPDM states. In this work,… ▽ More Cooper-pair density modulation (CPDM) states are superconducting phases in which the order parameter varies periodically in real space without breaking translational symmetry. Recently, moire superlattices in layered materials have emerged as powerful platforms for engineering charge density with tunable lattice symmetry, offering a new route to creating and controlling CPDM states. In this work, we demonstrate moire-induced CPDM states in a bilayer heterostructure formed by epitaxially stacking one quintuple layer (1 QL) of topological insulator Sb2Te3 on a six-unit-cell (6 UC) antiferromagnetic FeTe layer. Scanning tunneling microscopy and spectroscopy (STM/S) measurements reveal a moiré superlattice formed between the hexagonal Te lattice of Sb2Te3 and the square Te lattice of FeTe, which spatially modulates the two superconducting gaps of the 1 QL Sb2Te3/6 UC FeTe bilayer. Our Josephson STM/S measurements provide direct real-space imaging of the CPDM states with a wavelength corresponding to the periodicity of the moire superlattice. By substituting Sb2Te3 with Bi2Te3, we achieve control over both the periodicity and magnitude of the CPDM states. Our work demonstrates an epitaxial strategy for synthesizing moire superlattices from materials with different crystal symmetries and reveals a new mechanism for engineering CPDM states in designer bilayer heterostructures. △ Less

Submitted 26 February, 2026; originally announced February 2026.

Comments: 36 pages, 4 main figures, and 10 Extended Data figures. Accepted by Nature. Comments are very welcome

Journal ref: Nature 652, 335 (2026)

Interplay of Quantum Size Effect and Tensile Strain on Surface Morphology of Sn(100) Islands

Authors: Bing Xia, Xiaoyin Li, Hongyuan Chen, Bo Yang, Jie Cai, Stephen Paolini, Zihao Wang, Zi-Jie Yan, Hao Yang, Xiaoxue Liu, Liang Liu, Dandan Guan, Shiyong Wang, Yaoyi Li, Canhua Liu, Hao Zheng, Cui-Zu Chang, Feng Liu, Jinfeng Jia

Abstract: The quantum size effect (QSE) and strain effect are two key factors influencing the surface morphology of thin films, which can increase film surface roughness through QSE-induced thickness oscillation and strain-induced island formation, respectively. Surface roughness usually manifests in the early stages of film growth and diminishes beyond a critical thickness. In this work, we employ molecula… ▽ More The quantum size effect (QSE) and strain effect are two key factors influencing the surface morphology of thin films, which can increase film surface roughness through QSE-induced thickness oscillation and strain-induced island formation, respectively. Surface roughness usually manifests in the early stages of film growth and diminishes beyond a critical thickness. In this work, we employ molecular beam epitaxy (MBE) to grow Sn(100) islands with varying thickness N on bilayer graphene-terminated 6H-SiC(0001) substrates. Scanning tunneling microscopy and spectroscopy measurements reveal an inverse surface roughness effect that highlights the interplay of QSE and misfit strain in shaping the surface morphology of Sn(100) islands. For N =< 10, the islands exhibit flat surfaces, while for N >= 26, the island surfaces become corrugated and patterned. For the intermediate range, i.e., 12 =< N =<24, both flat and patterned surfaces coexist, with the percentage coverage of the patterned surface oscillating as a function of N. By performing density functional theory calculations, we demonstrate that the unusual surface pattern evolution in our MBE-grown Sn(100) islands is a result of the interplay between QSE-induced surface roughing and tensile strain-induced smoothening effect. △ Less

Submitted 9 February, 2026; originally announced February 2026.

Comments: 23 pages, 5 figures, comments are very much welcome

Journal ref: ACS Nano 20, 8289 (2026)

Investigating a Quantum-Inspired Method for Quantum Dynamics

Authors: Bo Xiao, Benedikt Kloss, E. Miles Stoudenmire

Abstract: Building on recent advances in quantum algorithms which measure and reuse qubits and in efficient classical simulation leveraging projective measurements, we extend these frameworks to real-time dynamics of quantum many-body systems undergoing discrete-time and continuous-time Hamiltonian evolution, and find improvements that significantly reduce sampling overhead. The approach exploits causal lig… ▽ More Building on recent advances in quantum algorithms which measure and reuse qubits and in efficient classical simulation leveraging projective measurements, we extend these frameworks to real-time dynamics of quantum many-body systems undergoing discrete-time and continuous-time Hamiltonian evolution, and find improvements that significantly reduce sampling overhead. The approach exploits causal light-cone structure by interleaving time and space evolution and applying projective measurements as soon as local subsystems reach the target physical time, suppressing entanglement growth. Comparing to time-evolving block decimation, the method reaches longer times per sample for the same resources. We also gain the ability to study dynamics of entanglement that would be occurring on quantum hardware when following similar protocols, such as the holographic quantum dynamics simulation framework. We show how to efficiently obtain local observables as well as equal-time and time-dependent correlation functions. Our findings show how optimizations for quantum hardware can benefit classical tensor network simulations and how such classical methods can yield insights into the utility of quantum simulations. △ Less

Submitted 4 December, 2025; originally announced December 2025.

Comments: 14+4 pages (main + appendix), 10+6 figures

Orbital Hybridization-Induced Ising-Type Superconductivity in a Confined Gallium Layer

Authors: Hemian Yi, Yunzhe Liu, Chengye Dong, Yiheng Yang, Zi-Jie Yan, Zihao Wang, Lingjie Zhou, Dingsong Wu, Houke Chen, Stephen Paolini, Bing Xia, Bomin Zhang, Xiaoda Liu, Hongtao Rong, Annie G. Wang, Saswata Mandal, Kaijie Yang, Benjamin N. Katz, Lunhui Hu, Jieyi Liu, Tien-Lin Lee, Vincent H. Crespi, Yuanxi Wang, Yulin Chen, Joshua A. Robinson , et al. (2 additional authors not shown)

Abstract: In low-dimensional superconductors, the interplay between quantum confinement and interfacial hybridization effects can reshape Cooper pair wavefunctions and induce novel forms of unconventional superconductivity. In this work, we employ a plasma-free, carbon buffer layer-assisted confinement epitaxy method to synthesize trilayer gallium (Ga) sandwiched between a graphene layer and a 6H-SiC(0001)… ▽ More In low-dimensional superconductors, the interplay between quantum confinement and interfacial hybridization effects can reshape Cooper pair wavefunctions and induce novel forms of unconventional superconductivity. In this work, we employ a plasma-free, carbon buffer layer-assisted confinement epitaxy method to synthesize trilayer gallium (Ga) sandwiched between a graphene layer and a 6H-SiC(0001) substrate, forming an air-stable graphene/trilayer Ga/SiC heterostructure. In this confined light-element Ga layer, we demonstrate interfacial Ising-type superconductivity driven by atomic orbital hybridization between the Ga layer and the SiC substrate. Electrical transport measurements reveal that the in-plane upper critical magnetic field u0Hc2,|| reaches ~21.98T at T=400 mK, approximately 3.38 times the Pauli paramagnetic limit (~6.51T). Angle-resolved photoemission spectroscopy (ARPES) measurements combined with theoretical calculations confirm the presence of split Fermi surfaces with Ising-type spin textures at the K and K' valleys of the confined Ga layer strongly hybridized with SiC. Moreover, by incorporating finite relaxation time induced by impurity scattering into an Ising-type superconductivity model, we reproduce the entire temperature-dependent u0Hc2,|| phase diagram. This work establishes a new strategy to realize unconventional pairing wavefunctions by combining quantum confinement and interfacial hybridization effects in superconducting thin films. It also opens new avenues for designing scalable superconducting quantum electronic and spintronic devices through interfacial engineering. △ Less

Submitted 6 September, 2025; originally announced September 2025.

Comments: 40 pages, 4 figures and 10 extended data figures, comments are welcome

Journal ref: Nature Mater. (2026)

Hybrid Particle Swarm Optimization for Fast and Reliable Parameter Extraction in Thermoreflectance

Authors: Bingjia Xiao, Tao Chen, Wenbin Zhang, Xin Qian, Puqing Jiang

Abstract: Frequency-domain thermoreflectance (FDTR) is a widely used technique for characterizing thermal properties of multilayer thin films. However, extracting multiple parameters from FDTR measurements presents a nonlinear inverse problem due to its high dimensionality and multimodal, non-convex solution space. This study evaluates four popular global optimization algorithms: Genetic Algorithm (GA), Qua… ▽ More Frequency-domain thermoreflectance (FDTR) is a widely used technique for characterizing thermal properties of multilayer thin films. However, extracting multiple parameters from FDTR measurements presents a nonlinear inverse problem due to its high dimensionality and multimodal, non-convex solution space. This study evaluates four popular global optimization algorithms: Genetic Algorithm (GA), Quantum Genetic Algorithm (QGA), Particle Swarm Optimization (PSO), and Fireworks Algorithm (FWA), for extracting parameters from FDTR measurements of a GaN/Si heterostructure. However, none achieve reliable convergence within 60 seconds. To improve convergence speed and accuracy, we propose an AI-driven hybrid optimization framework that combines each global algorithm with a Quasi-Newton local refinement method, resulting in four hybrid variants: HGA, HQGA, HPSO, and HFWA. Among these, HPSO outperforms all other methods, with 80% of trials reaching the target fitness value within 60 seconds, showing greater robustness and a lower risk of premature convergence. In contrast, only 30% of HGA and HQGA trials and 20% of HFWA trials achieve this threshold. We then evaluate the worst-case performance across 100 independent trials for each algorithm when the time is extended to 1000 seconds. Only HPSO, PSO, and HGA consistently reach the target accuracy, with HPSO converging five times faster than the others. HPSO provides a general-purpose solution for inverse problems in thermal metrology and can be readily extended to other model-fitting techniques. △ Less

Submitted 29 July, 2025; originally announced July 2025.

Comments: 28 pages, 8 figures

Journal ref: International Journal of Heat and Mass Transfer 256, 128109 (2026)

From weakly interacting spinons to tightly bound triplons in the frustrated quantum spin-Peierls chain

Authors: Pyeongjae Park, Bo Xiao, Karolina Górnicka, Andrew F. May, Jiaqiang Yan, Ryoichi Kajimoto, Mitsutaka Nakamura, Matthew B. Stone, Gábor B. Halász, Andrew D. Christianson

Abstract: Fractionalized quasiparticles and their confinement into emergent bound states lie at the heart of modern quantum magnetism. While the evolution into magnonic bound states has been well characterized, experimental insight into the analogous transition to triplons remains limited. Here, using high-resolution neutron spectroscopy and state-of-the-art spin dynamics simulations, we uncover the transfo… ▽ More Fractionalized quasiparticles and their confinement into emergent bound states lie at the heart of modern quantum magnetism. While the evolution into magnonic bound states has been well characterized, experimental insight into the analogous transition to triplons remains limited. Here, using high-resolution neutron spectroscopy and state-of-the-art spin dynamics simulations, we uncover the transformation from weakly interacting spinons to tightly bound triplons in the spin-Peierls compound CuGeO3. Quantitative comparisons between the measured spectra and tensor network simulations reveal substantial next-nearest-neighbor frustration and weak external dimerization, placing the system deep within the spontaneously dimerized regime and near the exactly solvable Majumdar-Ghosh point. We further show an energy- and temperature-dependent evolution between two contrasting quasiparticle regimes: deconfined spinons with markedly suppressed interactions by frustration, and coherent triplonic bound states with no observable spinon degrees of freedom. Remarkably, triplon character persists into the two-particle regime, forming a structured two-triplon continuum with a spectral feature associated with a van Hove singularity at its lower boundary. These findings challenge the conventional view that robust triplons require strong external dimerization and demonstrate how the interplay between frustration and dimerization can reshape fractionalization and confinement. △ Less

Submitted 6 August, 2025; v1 submitted 25 July, 2025; originally announced July 2025.

Comments: 13 pages, 4 figures

Robust Chiral Edge Dynamics of a Kitaev Honeycomb on a Trapped Ion Processor

Authors: Ammar Ali, Joe Gibbs, Keerthi Kumaran, Varadharajan Muruganandam, Bo Xiao, Paul Kairys, Gábor Halász, Arnab Banerjee, Phillip C. Lotshaw

Abstract: Kitaev's honeycomb model is a paradigmatic exactly solvable system hosting a quantum spin liquid with non-Abelian anyons and topologically protected edge modes, offering a platform for fault-tolerant quantum computation. However, real candidate Kitaev materials invariably include complex secondary interactions that obscure the realization of spin-liquid behavior and demand novel quantum computatio… ▽ More Kitaev's honeycomb model is a paradigmatic exactly solvable system hosting a quantum spin liquid with non-Abelian anyons and topologically protected edge modes, offering a platform for fault-tolerant quantum computation. However, real candidate Kitaev materials invariably include complex secondary interactions that obscure the realization of spin-liquid behavior and demand novel quantum computational approaches for efficient simulation. Here we report quantum simulations of a 22-site Kitaev honeycomb lattice on a trapped-ion quantum processor, without and with non-integrable Heisenberg interactions that are present in real materials. We develop efficient quantum circuits for ground-state preparation, achieving high accuracy with energy errors equivalent to an effective temperature of 0.2 (in units of the Kitaev interactions), consistent with the experimentally relevant spin-liquid regime. Starting from these states, we apply controlled perturbations and measure time-dependent spin correlations along the system's edge. In the non-Abelian phase, we observe chiral edge dynamics consistent with a non-zero Chern number, a hallmark of topological order, which vanishes upon transition to the Abelian toric code phase. Extending to the non-integrable Kitaev-Heisenberg model, we find that weak Heisenberg interactions preserve chiral edge dynamics, while stronger couplings suppress them, signaling the breakdown of topological protection. Our work demonstrates a viable route for probing dynamical signatures of topological order in quantum spin liquids using programmable quantum hardware, opening new pathways for quantum simulation of strongly correlated materials. △ Less

Submitted 25 November, 2025; v1 submitted 11 July, 2025; originally announced July 2025.

Comments: Added temperature analysis. 7+10 pages, 4+6 figures

Robustness of Vacancy-Bound Non-Abelian Anyons in the Kitaev Model in a Magnetic Field

Authors: Bo Xiao, Gonzalo Alvarez, Gábor B. Halász

Abstract: Non-Abelian anyons in quantum spin liquids (QSLs) provide a promising route to fault-tolerant topological quantum computation. In the exactly solvable Kitaev honeycomb model, such anyons of the QSL state can be bound to nonmagnetic spin vacancies and endowed with non-Abelian statistics by an infinitesimal magnetic field. Here, we investigate how this approach for stabilizing non-Abelian anyons ext… ▽ More Non-Abelian anyons in quantum spin liquids (QSLs) provide a promising route to fault-tolerant topological quantum computation. In the exactly solvable Kitaev honeycomb model, such anyons of the QSL state can be bound to nonmagnetic spin vacancies and endowed with non-Abelian statistics by an infinitesimal magnetic field. Here, we investigate how this approach for stabilizing non-Abelian anyons extends to a finite magnetic field represented by a proper Zeeman term. Through large-scale density-matrix renormalization group (DMRG) simulations, we compute the vacancy-anyon binding energy as a function of magnetic field for both the ferromagnetic (FM) and antiferromagnetic (AFM) Kitaev models. We find that anyon binding remains robust within the entire QSL phase for the FM Kitaev model but breaks down already inside this phase for the AFM Kitaev model. To compute a binding energy several orders of magnitude below the magnetic energy scale, we introduce both a refined definition and an extrapolation scheme based on carefully tailored perturbations. △ Less

Submitted 6 October, 2025; v1 submitted 10 March, 2025; originally announced March 2025.

Comments: Published version: 8 pages, 6 figures

Journal ref: Phys. Rev. Lett. 135, 096603 (2025)

Lattice Vibration, Raman Modes and Room-Temperature Spin-Phonon Coupling in Intrinsic 2D van der Waals Ferromagnetic Fe3GaTe2

Authors: Gaojie Zhang, Hao Wu, Li Yang, Wen Jin, Bichen Xiao, Wenfeng Zhang, Haixin Chang

Abstract: Two-dimensional (2D) van der Waals (vdW) magnets with spin-phonon coupling are crucial for next-generation spintronics. Among them, Fe3GaTe2 has attracted widespread attention due to above-room-temperature intrinsic ferromagnetism and large perpendicular magnetic anisotropy. However, the lattice vibrations and the interplay between ferromagnetism and lattice vibrations in Fe3GaTe2 are still unexpl… ▽ More Two-dimensional (2D) van der Waals (vdW) magnets with spin-phonon coupling are crucial for next-generation spintronics. Among them, Fe3GaTe2 has attracted widespread attention due to above-room-temperature intrinsic ferromagnetism and large perpendicular magnetic anisotropy. However, the lattice vibrations and the interplay between ferromagnetism and lattice vibrations in Fe3GaTe2 are still unexplored. Here, we report the lattice vibration, Raman modes, and room-temperature spin-phonon coupling in 2D Fe3GaTe2 with above-room-temperature Curie temperature (TC). Two typical Raman modes with out-of-plane lattice vibrations are identified: "A" _"1g" ^"1" and "A" _"1g" ^"2" , whose frequencies increase as the thickness decreases from bulk to 2D Fe3GaTe2 due to the weakening of interlayer vdW interactions and spin exchange coupling. Moreover, the difference between phonon band dispersions under ferromagnetic and nonmagnetic interlayer spin ordering indicates the existence of spin-phonon coupling. The phonon frequency diverges from the anharmonic model below TC and thus the strength of spin-phonon coupling is ~0.28 cm-1 at 300 K, which is the first experimental identification of room-temperature spin-phonon coupling in 2D vdW magnets. This work deepens the understanding of novel 2D vdW magnets and provides a basis for spintronic applications at and above room temperature. △ Less

Submitted 29 November, 2024; originally announced November 2024.

Thickness-dependent Topological Phases and Flat Bands in Rhombohedral Multilayer Graphene

Authors: H. B. Xiao, C. Chen, X. Sui, S. H. Zhang, M. Z. Sun, H. Gao, Q. Jiang, Q. Li, L. X. Yang, M. Ye, F. Y. Zhu, M. X. Wang, J. P. Liu, Z. B. Zhang, Z. J. Wang, Y. L. Chen, K. H. Liu, Z. K. Liu

Abstract: Rhombohedral multilayer graphene has emerged as an extraordinary platform for investigating exotic quantum states, such as superconductivity and fractional quantum anomalous Hall effects, mainly due to the existence of topological surface flatbands. Despite extensive research efforts, a systematic spectroscopic investigation on the evolution of its electronic structure from thin layers to bulk rem… ▽ More Rhombohedral multilayer graphene has emerged as an extraordinary platform for investigating exotic quantum states, such as superconductivity and fractional quantum anomalous Hall effects, mainly due to the existence of topological surface flatbands. Despite extensive research efforts, a systematic spectroscopic investigation on the evolution of its electronic structure from thin layers to bulk remains elusive. Using state-of-the-art angle-resolved photoemission spectroscopy with submicron spatial resolution, we directly probe and trace the thickness evolution of the topological electronic structures of rhombohedral multilayer graphene. As the layer number increases, the gapped subbands transform into the 3D Dirac nodes that spirals in the momentum space; while the flatbands are constantly observed around Fermi level, and eventually evolve into the topological drumhead surface states. This unique thickness-dependent topological phase transition can be well captured by the 3D generalization of 1D Su-Schrieffer-Heeger chain in thin layers, to the topological Dirac nodal spiral semimetal in the bulk limit. Our findings establish a solid foundation for exploring the exotic quantum phases with nontrivial topology and correlation effects in rhombohedral multilayer graphene. △ Less

Submitted 25 November, 2024; v1 submitted 18 November, 2024; originally announced November 2024.

Comments: 15 pages, 4 figures, under review. A note added

Journal ref: Science Bulletin (2025)

Tunable topological edge states in black phosphorus-like Bi(110)

Authors: Chen Liu, Shengdan Tao, Guanyong Wang, Hongyuan Chen, Bing Xia, Hao Yang, Xiaoxue Liu, Liang Liu, Yaoyi Li, Shiyong Wang, Hao Zheng, Canhua Liu, Dandan Guan, Yunhao Lu, Jin-feng Jia

Abstract: We have investigated the structures and electronic properties of ultra-thin Bi(110) films grown on an s-wave superconductor substrate using low-temperature scanning tunneling microscopy and spectroscopy. Remarkably, our experimental results validate the theoretical predictions that the manipulation of Bi(110) surface atom buckling can control the topological phase transition. Notably, we have obse… ▽ More We have investigated the structures and electronic properties of ultra-thin Bi(110) films grown on an s-wave superconductor substrate using low-temperature scanning tunneling microscopy and spectroscopy. Remarkably, our experimental results validate the theoretical predictions that the manipulation of Bi(110) surface atom buckling can control the topological phase transition. Notably, we have observed robust unreconstructed edge states at the edges of both 3-bilayer (BL) and 4-BL Bi(110) films, with the 4-BL film displaying stronger edge state intensity and a smaller degree of atomic buckling. First-principle calculations further substantiate these findings, demonstrating a gradual reduction in buckling as the film thickness increases, with average height differences between two Bi atoms of approximately 0.19 Å, 0.10 Å, 0.05 Å, and 0.00 Å for the 1-BL, 2-BL, 3-BL, and 4-BL Bi(110) films, respectively. When Bi films are larger than 2 layers, the system changes from a trivial to a non-trivial phase. This research sets the stage for the controlled realization of topological superconductors through the superconducting proximity effect, providing a significant platform for investigating Majorana zero modes and fabricating quantum devices. △ Less

Submitted 25 October, 2024; originally announced October 2024.

Tunable Second-Order Structural Transition in As-Deficient MnAs

Authors: B. D. White, K. Huang, I. L. Fipps, J. J. Hamlin, S. Jang, G. J. Smith, B. Xia, J. W. Simonson, C. S. Nelson, M. C. Aronson, M. B. Maple

Abstract: We report measurements of magnetization, specific heat, and thermal expansion performed on As-deficient MnAs single crystals (MnAs$_{0.968}$). Ferromagnetic order is observed near $T_C \simeq$ 306 K on warming and $T_C \simeq$ 302 K on cooling, which is consistent with previously-reported values for stoichiometric MnAs samples. In contrast, the second-order structural phase transition is observed… ▽ More We report measurements of magnetization, specific heat, and thermal expansion performed on As-deficient MnAs single crystals (MnAs$_{0.968}$). Ferromagnetic order is observed near $T_C \simeq$ 306 K on warming and $T_C \simeq$ 302 K on cooling, which is consistent with previously-reported values for stoichiometric MnAs samples. In contrast, the second-order structural phase transition is observed at $T_S \simeq$ 353 K, which is nearly 50 K lower than in the stoichiometric compound. We observe differences in the thermal expansion of our samples when compared to reports of stoichiometric MnAs including: (1) the $\sim$1.5% volume decrease at $T_C$ is smaller than the expected value of 1.9%, (2) the lattice parameters perpendicular to the basal plane exhibit a discontinuous jump of $\sim$1.1% at $T_C$ instead of being continuous across $T_C$, and (3) thermal expansion perpendicular to the basal plane for $T_C \le T \le$ 315 K is negative rather than positive. We also observe a correlation between the ratio of hexagonal lattice parameters, $c/a$, and $T_S$, strongly suggesting that the degree of structural anisotropy in MnAs could play an important role in tuning $T_S$. △ Less

Submitted 14 August, 2024; originally announced August 2024.

Above-room-temperature intrinsic ferromagnetism in ultrathin van der Waals crystal Fe$_{3+x}$GaTe$_2$

Authors: Gaojie Zhang, Jie Yu, Hao Wu, Li Yang, Wen Jin, Bichen Xiao, Wenfeng Zhang, Haixin Chang

Abstract: Two-dimensional (2D) van der Waals (vdW) magnets are crucial for ultra-compact spintronics. However, so far, no vdW crystal has exhibited tunable above-room-temperature intrinsic ferromagnetism in the 2D ultrathin regime. Here, we report the tunable above-room-temperature intrinsic ferromagnetism in ultrathin vdW crystal Fe$_{3+x}$GaTe$_2$ ($x$ = 0 and 0.3). By increasing the Fe content, the Curie… ▽ More Two-dimensional (2D) van der Waals (vdW) magnets are crucial for ultra-compact spintronics. However, so far, no vdW crystal has exhibited tunable above-room-temperature intrinsic ferromagnetism in the 2D ultrathin regime. Here, we report the tunable above-room-temperature intrinsic ferromagnetism in ultrathin vdW crystal Fe$_{3+x}$GaTe$_2$ ($x$ = 0 and 0.3). By increasing the Fe content, the Curie temperature (TC) and room-temperature saturation magnetization of bulk Fe$_{3+x}$GaTe$_2$ crystals are enhanced from 354 to 376 K and 43.9 to 50.4 emu/g, respectively. Remarkably, the robust anomalous Hall effect in 3-nm Fe$_{3.3}$GaTe$_2$ indicate a record-high TC of 340 K and a large room-temperature perpendicular magnetic anisotropy energy of 6.6 * 10^5 J/m$^3$, superior to other ultrathin vdW ferromagnets. First-principles calculations reveal the asymmetric density of states and an additional large spin exchange interaction in ultrathin Fe$_{3+x}$GaTe$_2$ responsible for robust intrinsic ferromagnetism and higher Tc. This work opens a window for above-room-temperature ultrathin 2D magnets in vdW-integrated spintronics. △ Less

Submitted 5 August, 2024; originally announced August 2024.

Journal ref: Applied Physics Letters, 2024

Multi-task learning for molecular electronic structure approaching coupled-cluster accuracy

Authors: Hao Tang, Brian Xiao, Wenhao He, Pero Subasic, Avetik R. Harutyunyan, Yao Wang, Fang Liu, Haowei Xu, Ju Li

Abstract: Machine learning (ML) plays an important role in quantum chemistry, providing fast-to-evaluate predictive models for various properties of molecules. However, most existing ML models for molecular electronic properties use density functional theory (DFT) databases as ground truth in training, and their prediction accuracy cannot surpass that of DFT. In this work, we developed a unified ML method f… ▽ More Machine learning (ML) plays an important role in quantum chemistry, providing fast-to-evaluate predictive models for various properties of molecules. However, most existing ML models for molecular electronic properties use density functional theory (DFT) databases as ground truth in training, and their prediction accuracy cannot surpass that of DFT. In this work, we developed a unified ML method for electronic structures of organic molecules using the gold-standard CCSD(T) calculations as training data. Tested on hydrocarbon molecules, our model outperforms DFT with the widely-used hybrid and double hybrid functionals in computational costs and prediction accuracy of various quantum chemical properties. As case studies, we apply the model to aromatic compounds and semiconducting polymers on both ground state and excited state properties, demonstrating its accuracy and generalization capability to complex systems that are hard to calculate using CCSD(T)-level methods. △ Less

Submitted 24 June, 2024; v1 submitted 9 May, 2024; originally announced May 2024.

Tunable high-temperature tunneling magnetoresistance in all-van der Waals antiferromagnet/semiconductor/ferromagnet junctions

Authors: Wen Jin, Xinlu Li, Gaojie Zhang, Hao Wu, Xiaokun Wen, Li Yang, Jie Yu, Bichen Xiao, Wenfeng Zhang, Jia Zhang, Haixin Chang

Abstract: Magnetic tunnel junctions (MTJs) have been widely applied in spintronic devices for efficient spin detection through the imbalance of spin polarization at the Fermi level. The van der Waals (vdW) nature of two-dimensional (2D) magnets with atomic-scale flat surfaces and negligible surface roughness greatly facilitates the development of MTJs, yet is only restricted to ferromagnets. Here, we report… ▽ More Magnetic tunnel junctions (MTJs) have been widely applied in spintronic devices for efficient spin detection through the imbalance of spin polarization at the Fermi level. The van der Waals (vdW) nature of two-dimensional (2D) magnets with atomic-scale flat surfaces and negligible surface roughness greatly facilitates the development of MTJs, yet is only restricted to ferromagnets. Here, we report A-type antiferromagnetism in 2D vdW single-crystal (Fe0.8Co0.2)3GaTe2 with TN~203 K in bulk and ~185 K in 9-nm nanosheets. The metallic nature and out-of-plane magnetic anisotropy make it a suitable candidate for MTJ electrodes. By constructing heterostructures based on (Fe0.8Co0.2)3GaTe2/WSe2/Fe3GaTe2, we obtain a large tunneling magnetoresistance (TMR) ratio of 180% at low temperature and the TMR retains at near-room temperature 280 K. Moreover, the TMR is tunable by the electric field down to 1 mV, implying the potential in energy-efficient spintronic devices. Our work provides new opportunities for 2D antiferromagnetic spintronics and quantum devices. △ Less

Submitted 30 January, 2024; originally announced January 2024.

Graph Theorem for Chiral Exact Flat Bands at Charge Neutrality

Authors: Gurjyot Sethi, Bowen Xia, Dongwook Kim, Hang Liu, Xiaoyin Li, Feng Liu

Abstract: Chiral exact flat bands (FBs) at charge neutrality have attracted much recent interest, presenting an intriguing condensed-matter system to realize exact many-body phenomena, as specifically shown in "magic angle" twisted bilayer graphene for superconductivity and triangulene-based superatomic graphene for excitonic condensation. Yet, no generic physical model to realize such FBs has been develope… ▽ More Chiral exact flat bands (FBs) at charge neutrality have attracted much recent interest, presenting an intriguing condensed-matter system to realize exact many-body phenomena, as specifically shown in "magic angle" twisted bilayer graphene for superconductivity and triangulene-based superatomic graphene for excitonic condensation. Yet, no generic physical model to realize such FBs has been developed. Here we present a new mathematical theorem, called bipartite double cover (BDC) theorem, and prove that the BDC of line-graph (LG) lattices hosts at least two chiral exact FBs of opposite chirality, i.e., yin-yang FBs, centered-around/at charge neutrality (E = 0) akin to the "chiral limit" of twisted bilayer graphene. We illustrate this theorem by mapping it exactly onto tight-binding lattice models of the BDC of LGs of hexagonal lattice for strong topological and of triangular lattice for fragile topological FBs, respectively. Moreover, we use orbital design principle to realize such exotic yin-yang FBs in non-BDC lattices to instigate their real material discovery. This work not only enables the search for exact chiral FBs at zero energy beyond moiré heterostructures, but also opens the door to discovering quantum semiconductor features with FB-enabled strongly correlated carriers. △ Less

Submitted 19 December, 2023; originally announced December 2023.

Robust magnetic proximity induced anomalous Hall effect in a room temperature van der Waals ferromagnetic semiconductor based 2D heterostructure

Authors: Hao Wu, Li Yang, Gaojie Zhang, Wen Jin, Bichen Xiao, Wenfeng Zhang, Haixin Chang

Abstract: Developing novel high-temperature van der Waals ferromagnetic semiconductor materials and investigating their interface coupling effects with two-dimensional topological semimetals are pivotal for advancing next-generation spintronic and quantum devices. However, most van der Waals ferromagnetic semiconductors exhibit ferromagnetism only at low temperatures, limiting the proximity research on thei… ▽ More Developing novel high-temperature van der Waals ferromagnetic semiconductor materials and investigating their interface coupling effects with two-dimensional topological semimetals are pivotal for advancing next-generation spintronic and quantum devices. However, most van der Waals ferromagnetic semiconductors exhibit ferromagnetism only at low temperatures, limiting the proximity research on their interfaces with topological semimetals. Here, we report an intrinsic, van der Waals layered room-temperature ferromagnetic semiconductor crystal, FeCr0.5Ga1.5Se4 (FCGS), with a Curie temperature as high as 370 K, setting a new record for van der Waals ferromagnetic semiconductors. The saturation magnetization at low temperature (2 K) and room temperature (300 K) reaches 8.2 emu/g and 2.7 emu/g, respectively. Furthermore, FCGS possesses a bandgap of approximately 1.2 eV, which is comparable to the widely used commercial silicon. The FCGS/graphene heterostructure exhibits an impeccably smooth and gapless interface, thereby inducing a robust magnetic proximity coupling effect between FCGS and graphene. After the proximity coupling, graphene undergoes a charge carrier transition from electrons to holes, accompanied by a transition from non-magnetic to ferromagnetic transport behavior with robust anomalous Hall effect. Notably, the anomalous Hall effect remains robust even temperatures up to 400 K. △ Less

Submitted 13 November, 2023; originally announced November 2023.

Journal ref: Small Methods, 2024

ScaleLat: A chemical structure matching algorithm for mapping atomic structure of multi-phase system and high entropy alloys

Authors: Nan Li, Junming Guo, Sateng Li, Haoliang Liu, Qianwu Li, Fangjie Shi, Yefei Li, Bing Xiao

Abstract: ScaleLat (Scale Lattice) is a computer program written in C for performing the atomic structure analysis of multi-phase system or high entropy alloys (HEAs). The program implements an atomic cluster extraction algorithm to obtain all independent and symmetry-reduced characteristic chemical structures for the complex atomic configurations which are usually obtained from molecular dynamics or kineti… ▽ More ScaleLat (Scale Lattice) is a computer program written in C for performing the atomic structure analysis of multi-phase system or high entropy alloys (HEAs). The program implements an atomic cluster extraction algorithm to obtain all independent and symmetry-reduced characteristic chemical structures for the complex atomic configurations which are usually obtained from molecular dynamics or kinetic Monte-Carlo simulations for supercell containing more than 104 atoms. ScaleLat employes an efficient and unique chemical structure matching algorithm to map all extracted atomic clusters from a large supercell (>10^4 atoms) to a representative small one (~ 10^3 or less), providing the possibility to directly use the highly accurate quantum mechanical methods to study the electronic, magnetic, and mechanical properties of multi-component alloys with complex microstructures. We demonstrate the capability of ScaleLat code by conducting both the atomic structure analysis and chemical structure matching procedure for Fe-12.8 at.% Cr binary alloy and equiatomic CrFeCoNiCu high entropy alloy, and by successfully obtaining the representatively supercells containing 10^2~10^3 atoms of the two alloys. Overall, ScaleLat program provides a universal platform to efficiently project all essential chemical structures of large complex atomic structures to a relatively easy-handling small supercell for quantum mechanical calculations of various user interested properties. △ Less

Submitted 7 October, 2023; originally announced October 2023.

Assessing r2SCAN meta-GGA functional for structural parameters, cohesive energy, mechanical modulus and thermophysical properties of 3d, 4d and 5d transition metals

Authors: Haoliang Liu, Xue Bai, Jingliang Ning, Yuxuan Hou, Zifeng Song, Akilan Ramasamy, Ruiqi Zhang, Yefei Li, Jianwei Sun, Bing Xiao

Abstract: The recent development of the accurate and efficient semilocal density functionals on the third rung of Jacob's ladder of density functional theory such as the revised regularized strongly constrained and appropriately normed (r2SCAN) density functional could enable the rapid and highly reliable prediction of the elasticity and temperature dependence of thermophysical parameters of refractory elem… ▽ More The recent development of the accurate and efficient semilocal density functionals on the third rung of Jacob's ladder of density functional theory such as the revised regularized strongly constrained and appropriately normed (r2SCAN) density functional could enable the rapid and highly reliable prediction of the elasticity and temperature dependence of thermophysical parameters of refractory elements and their intermetallic compounds using quasi-harmonic approximation (QHA). Here, we present a comparative evaluation of the equilibrium cell volumes, cohesive energy, mechanical moduli, and thermophysical properties (Debye temperature and thermal expansion coefficient) for 22 transition metals using semilocal density functionals, including local density approximation (LDA), the Perdew-Burke-Ernzerhof (PBE) and PBEsol generalized gradient approximations (GGA), and the r2SCAN meta-GGA. PBEsol and r2SCAN deliver the same level of accuracies for structural, mechanical and thermophysical properties. Otherwise, PBE and r2SCAN perform better than LDA and PBEsol for calculating cohesive energies of transition metals. Among the tested density functionals, r2SCAN provides an overall well-balanced performance for reliably computing the cell volumes, cohesive energies, mechanical properties, and thermophysical properties of various 3d, 4d, and 5d transition metals using QHA. Therefore, we recommend that r2SCAN could be employed as a workhorse method to evaluate the thermophysical properties of transition metal compounds and alloys in the high throughput workflows. △ Less

Submitted 21 September, 2023; originally announced September 2023.

Room-Temperature Highly-Tunable Coercivity and Highly-Efficient Nonvolatile Multi-States Magnetization Switching by Small Current in Single 2D Ferromagnet Fe$_3$GaTe$_2$

Authors: Gaojie Zhang, Hao Wu, Li Yang, Wen Jin, Bichen Xiao, Wenfeng Zhang, Haixin Chang

Abstract: Room-temperature electrically-tuned coercivity and nonvolatile multi-states magnetization switching is crucial for next-generation low-power 2D spintronics. However, most methods have limited ability to adjust the coercivity of ferromagnetic systems, and room-temperature electrically-driven magnetization switching shows high critical current density and high power dissipation. Here, highly-tunable… ▽ More Room-temperature electrically-tuned coercivity and nonvolatile multi-states magnetization switching is crucial for next-generation low-power 2D spintronics. However, most methods have limited ability to adjust the coercivity of ferromagnetic systems, and room-temperature electrically-driven magnetization switching shows high critical current density and high power dissipation. Here, highly-tunable coercivity and highly-efficient nonvolatile multi-states magnetization switching are achieved at room temperature in single-material based devices by 2D van der Waals itinerant ferromagnet Fe$_3$GaTe$_2$. The coercivity can be readily tuned up to ~98.06% at 300 K by a tiny in-plane electric field that is 2-5 orders of magnitude smaller than that of other ferromagnetic systems. Moreover, the critical current density and power dissipation for room-temperature magnetization switching in 2D Fe$_3$GaTe$_2$ are down to ~1.7E5 A cm$^{-2}$ and ~4E12 W m$^{-3}$, respectively. Such switching power dissipation is 2-6 orders of magnitude lower than that of other 2D ferromagnetic systems. Meanwhile, multi-states magnetization switching are presented by continuously controlling the current, which can dramatically enhance the information storage capacity and develop new computing methodology. This work opens the avenue for room-temperature electrical control of ferromagnetism and potential applications for vdW-integrated 2D spintronics. △ Less

Submitted 23 August, 2023; originally announced August 2023.

Journal ref: ACS Materials Letters, 2024

Observation of abnormal resistance-temperature behavior along with diamagnetic transition in Pb$_{10-x}$Cu$_x$(PO$_4$)$_6$O-based composite

Authors: Hao Wu, Li Yang, Jie Yu, Gaojie Zhang, Bichen Xiao, Haixin Chang

Abstract: Recently, Sukbae Lee et al.reported that material Pb$_{10-x}$Cu$_x$(PO$_4$)$_6$O (LK-99) has a series of characteristics of room temperature superconductors, including diamagnetic transition, resistance jump, nearly zero-resistance, magnetic field-dependent IV characteristics and so on (10.6111/JKCGCT.2023.33.2.061, arXiv:2307.12008, arXiv:2307.12037). However, whether LK-99 is really a room tempe… ▽ More Recently, Sukbae Lee et al.reported that material Pb$_{10-x}$Cu$_x$(PO$_4$)$_6$O (LK-99) has a series of characteristics of room temperature superconductors, including diamagnetic transition, resistance jump, nearly zero-resistance, magnetic field-dependent IV characteristics and so on (10.6111/JKCGCT.2023.33.2.061, arXiv:2307.12008, arXiv:2307.12037). However, whether LK-99 is really a room temperature superconductor is still controversial. On the one hand, some people think that the relatively weak diamagnetism of LK-99 reported by Sukbae Lee et al. is not the Meissner effect. On the other hand, there are doubts about the authenticity of its zero-resistance test results. Global replication studies have shown that LK-99 does have a large diamagnetic (arXiv:2308.01516), and also found a zero-resistance behavior at a low temperature of 110 $^\circ$K (arXiv:2308.01192). However, up to now, there is still no direct reproducible evidence to support Sukbae Lee et al.'s conclusion that LK-99 is a room temperature superconductor. Here, a distinct resistance jump was observed at about 387 $^\circ$K under ambient pressure in our experiment for unclear reason including possible impurity's contribution. The overall resistance of the test LK-99 sample still shows semiconductivity, and the resistance cannot really drop to zero. Our findings indicate that to identify the true potential of LK-99, high quality crystals without impurity are very important. △ Less

Submitted 9 August, 2023; originally announced August 2023.

Successful growth and room temperature ambient-pressure magnetic levitation of LK-99

Authors: Hao Wu, Li Yang, Bichen Xiao, Haixin Chang

Abstract: Recently, Sukbae Lee et al. reported inspiring experimental findings on the atmospheric superconductivity of a modified lead apatite crystal (LK-99) at room temperature (10.6111/JKCGCT.2023.33.2.061, arXiv: 2307.12008, arXiv: 2307.12037). They claimed that the synthesized LK-99 materials exhibit the Meissner levitation phenomenon of superconductors and have a superconducting transition temperature… ▽ More Recently, Sukbae Lee et al. reported inspiring experimental findings on the atmospheric superconductivity of a modified lead apatite crystal (LK-99) at room temperature (10.6111/JKCGCT.2023.33.2.061, arXiv: 2307.12008, arXiv: 2307.12037). They claimed that the synthesized LK-99 materials exhibit the Meissner levitation phenomenon of superconductors and have a superconducting transition temperature (Tc) higher than 400 K. Here, for the first time, we successfully verify and synthesize the LK-99 crystals which can be magnetically levitated with larger levitated angle than Sukbae Lee's sample at room temperature. It is expected to realize the true potential of room temperature, non-contact superconducting magnetic levitation in near future. △ Less

Submitted 2 August, 2023; originally announced August 2023.

Topological materials or structures: Origin of higher-order topological states

Authors: Shengjie Zheng, Guiju Duan, Jianting Liu, Baizhan Xia

Abstract: Higher-order topological states (HOTS) have been extensively investigated in classical wave systems. They do not exist in the band gaps of infinite materials, while exhibit as the in-gap localized modes once the infinite materials are truncated to be the finite structures. Here, we will experimentally reveal the origin of HOTSs in acoustic systems. We design the hollow acoustic structures exclusiv… ▽ More Higher-order topological states (HOTS) have been extensively investigated in classical wave systems. They do not exist in the band gaps of infinite materials, while exhibit as the in-gap localized modes once the infinite materials are truncated to be the finite structures. Here, we will experimentally reveal the origin of HOTSs in acoustic systems. We design the hollow acoustic structures exclusively composed of hinge and corner resonators. We present the experimental proof that, despite the lack of surfaces and bulks, the hollow acoustic structures can still support the topologically protected hinge and corner states, indicating that the local configurations of boundaries are the sources for the generation of HOTSs. We then get the composite structures by assembling the 2D and 3D topological hollow acoustic structures, and experimentally observe the robust HOTSs in them. Our results provide a fundamental perspective on HOTSs in periodic structures, and we foresee that these findings will pave the way toward designing new topological devices for energy recovering, information processing, non-destructive testing and acoustic sensing. △ Less

Submitted 11 February, 2023; originally announced February 2023.

Functional building blocks for scalable multipartite entanglement in optical lattices

Authors: Wei-Yong Zhang, Ming-Gen He, Hui Sun, Yong-Guang Zheng, Ying Liu, An Luo, Han-Yi Wang, Zi-Hang Zhu, Pei-Yue Qiu, Ying-Chao Shen, Xuan-Kai Wang, Wan Lin, Song-Tao Yu, Bin-Chen Li, Bo Xiao, Meng-Da Li, Yu-Meng Yang, Xiao Jiang, Han-Ning Dai, You Zhou, Xiongfeng Ma, Zhen-Sheng Yuan, Jian-Wei Pan

Abstract: Featuring excellent coherence and operated parallelly, ultracold atoms in optical lattices form a competitive candidate for quantum computation. For this, a massive number of parallel entangled atom pairs have been realized in superlattices. However, the more formidable challenge is to scale-up and detect multipartite entanglement due to the lack of manipulations over local atomic spins in retro-r… ▽ More Featuring excellent coherence and operated parallelly, ultracold atoms in optical lattices form a competitive candidate for quantum computation. For this, a massive number of parallel entangled atom pairs have been realized in superlattices. However, the more formidable challenge is to scale-up and detect multipartite entanglement due to the lack of manipulations over local atomic spins in retro-reflected bichromatic superlattices. Here we developed a new architecture based on a cross-angle spin-dependent superlattice for implementing layers of quantum gates over moderately-separated atoms incorporated with a quantum gas microscope for single-atom manipulation. We created and verified functional building blocks for scalable multipartite entanglement by connecting Bell pairs to one-dimensional 10-atom chains and two-dimensional plaquettes of $2\times4$ atoms. This offers a new platform towards scalable quantum computation and simulation. △ Less

Submitted 6 October, 2022; originally announced October 2022.

Journal ref: Phys. Rev. Lett. 131, 073401 (2023)

Extracting Off-Diagonal Order from Diagonal Basis Measurements

Authors: Bo Xiao, Javier Robledo Moreno, Matthew Fishman, Dries Sels, Ehsan Khatami, Richard Scalettar

Abstract: Quantum gas microscopy has developed into a powerful tool to explore strongly correlated quantum systems. However, discerning phases with topological or off-diagonal long range order requires the ability to extract these correlations from site-resolved measurements. Here, we show that a multi-scale complexity measure can pinpoint the transition to and from the bond ordered wave phase of the one-di… ▽ More Quantum gas microscopy has developed into a powerful tool to explore strongly correlated quantum systems. However, discerning phases with topological or off-diagonal long range order requires the ability to extract these correlations from site-resolved measurements. Here, we show that a multi-scale complexity measure can pinpoint the transition to and from the bond ordered wave phase of the one-dimensional extended Hubbard model with an off-diagonal order parameter, sandwiched between diagonal charge and spin density wave phases, using only diagonal descriptors. We study the model directly in the thermodynamic limit using the recently developed variational uniform matrix product states algorithm, and draw our samples from degenerate ground states related by global spin rotations, emulating the projective measurements that are accessible in experiments. Our results will have important implications for the study of exotic phases using optical lattice experiments. △ Less

Submitted 7 August, 2024; v1 submitted 21 September, 2022; originally announced September 2022.

Comments: 11 pages including supplementary materials, 8 figures

Journal ref: Phys. Rev. Research 6, L022064 (2024)

Negative Interatomic Spring Constant Manifested by Topological Phonon Flat Band

Authors: Bowen Xia, Hang Liu, Feng Liu

Abstract: Phonons as bosons are different from electrons as fermions. Unlike interatomic electron hopping that can be either positive or negative and further tuned by spin-orbit coupling, interatomic spring constant is positive, or the structure of atomic lattices would be dynamically unstable. Surprisingly, we found that topological phonon flat bands (FBs) can manifest either a positive or negative interat… ▽ More Phonons as bosons are different from electrons as fermions. Unlike interatomic electron hopping that can be either positive or negative and further tuned by spin-orbit coupling, interatomic spring constant is positive, or the structure of atomic lattices would be dynamically unstable. Surprisingly, we found that topological phonon flat bands (FBs) can manifest either a positive or negative interatomic spring constant that couples the FB-modes of opposite chirality, as exemplified by first-principles calculations of a 2D material of Kagome-BN. To reveal its physical origin, we first establish a fundamental correspondence between a collective lattice-coupling (CLC) variable of two quasi-particle states (e.g., electronic states or phonon modes) of opposite parity in a periodic lattice with band topology. Topological semimetals arise with zero CLC at special k-points protected by symmetry; while positive and negative CLC at these k-points gives rise to normal and topological insulators, respectively. Then, we show topological FB has a special form of CLC that vanishes at all k-points as characterized by its real-space wave function, and multi-atom FB phonon mode can manifest effectively a negative interatomic spring constant. Our findings shed new light on our fundamental understanding of topology and provide a practical design principle for creating artificial bosonic topological states. △ Less

Submitted 2 September, 2022; v1 submitted 24 August, 2022; originally announced August 2022.

Elemental (im-)miscibility determines phase formation of multinary nanoparticles co-sputtered in ionic liquids

Authors: Michael Meischein, Alba Garzón-Manjón, Thomas Hammerschmidt, Bin Xiao, Siyuan Zhang, Lamya Abdellaoui, Christina Scheu, Alfred Ludwig

Abstract: Non-equilibrium synthesis methods allow to alloy bulk-immiscible elements into multinary nanoparticles, which broadens the design space for new materials. Whereas sputtering onto solid substrates can combine immiscible elements into thin film solid solutions, this is not clear for sputtering of nanoparticles in ionic liquids. Thus, the suitability of sputtering in ionic liquids for producing nanop… ▽ More Non-equilibrium synthesis methods allow to alloy bulk-immiscible elements into multinary nanoparticles, which broadens the design space for new materials. Whereas sputtering onto solid substrates can combine immiscible elements into thin film solid solutions, this is not clear for sputtering of nanoparticles in ionic liquids. Thus, the suitability of sputtering in ionic liquids for producing nanoparticles of immiscible elements is investigated by co-sputtering the systems Au-Cu (miscible), Au-Ru and Cu-Ru (both immiscible), and Au-Cu-Ru on the surface of the ionic liquid 1-butyl-3-methylimidazolium bis-trifluoromethylsulfonyl)imide [Bmim][(Tf)2N]. The sputtered nanoparticles were analyzed to obtain (i) knowledge concerning the general formation process of nanoparticles when sputtering onto ionic liquid surfaces and (ii) information, if alloy nanoparticles of immiscible elements can be synthesized as well as (iii) evidence if the Hume-Rothery rules for solid solubility are valid for sputtered nanoparticles. Accompanying atomistic simulations using density-functional theory for clusters of different size and ordering confirm that the miscibility of Au-Cu and the immiscibility of Au-Ru and Cu-Ru govern the thermodynamic stability of the nanoparticles. Based on the matching experimental and theoretical results for the NP/IL-systems concerning NP stability, a formation model of multinary NPs in ILs was developed. △ Less

Submitted 10 June, 2022; originally announced June 2022.

Observation of fractal topological states in acoustic metamaterials

Authors: Shengjie Zheng, Xianfeng Man, Ze-Lin Kong, Zhi-Kang Lin, Guiju Duan, Ning Chen, Dejie Yu, Jian-Hua Jiang, Baizhan Xia

Abstract: Topological phases of matter have been extensively investigated in solid state materials and classical wave systems with integer dimensions. However, topological states in non-integer dimensions remain largely unexplored. Fractals, being nearly the same at different scales, are one of the intriguing complex geometries with non-integer dimensions. Here, we demonstrate acoustic Sierpiński fractal to… ▽ More Topological phases of matter have been extensively investigated in solid state materials and classical wave systems with integer dimensions. However, topological states in non-integer dimensions remain largely unexplored. Fractals, being nearly the same at different scales, are one of the intriguing complex geometries with non-integer dimensions. Here, we demonstrate acoustic Sierpiński fractal topological insulators with unconventional higher-order topological phenomena via consistent theory and experiments. We discover abundant topological edge and corner states emerging in our acoustic systems due to the rich edge and corner boundaries inside the fractals. Interestingly, the numbers of the edge and corner states scale the same as the bulk states with the system size and the exponents coincide with the Hausdorff fractal dimension of the Sierpiński carpet. Furthermore, the emergent corner states exhibit unconventional spectrum and wave patterns. Our study opens a pathway toward topological states in fractal geometries. △ Less

Submitted 9 May, 2022; originally announced May 2022.

Comments: 14 Pages, 4 Figures

Temperature Dependence of Spin and Charge Orders in the Doped Two-Dimensional Hubbard Model

Authors: Bo Xiao, Yuan-Yao He, Antoine Georges, Shiwei Zhang

Abstract: Competing and intertwined orders including inhomogeneous patterns of spin and charge are observed in many correlated electron materials, such as high-temperature superconductors. Introducing a new development of the constrained-path auxiliary-field quantum Monte Carlo (AFQMC) method, we study the interplay between thermal and quantum fluctuations in the two-dimensional Hubbard model. We obtain an… ▽ More Competing and intertwined orders including inhomogeneous patterns of spin and charge are observed in many correlated electron materials, such as high-temperature superconductors. Introducing a new development of the constrained-path auxiliary-field quantum Monte Carlo (AFQMC) method, we study the interplay between thermal and quantum fluctuations in the two-dimensional Hubbard model. We obtain an accurate and systematic characterization of the evolution of the spin and charge correlations as a function of temperature $T$ and how it connects to the ground state, at three representative doping levels $δ= 1/5$, $1/8$, and $1/10$. We find increasing short-range commensurate antiferromagnetic correlations as $T$ is lowered. As the correlation length grows sufficiently large, a modulated spin-density-wave (SDW) appears. At $δ= 1/5$, the SDW saturates and remains short-ranged as $T \rightarrow 0$. In contrast, at $δ= 1/8$ and $1/10$ this evolves into a ground-state stripe phase. We study the relation between spin and charge orders and find that formation of charge order appears to be driven by that of the spin order. We identify a finite-temperature phase transition below which charge ordering sets in and discuss the implications of our results for the nature of this transition. △ Less

Submitted 22 February, 2023; v1 submitted 23 February, 2022; originally announced February 2022.

Comments: 21 pages, 15 figures

Journal ref: Phys. Rev. X 13, 011007 (2023)

Deconvolving the components of the sign problem

Authors: S. Tarat, Bo Xiao, R. Mondaini, R. T. Scalettar

Abstract: Auxiliary field Quantum Monte Carlo simulations of interacting fermions require sampling over a Hubbard-Stratonovich field $h$ introduced to decouple the interactions. The weight for a given configuration involves the products of the determinant of matrices $M_σ(h)$, where $σ$ labels the species, and hence is typically not positive definite. Indeed, the average sign $\langle {\cal S} \rangle$ of t… ▽ More Auxiliary field Quantum Monte Carlo simulations of interacting fermions require sampling over a Hubbard-Stratonovich field $h$ introduced to decouple the interactions. The weight for a given configuration involves the products of the determinant of matrices $M_σ(h)$, where $σ$ labels the species, and hence is typically not positive definite. Indeed, the average sign $\langle {\cal S} \rangle$ of the determinants goes to zero exponentially with increasing spatial size and decreasing temperature for most Hamiltonians of interest. This statement, however, does not explicitly separate two possible origins for the vanishing of $\langle {\cal S} \rangle$. Does $\langle {\cal S} \rangle \rightarrow 0$ because {\it randomly} chosen field configurations have ${\rm det}\big(M(h)\big) < 0$, or does the `sign problem' arise because the specific subset of configurations chosen by the weighting function have a greater preponderance of negative values? In the latter case, the process of weighting the configurations with $|{\rm det}\big(M(h)\big)|$ might steer the simulation to a region of configuration space of $h$ where positive and negative determinants are equally likely, even though randomly chosen $h$ would preferentially have determinants with a single dominant sign. In this paper we address the relative importance of these two mechanisms for the vanishing of $\langle {\cal S} \rangle$ in quantum simulations. △ Less

Submitted 2 December, 2021; v1 submitted 1 August, 2021; originally announced August 2021.

Comments: 14 pages, 17 figures, Title changed, several small modifications made in the text

Combinatorial materials discovery strategy for high entropy alloy electrocatalysts using deposition source permutations

Authors: Lars Banko, Olga A. Krysiak, Bin Xiao, Tobias Löffler, Alan Savan, Jack Kirk Pedersen, Jan Rossmeisl, Wolfgang Schuhmann, Alfred Ludwig

Abstract: High entropy alloys offer a huge search space for new electrocatalysts. Searching for a global property maximum in one quinary system could require, depending on compositional resolution, the synthesis of up to 10E6 samples which is impossible using conventional approaches. Co-sputtered materials libraries address this challenge by synthesis of controlled composition gradients of each element. How… ▽ More High entropy alloys offer a huge search space for new electrocatalysts. Searching for a global property maximum in one quinary system could require, depending on compositional resolution, the synthesis of up to 10E6 samples which is impossible using conventional approaches. Co-sputtered materials libraries address this challenge by synthesis of controlled composition gradients of each element. However, even such a materials library covers less than 1% of the composition space of a quinary system. We present a new strategy using deposition source permutations optimized for highest improvement of the covered new compositions. Using this approach, the composition space can be sampled in different subsections allowing identification of the contribution of individual elements and their combinations on electrochemical activity. Unsupervised machine learning reveals that electrochemical activity is governed by the complex interplay of chemical and structural factors. Out of 2394 measured compositions, a new highly active composition for the oxygen reduction reaction around Ru17Rh5Pd19Ir29Pt30 was identified. △ Less

Submitted 16 June, 2021; originally announced June 2021.

Bayesian Optimization of High-Entropy Alloy Compositions for Electrocatalytic Oxygen Reduction

Authors: Jack K. Pedersen, Christian M. Clausen, Olga A. Krysiak, Bin Xiao, Thomas A. A. Batchelor, Tobias Löffler, Vladislav A. Mints, Lars Banko, Matthias Arenz, Alan Savan, Wolfgang Schuhmann, Alfred Ludwig, Jan Rossmeisl

Abstract: Active, selective and stable catalysts are imperative for sustainable energy conversion, and engineering materials with such properties are highly desired. High-entropy alloys (HEAs) offer a vast compositional space for tuning such properties. Too vast, however, to traverse without the proper tools. Here, we report the use of Bayesian optimization on a model based on density functional theory (DFT… ▽ More Active, selective and stable catalysts are imperative for sustainable energy conversion, and engineering materials with such properties are highly desired. High-entropy alloys (HEAs) offer a vast compositional space for tuning such properties. Too vast, however, to traverse without the proper tools. Here, we report the use of Bayesian optimization on a model based on density functional theory (DFT) to predict the most active compositions for the electrochemical oxygen reduction reaction (ORR) with the least possible number of sampled compositions for the two HEAs Ag-Ir-Pd-Pt-Ru and Ir-Pd-Pt-Rh-Ru. The discovered optima are then scrutinized with DFT and subjected to experimental validation where optimal catalytic activities are verified for Ag-Pd, Ir-Pt, and Pd-Ru binary systems. This study offers insight into the number of experiments needed for exploring the vast compositional space of multimetallic alloys which has been determined to be on the order of 50 for ORR on these HEAs. △ Less

Submitted 15 June, 2021; originally announced June 2021.

Landau Levels and van der Waals Interfaces of Acoustics in Moiré Phononic Lattices

Authors: Shengjie Zheng, Jie Zhang, Guiju Duan, Zihan Jiang, Xianfeng Man, Dejie Yu, Baizhan Xia

Abstract: Moiré lattices which consist of parallel but staggered periodic lattices have been extensively explored due to their salient physical properties, such as van Hove singularities[1, 2], commensurable incommensurable transitions[3], non-Abelian gauge potentials[4], fractional quantum Hall effects[5-7], van der Waals interfaces[8, 9] and unconventional superconductivity[10, 11]. However, there are lim… ▽ More Moiré lattices which consist of parallel but staggered periodic lattices have been extensively explored due to their salient physical properties, such as van Hove singularities[1, 2], commensurable incommensurable transitions[3], non-Abelian gauge potentials[4], fractional quantum Hall effects[5-7], van der Waals interfaces[8, 9] and unconventional superconductivity[10, 11]. However, there are limited demonstrations of such concepts for classical wave systems. Here, we realized gauge fields in one-dimensional Moiré phononic lattices consisting of two superimposed periodic patterns which mismatched with each other along one direction. Benefiting from gauge fields, we generated Landau level flat bands near the Dirac cone and experimentally measured their spatial localization in pressure-field distributions. Then, by mismatching lattices along both directions, we constructed two-dimensional Moiré phononic lattices with van der Waals interfaces. We found that acoustic waves efficiently transported along van der Waals interfaces behaving as metallic networks. As mismatched lattices are well-controllable, our study offers a novel path to manipulate sound waves which are inaccessible in traditional periodic acoustic systems, and can be easily extended to mechanics, optics, electromagnetics and electronics. △ Less

Submitted 22 March, 2021; originally announced March 2021.

Comments: 17 pages, 9 figures

Three-dimensional Dirac Phonons with Inversion Symmetry

Authors: Zhongjia Chen, Rui Wang, Bowen Xia, Baobing Zheng, Yuanjun Jin, Yu-Jun Zhao, Hu Xu

Abstract: Dirac semimetals associated with bulk Dirac fermions are well-known in topological electronic systems. In sharp contrast, three-dimensional (3D) Dirac phonons in crystalline solids are still unavailable. Here we perform symmetry arguments and first-principles calculations to systematically investigate 3D Dirac phonons in all space groups with inversion symmetry. The results show that there are two… ▽ More Dirac semimetals associated with bulk Dirac fermions are well-known in topological electronic systems. In sharp contrast, three-dimensional (3D) Dirac phonons in crystalline solids are still unavailable. Here we perform symmetry arguments and first-principles calculations to systematically investigate 3D Dirac phonons in all space groups with inversion symmetry. The results show that there are two categories of 3D Dirac phonons depending on their protection mechanisms and positions in momentum space. The first category originates from the four-dimensional irreducible representations at the high symmetry points. The second category arises from the phonon branch inversion, and the symmetry guarantees Dirac points to be located along the high symmetry lines. Furthermore, we reveal that non-symmorphic symmetries and the combination of inversion and time-reversal symmetries play essential roles in the emergence of 3D Dirac phonons. Our work not only offers a comprehensive understanding of 3D Dirac phonons but also provides significant guidance for exploring Dirac bosons in both phononic and photonic systems. △ Less

Submitted 1 March, 2021; originally announced March 2021.

Journal ref: Phys. Rev. Lett. 126, 185301 (2021)

Topological defect states in elastic phononic plates

Authors: Baizhan Xia, Liang Tong, Jie Zhang, Shengjie Zheng, Xianfeng Man

Abstract: Topological defects (including disclinations and dislocations) which commonly exist in various materials have shown an amazing ability to produce excellent mechanical and physical properties of matters. In this paper, disclinations and dislocations are firstly introduced into the valley-polarized elastic phononic plate. Deformation of the lattice yields the interface expressing as the topologicall… ▽ More Topological defects (including disclinations and dislocations) which commonly exist in various materials have shown an amazing ability to produce excellent mechanical and physical properties of matters. In this paper, disclinations and dislocations are firstly introduced into the valley-polarized elastic phononic plate. Deformation of the lattice yields the interface expressing as the topologically protected wave guiding, due to the valley-polarized phase transition of phononic crystals (PnCs) across the interface. Then, disclinations are introduced into the Wannier-type elastic phononic plate. The deformation of the lattice yielded by disclinations produces a pentagonal core with the local five-fold symmetry. The topological bound states are well localized around the boundaries of the pentagonal cores with and without the hollow regions. The topological interface state and the topological bound state immunize against the finite sizes and the moderate disturbances of plates, essentially differing from the trivial defect states. The discovery of topological defect states unveils a new horizon in topological mechanics and physics, and it provides a novel platform to implement large-scale elastic devices with robust topological waveguides and resonators. △ Less

Submitted 15 February, 2021; originally announced February 2021.

Comments: 26 pages, 18 figures

Anomalous skin effect study of superconducting film

Authors: Binping Xiao, M. Blaskiewicz, T. Xin

Abstract: The field distribution inside the superconducting radiofrequency (SRF) film with different mean free path is studied using niobium (Nb) as an example. The surface resistance of clean Nb film with different substrate and different film thickness is calculated. We also show the study of a special structured multilayer superconducting film called Superconductor-Insulator-Superconductor (SIS) structur… ▽ More The field distribution inside the superconducting radiofrequency (SRF) film with different mean free path is studied using niobium (Nb) as an example. The surface resistance of clean Nb film with different substrate and different film thickness is calculated. We also show the study of a special structured multilayer superconducting film called Superconductor-Insulator-Superconductor (SIS) structure. △ Less

Submitted 19 March, 2021; v1 submitted 27 January, 2021; originally announced January 2021.

Strain Tunable Semimetal-Topological-Insulator Transition in Monolayer 1T'-WTe2

Authors: Chenxiao Zhao, Mengli Hu, Jin Qin, Bing Xia, Canhua Liu, Shiyong Wang, Dandan Guan, Yaoyi Li, Hao Zheng, Junwei Liu, Jinfeng Jia

Abstract: A quantum spin hall insulator(QSHI) is manifested by its conducting edge channels that originate from the nontrivial topology of the insulating bulk states. Monolayer 1T'-WTe2 exhibits this quantized edge conductance in transport measurements, but because of its semimetallic nature, the coherence length is restricted to around 100 nm. To overcome this restriction, we propose a strain engineering t… ▽ More A quantum spin hall insulator(QSHI) is manifested by its conducting edge channels that originate from the nontrivial topology of the insulating bulk states. Monolayer 1T'-WTe2 exhibits this quantized edge conductance in transport measurements, but because of its semimetallic nature, the coherence length is restricted to around 100 nm. To overcome this restriction, we propose a strain engineering technique to tune the electronic structure, where either a compressive strain along a axis or a tensile strain along b axis can drive 1T'-WTe2 into an full gap insulating phase. A combined study of molecular beam epitaxy and in-situ scanning tunneling microscopy/spectroscopy then confirmed such a phase transition. Meanwhile, the topological edge states were found to be very robust in the presence of strain. △ Less

Submitted 23 September, 2020; originally announced September 2020.

Comments: 15 pages, 5 figure and 21 pages of supplementary material

Journal ref: Phys. Rev. Lett. 125, 046801 (2020)

Complex solid solution electrocatalyst discovery by prediction and high-throughput experimentation

Authors: Thomas A. A. Batchelor, Tobias Löffler, Bin Xiao, Olga A. Krysiak, Valerie Strotkötter, Jack K. Pedersen, Christian M. Clausen, Alan Savan, Wolfgang Schuhmann, Jan Rossmeisl, Alfred Ludwig

Abstract: Efficient discovery of electrocatalysts for electrochemical energy conversion reactions is of utmost importance to combat climate change. With the example of the oxygen reduction reaction we show that by utilising a data-driven discovery cycle, the multidimensionality challenge offered by compositionally complex solid solution (high entropy alloy) electrocatalysts can be mastered. Iteratively refi… ▽ More Efficient discovery of electrocatalysts for electrochemical energy conversion reactions is of utmost importance to combat climate change. With the example of the oxygen reduction reaction we show that by utilising a data-driven discovery cycle, the multidimensionality challenge offered by compositionally complex solid solution (high entropy alloy) electrocatalysts can be mastered. Iteratively refined computational models predict activity trends for quinary target compositions, around which continuous composition spread thin-film libraries are synthesized. High-throughput characterisation datasets are then input for refinement of the model. The refined model correctly predicts activity maxima of the exemplary model system Ag-Ir-Pd-Pt-Ru for the oxygen reduction reaction. The method can identify optimal complex solid solutions for electrochemical reactions in an unprecedented manner. △ Less

Submitted 17 September, 2020; originally announced September 2020.

Combining quantum spin hall effect and superconductivity in few-layer stanene

Authors: Chenxiao Zhao, Jin Qin, Bing Xia, Bo Yang, Hao Zheng, Shiyong Wang, Canhua liu, Yaoyi Li, Dandan Guan, Jinfeng Jia

Abstract: Stanene was proposed to be a quantum spin hall insulator containing topological edges states and a time reversal invariant topological superconductor hosting helical Majorana edge mode. Recently, experimental evidences of existence of topological edge states have been found in monolayer stanene films and superconductivity has been observed in few-layer stanene films excluding single layer. An inte… ▽ More Stanene was proposed to be a quantum spin hall insulator containing topological edges states and a time reversal invariant topological superconductor hosting helical Majorana edge mode. Recently, experimental evidences of existence of topological edge states have been found in monolayer stanene films and superconductivity has been observed in few-layer stanene films excluding single layer. An integrated system with both topological edge states and superconductivity are higly pursued as a possible platform to realize topological superconductivity. Few-layer stanene show great potential to meet this requirement and is highly desired in experiment. Here we successfully grow few-layer stanene on bismuth (111) substrate. Both topological edge states and superconducting gaps are observed by in-situ scanning tunneling microscopy/spectroscopy (STM/STS). Our results take a further step towards topological superconductivity by stanene films. △ Less

Submitted 17 June, 2020; originally announced June 2020.

Comments: 14 pages,4 figures

Experimental realization of topological on-chip acoustic tweezers

Authors: Hongqing Dai, Linbo Liu, Baizhan Xia, Dejie Yu

Abstract: Acoustic tweezers are gaining increasing attention due to their excellent biological compatibility. Recently, the concept of topology has been expanded from condensed matter physics into acoustics, giving rise to a robust wave manipulation against defects and sharp turns. So far, topological acoustics have not been experimentally realized in on-chip level which can be worked as tweezers for microp… ▽ More Acoustic tweezers are gaining increasing attention due to their excellent biological compatibility. Recently, the concept of topology has been expanded from condensed matter physics into acoustics, giving rise to a robust wave manipulation against defects and sharp turns. So far, topological acoustics have not been experimentally realized in on-chip level which can be worked as tweezers for microparticle manipulations. Here, we achieved a topological on-chip acoustic tweezer based on the topologically protected phononic mode. This tweezer consisted of one-dimensional arrays of Helmholtz resonant air cavities. Strong microfluidic oscillations induced by acoustic waves were experimentally observed at water-air surfaces of Helmholtz resonant air cavities at the topological interface. Acoustic radiation force induced by these microfluidic oscillations captured microparticles whose sizes were up to 20 um and made them do orbital rotations. Our topological on-chip acoustic tweezer realized non-contact label-free microparticle manipulations in microfluidics and exhibited enormous application potential in the biomedical field. △ Less

Submitted 2 April, 2020; originally announced April 2020.

Comments: 14 pages, 5 figures

Journal ref: Phys. Rev. Applied 15, 064032 (2021)

Three-dimensional higher-order topological acoustic system with multidimensional topological states

Authors: Baizhan Xia, Shengjie Zheng, Liang Tong, Junrui Jiao, Guiju Duan, Dejie Yu

Abstract: Topologically protected gapless edge/surface states are phases of quantum matter which behave as massless Dirac fermions, immunizing against disorders and continuous perturbations. Recently, a new class of topological insulators (TIs) with gapped edge states and in-gap corner states have been theoretically predicted in electric systems 1,2, and experimentally realized in two-dimensional (2D) mecha… ▽ More Topologically protected gapless edge/surface states are phases of quantum matter which behave as massless Dirac fermions, immunizing against disorders and continuous perturbations. Recently, a new class of topological insulators (TIs) with gapped edge states and in-gap corner states have been theoretically predicted in electric systems 1,2, and experimentally realized in two-dimensional (2D) mechanical and electromagnetic systems 3,4, electrical circuits 5, optical and sonic crystals 6-11, and elastic phononic plates 12. Here, we elaborately design a strong three-dimensional (3D) topological acoustic system, by arranging acoustic meta-atoms in a simple cubic lattice. Under the direct field measurements, besides of the 2D surface propagations on all of the six surfaces, the 1D hinge propagations behaving as robust acoustic fibers along the twelve hinges and the 0D corner modes working as robust localized resonances at the eight corners are experimentally confirmed. As these multidimensional topological states are activated in different frequencies and independent spaces, our works pave feasible ways for applications in the topological acoustic cavities, communications and signal-processing. △ Less

Submitted 18 December, 2019; originally announced December 2019.

Comments: 23 pages,6 figures

Journal ref: Phys. Rev. B 102, 104113 (2020)

Charge Density Wave and Superconductivity in the Disordered Holstein Model

Authors: Bo Xiao, Natanael C. Costa, Ehsan Khatami, George G. Batrouni, Richard T. Scalettar

Abstract: The interplay between electron-electron correlations and disorder has been a central theme of condensed matter physics over the last several decades, with particular interest in the possibility that interactions might cause delocalization of an Anderson insulator into a metallic state, and the disrupting effects of randomness on magnetic order and the Mott phase. Here we extend this physics to exp… ▽ More The interplay between electron-electron correlations and disorder has been a central theme of condensed matter physics over the last several decades, with particular interest in the possibility that interactions might cause delocalization of an Anderson insulator into a metallic state, and the disrupting effects of randomness on magnetic order and the Mott phase. Here we extend this physics to explore electron-phonon interactions and show, via exact quantum Monte Carlo simulations, that the suppression of the charge density wave correlations in the half-filled Holstein model by disorder can stabilize a superconducting phase. Our simulations thus capture qualitatively the suppression of charge ordered phases and emergent superconductivity recently seen experimentally. △ Less

Submitted 10 March, 2021; v1 submitted 19 October, 2019; originally announced October 2019.

Journal ref: Phys. Rev. B 103, 060501 (2021)

Symmetry-Protected Topological Triangular Weyl Complex

Authors: R. Wang, B. W. Xia, Z. J. Chen, B. B. Zheng, Y. J. Zhao, H. Xu

Abstract: Weyl points are often believed to appear in pairs with opposite chirality. In this work, we show by first-principles calculations and symmetry analysis that single Weyl phonons with linear dispersion and double Weyl phonons with quadratic dispersion are simultaneously present between two specific phonon branches in realistic materials with trigonal or hexagonal lattices. These phonon Weyl points a… ▽ More Weyl points are often believed to appear in pairs with opposite chirality. In this work, we show by first-principles calculations and symmetry analysis that single Weyl phonons with linear dispersion and double Weyl phonons with quadratic dispersion are simultaneously present between two specific phonon branches in realistic materials with trigonal or hexagonal lattices. These phonon Weyl points are guaranteed to locate at high-symmetry points due to the screw rotational symmetry, forming a unique triangular Weyl complex. In sharp contrast to conventional Weyl systems with surface arcs terminated at the projections of a pair of Weyl points with opposite chirality, the phonon surface arcs of the unconventional triangular Weyl complex connect the projections of one double Weyl point and two single Weyl points. Importantly, the phonon surface arcs originating from the triangular Weyl complex are extremely long and span the entire surface Brillouin-zone. Furthermore, there are only nontrivial phonon surface states across the iso-frequency surface, which facilitates their detection in experiments and further applications. Our work not only offers the promising triangular phonon Weyl complex but also provides guidance for exploring triangular Weyl bosons in both phononic and photonic systems. △ Less

Submitted 17 March, 2020; v1 submitted 25 August, 2019; originally announced August 2019.

Comments: 5 pages, 4 figures

Journal ref: Phys. Rev. Lett. 124, 105303 (2020)

Classification and prediction of wave chaotic systems with machine learning techniques

Authors: Shukai Ma, Bo Xiao, Ron Hong, Bisrat Addissie, Zachary Drikas, Thomas Antonsen, Edward Ott, Steven Anlage

Abstract: The wave properties of complex scattering systems that are large compared to the wavelength, and show chaos in the classical limit, are extremely sensitive to system details. A solution to the wave equation for a specific configuration can change substantially under small perturbations. Due to this extreme sensitivity, it is difficult to discern basic information about a complex system simply from… ▽ More The wave properties of complex scattering systems that are large compared to the wavelength, and show chaos in the classical limit, are extremely sensitive to system details. A solution to the wave equation for a specific configuration can change substantially under small perturbations. Due to this extreme sensitivity, it is difficult to discern basic information about a complex system simply from scattering data as a function of energy or frequency, at least by eye. In this work, we employ supervised machine learning algorithms to reveal and classify hidden information about the complex scattering system presented in the data. As an example we are able to distinguish the total number of connected cavities in a linear chain of weakly coupled lossy enclosures from measured reflection data. A predictive machine learning algorithm for the future states of a perturbed complex scattering system is also trained with a recurrent neural network. Given a finite training data series, the reflection/transmission properties can be forecast by the proposed algorithm. △ Less

Submitted 13 August, 2019; originally announced August 2019.

Pseudospin-valley-coupled phononic topological insulator with edge and corner states

Authors: Haiyan Fan, Baizhan Xia, Shengjie Zheng, Liang Tong

Abstract: Topologically protected gapless edge states are phases of quantum matter which behave as massless Dirac fermions, immunizing against disorders and continuous perturbations. Recently, a new class of topological insulators (TIs) with topological corner states have been theoretically predicted in electric systems, and experimentally realized in two-dimensional (2D) mechanical and electromagnetic syst… ▽ More Topologically protected gapless edge states are phases of quantum matter which behave as massless Dirac fermions, immunizing against disorders and continuous perturbations. Recently, a new class of topological insulators (TIs) with topological corner states have been theoretically predicted in electric systems, and experimentally realized in two-dimensional (2D) mechanical and electromagnetic systems, electrical circuits, optical and sonic crystals, and elastic phononic plates. Here, we demonstrate a pseudospin-valley-coupled phononic TI, which simultaneously exhibits gapped edge states and topological corner states. Pseudospin-orbit coupling edge states and valley-polarized edge state are respectively induced by the lattice deformation and the symmetry breaking. When both of them coexist, these topological edge states will be greatly gapped and the topological corner state emerges. Under direct field measurements, the robust edge propagation behaving as an elastic waveguide and the topological corner mode working as a robust localized resonance are experimentally confirmed. The pseudospin-valley coupling in our phononic TIs can be well-controlled which provides a reconfigurable platform for the multiple edge and corner states, and exhibits well applications in the topological elastic energy recovery and the highly sensitive sensing. △ Less

Submitted 9 August, 2019; originally announced August 2019.

Comments: 18 pages, 5 figures