Showing 1–50 of 130 results for author: Xia, X

From Disorder to Function: Entropy-Engineered Broadband Photonics with Ion-Transport-Stabilized Spectral Fidelity

Authors: Yuxiang Xin, Chen-Xin Yu, Jianru Wang, Jianbo Jin, Minliang Lai, Yinan Wang, Shuwen Yan, Gu-wen Chen, Liang Fan, Xiachu Xiao, Yutao Yang, Luying Li, Han Wang, Zhi-Pan Liu, Jiang Tang, Li-Ming Yang, Zhuolei Zhang

Abstract: The high-entropy halide-perovskite field has expanded rapidly, yet a key gap remains: configurational entropy is not yet a reliable, designable lever to co-deliver expanded photonic functionality and operational robustness with a composition-transferable mechanistic basis. Here we develop entropy-engineered rare-earth halide double-perovskite single crystals, Cs2Na(Sb, RE)Cl6 (RE3+ = Sc3+, Er3+, Y… ▽ More The high-entropy halide-perovskite field has expanded rapidly, yet a key gap remains: configurational entropy is not yet a reliable, designable lever to co-deliver expanded photonic functionality and operational robustness with a composition-transferable mechanistic basis. Here we develop entropy-engineered rare-earth halide double-perovskite single crystals, Cs2Na(Sb, RE)Cl6 (RE3+ = Sc3+, Er3+, Yb3+, Tm3+), that simultaneously expand near-infrared (NIR) functionality and establish a mechanistic stability rule. Near-equiatomic B(III)-site alloying yields a single-phase high-entropy solid solution (Delta_Sconfig about 1.6R). Sb3+ serves as a sensitizer that unifies excitation and cooperatively activates multiple lanthanide channels, transforming the parent single-mode response into a broadband NIR output (~850-1600 nm) with three spectrally orthogonal fingerprint bands at 996, 1220, and 1540 nm. This tri-peak, self-referenced output enables redundancy-based ratiometric solvent identification and quantitative mixture sensing with reduced susceptibility to intensity drift. Accelerated aging under humidity and oxygen shows improved phase and emission stability versus single-component analogues. DFT and molecular dynamics attribute the robustness to strongly suppressed RE$^{3+}$/Cl$^-$ self-diffusion despite comparable H$_2$O/O$_2$ adsorption, kinetically impeding ion-migration-assisted reconstruction and degradation. Integration into a phosphor-converted LED delivers spectrally stable, broadband NIR illumination, establishing entropy engineering as a practical handle to couple expanded photonic functionality with mechanistically accountable durability in metal-halide photonics. △ Less

Submitted 10 February, 2026; v1 submitted 30 December, 2025; originally announced December 2025.

Atomic-scale spin sensing of a 2D $d$-wave altermagnet via helical tunneling

Authors: Zhuying Wang, Shuikang Yu, Xingkai Cheng, Xiaoyu Xiao, Wanru Ma, Feixiong Quan, Hongxi Song, Kunming Zhang, Yunmei Zhang, Yitian Ma, Wenhao Liu, Priti Yadav, Xiangbiao Shi, Zhijun Wang, Qian Niu, Yang Gao, Bin Xiang, Junwei Liu, Zhenyu Wang, Xianhui Chen

Abstract: Altermagnetism simultaneously possesses nonrelativistic spin responses and zero net magnetization, thus combining advantages of ferromagnetism and antiferromagnetism. This superiority originates from its unique dual feature, i.e., opposite-magnetic sublattices in real space and alternating spin polarization in momentum space enforced by the same crystal symmetry. Therefore, the determination of an… ▽ More Altermagnetism simultaneously possesses nonrelativistic spin responses and zero net magnetization, thus combining advantages of ferromagnetism and antiferromagnetism. This superiority originates from its unique dual feature, i.e., opposite-magnetic sublattices in real space and alternating spin polarization in momentum space enforced by the same crystal symmetry. Therefore, the determination of an altermagnetic order and its unique spin response inherently necessitates atomic-scale spin-resolved measurements in real and momentum spaces, an experimental milestone yet to be achieved. Here, via utilizing the helical edge (hinge) modes of a higher order topological insulator as the spin sensor, we realize spin-resolved scanning tunneling microscopy which enables us to pin down the dual-space feature of a layered $d$-wave altermagnet, KV$_2$Se$_2$O. In real space, atomic-registered mapping demonstrates the checkerboard antiferromagnetic order together with density-wave lattice modulation, and in momentum space, spin-resolved spectroscopic imaging provides a direct visualization of d-wave spin splitting of the band structure. Critically, using this new topology-guaranteed spin filter we directly reveal the unidirectional, spin-polarized quasiparticle excitations originating from the crystal symmetry-paired X and Y valleys around opposite magnetic sublattices simultaneously --the unique spin response for $d$-wave altermagnetism. Our experiments establish a solid basis for the exploration and utilization of altermagnetism in layered materials and further facilitate access to atomic-scale spin sensing and manipulating of 2D quantum materials. △ Less

Submitted 29 December, 2025; originally announced December 2025.

Comments: 21 pages and 5 figures. Extended data figures and Supplementary notes are available from the corresponding author upon request. Comments are welcome

Green's Function Methods for Computing Supercurrents in Josephson Junctions

Authors: Eduardo R. Mucciolo, Jouko Nieminen, Xiao Xiao, Wei-Chi Chiu, Michael N. Leuenberger, Arun Bansil

Abstract: Interest in Josephson junctions (JJs) has increased rapidly in recent years not only because of their use in qubits and other quantum devices but also due to the unique physics supported by the JJs. The advent of various novel quantum materials for both the barrier region and the superconducting leads has led to the possibility of adding new functionalities to the JJs. Thus, there is a growing nee… ▽ More Interest in Josephson junctions (JJs) has increased rapidly in recent years not only because of their use in qubits and other quantum devices but also due to the unique physics supported by the JJs. The advent of various novel quantum materials for both the barrier region and the superconducting leads has led to the possibility of adding new functionalities to the JJs. Thus, there is a growing need for accurate modeling of the JJs and related systems to enable their predictive control and atomistic level understanding. This review presents an in-depth discussion of a Green's function-based formalism for computing supercurrents in JJs. The formulation is tailored for large-scale atomistic simulations and encompasses both dc and ac supercurrents. We hope that this review will provide a timely and comprehensive reference for researchers as well as beginning practitioners interested in Green's-function-based methods to model supercurrents in JJs. △ Less

Submitted 21 February, 2026; v1 submitted 8 September, 2025; originally announced September 2025.

Comments: 33 pages, 5 figures

CrystalDiT: A Diffusion Transformer for Crystal Generation

Authors: Xiaohan Yi, Guikun Xu, Xi Xiao, Zhong Zhang, Liu Liu, Yatao Bian, Peilin Zhao

Abstract: We present CrystalDiT, a diffusion transformer for crystal structure generation that achieves state-of-the-art performance by challenging the trend of architectural complexity. Instead of intricate, multi-stream designs, CrystalDiT employs a unified transformer that imposes a powerful inductive bias: treating lattice and atomic properties as a single, interdependent system. Combined with a periodi… ▽ More We present CrystalDiT, a diffusion transformer for crystal structure generation that achieves state-of-the-art performance by challenging the trend of architectural complexity. Instead of intricate, multi-stream designs, CrystalDiT employs a unified transformer that imposes a powerful inductive bias: treating lattice and atomic properties as a single, interdependent system. Combined with a periodic table-based atomic representation and a balanced training strategy, our approach achieves 8.78% SUN (Stable, Unique, Novel) rate on MP-20, substantially outperforming recent methods including FlowMM (4.21%) and MatterGen (3.66%). Notably, CrystalDiT generates 63.28% unique and novel structures while maintaining comparable stability rates, demonstrating that architectural simplicity can be more effective than complexity for materials discovery. Our results suggest that in data-limited scientific domains, carefully designed simple architectures outperform sophisticated alternatives that are prone to overfitting. △ Less

Submitted 30 December, 2025; v1 submitted 13 August, 2025; originally announced August 2025.

Comments: 18 pages, 18 figures. Code available at https://github.com/hanyi2021/CrystalDiT.git. Updated to remove copyright notice

ACM Class: I.2.6; I.2.10

Entropy-driven physical amplification in multivalent biosensing

Authors: Xiuyang Xia, Yuhan Peng, Ran Ni

Abstract: Sensitive detection of low-abundance molecular targets is widely assumed to require enzymatic amplification, such as PCR, to achieve low detection limits. In amplification-free platforms, sensitivity is traditionally constrained by equilibrium binding affinity. Here we show that multivalent linker entropy provides a distinct physical route to exponential sensitivity enhancement in purely equilibri… ▽ More Sensitive detection of low-abundance molecular targets is widely assumed to require enzymatic amplification, such as PCR, to achieve low detection limits. In amplification-free platforms, sensitivity is traditionally constrained by equilibrium binding affinity. Here we show that multivalent linker entropy provides a distinct physical route to exponential sensitivity enhancement in purely equilibrium sensing architectures. Using a statistical-mechanical theory supported by grand canonical Monte Carlo simulations, we demonstrate that redistributing a fixed total interaction strength over increasing linker valency exponentially lowers adsorption thresholds. This scaling emerges not from stronger energetic affinity, but from the rapid growth of combinatorial binding configurations, revealing entropy as an intrinsic amplification mechanism. Consequently, detection limits can be tuned independently of bond strength, enabling ultrasensitive responses without enzymatic replication. Our results establish a general physical design principle for engineering amplification-free detection systems capable of approaching PCR-level sensitivities through entropy-driven collective effects. △ Less

Submitted 24 February, 2026; v1 submitted 1 August, 2025; originally announced August 2025.

Beyond Constant-Temperature Reservoirs: A Stirling Cycle with Constant Heat-Generation Rate

Authors: Xinshu Xia, Hongbo Huang, Hui Dong

Abstract: Conventional heat-engine models typically assume two heat reservoirs at fixed temperatures. In contrast, radioisotope power systems introduce a fundamentally different paradigm in which the hot sources supply heat at a constant generation rate rather than maintaining a constant temperature. We develop a theoretical framework for finite-time heat engines operating between constant heat-generation-r… ▽ More Conventional heat-engine models typically assume two heat reservoirs at fixed temperatures. In contrast, radioisotope power systems introduce a fundamentally different paradigm in which the hot sources supply heat at a constant generation rate rather than maintaining a constant temperature. We develop a theoretical framework for finite-time heat engines operating between constant heat-generation-rate hot sources and constant-temperature cold reservoirs. A universal proportion between average output power and efficiency is established, independent of the specific cycle configuration or working substance. As a representative case, we analyze a finite-time Stirling cycle employing a tailored control protocol that maintains the working substance at constant temperatures during the quasi-isothermal processes. An intrinsic oscillatory behavior emerges in the temperature dynamics of the hot source, reflecting the interplay between heat accumulation and release. We further quantify the long-term decline in engine performance resulting from radioactive decay and demonstrate its impact over the system's operational lifespan. This work establishes a new theoretical prototype for heat engines and shall provide guidings for the analysis and design of radioisotope power systems. △ Less

Submitted 25 June, 2025; originally announced June 2025.

Comments: 6 pages, 4 figures

Highly reliable, ultra-wideband, isolator-free quantum-dot mode-locked frequency combs for optical interconnects beyond 3.2Tb/s

Authors: Shujie Pan, Victoria Cao, Yiheng Feng, Dingyi Wu, Jie Yan, Junjie Yang, Chao Zhao, Xi Xiao, Siming Chen

Abstract: Quantum dot mode-locked laser-based optical frequency combs are emerging as a critical solution for achieving low-cost, high-efficiency, and large-capacity optical interconnects. The practical implementation of wavelength division multiplexing interconnects necessitates a temperature-stable OFC source with a minimum 100 GHz channel spacing to enable high-bandwidth modulation while mitigating the c… ▽ More Quantum dot mode-locked laser-based optical frequency combs are emerging as a critical solution for achieving low-cost, high-efficiency, and large-capacity optical interconnects. The practical implementation of wavelength division multiplexing interconnects necessitates a temperature-stable OFC source with a minimum 100 GHz channel spacing to enable high-bandwidth modulation while mitigating the complexity of optical filtering and detection. By leveraging the advanced co-doping technique and a colliding pulse mode-locking scheme, here, we report a compact, ultra-wideband, highly reliable, isolator-free 100 GHz-spacing InAs/GaAs QD OFC source operating up to a record temperature of 140 °C. The comb source delivers a record 3 dB optical bandwidth of 14.312 nm, containing flat-top comb lines, each supporting 128 Gb/s PAM-4 modulation, which results in a total throughput of 3.328 Tb/s with an extremely low power consumption of 0.394 pJ/bit at 25°C. Performance remains stable at 85 °C, with negligible degradation of device critical metrics. Remarkably, accelerated aging tests under harsh conditions (85 °C with 8x threshold current injection) revealed a mean time to failure of approximately 207 years. The QD OFC source demonstrated in this work, for the first time, establishes a concrete link between fundamental research on comb sources and their practical deployment in next-generation, high-density optical interconnect systems. △ Less

Submitted 2 June, 2025; originally announced June 2025.

Magnetic vortex writing and local reversal seeding in artificial spin-vortex ice via all-optical and surface-probe control

Authors: Holly Holder, Jack C. Gartside, Alex Vanstone, Troy Dion, Xiaofei Xiao, Kilian D. Stenning, Tingjun Zheng, Daniel Bromley, Tobias Farchy, Rupert F. Oulton, Will R. Branford

Abstract: Artificial spin-vortex ice ('ASVI') is a reconfigurable nanomagnetic metamaterial consisting of magnetic nanoislands tailored to support both Ising macrospin and vortex textures. ASVI has recently shown functional applications including reconfigurable magnonics and neuromorphic computing, where the introduction of vortex textures broadens functionality beyond conventional artificial spin ice which… ▽ More Artificial spin-vortex ice ('ASVI') is a reconfigurable nanomagnetic metamaterial consisting of magnetic nanoislands tailored to support both Ising macrospin and vortex textures. ASVI has recently shown functional applications including reconfigurable magnonics and neuromorphic computing, where the introduction of vortex textures broadens functionality beyond conventional artificial spin ice which generally supports macrospin states. However, local control of writing vortex states in ASVI remains an open challenge. Here we demonstrate techniques for field-free magnetic vortex writing in ASVI. We expand ASVI to support metastable macrospin, single-vortex and double-vortex states. All-optical writing via focused laser illumination can locally write double-vortex textures, and surface-probe writing using an MFM tip can locally write single vortex states. We leverage this writing to tailor and explore the reconfigurable energy landscape of ASVI, demonstrating programmable local seeding of avalanche-like reversal events. The global field-free texture selective writing techniques reported here expand the suite of nanomagnetic control techniques, with a host of future applications including fundamental studies of avalanche dynamics, physical memory, and direct writing of nanomagnetic 'weights' in physical neuromorphic neural networks. △ Less

Submitted 22 May, 2025; originally announced May 2025.

Quantum spin excitations in a dual-core magnetic molecule

Authors: Wenbin Li, Wenwen Shi, Xiaoxiao Xiao, Haiyan Zhu, Cai Cheng, Dongfei Wang, Lan Chen, Masahiro Haze, Huixia Fu, Xiao Zheng, Yang Guo, Zhendong Li, Yukio Hasegawa

Abstract: Magnetic excitations are important quantum phenomena in magnetic systems and have been widely studied in individual magnetic atoms and molecules as well as their assembled structures over the past few decades. Using scanning tunneling microscopy/spectroscopy (STM/S) combined with density functional theory (DFT) and the state-of-the-art ab initio wavefunction calculations, we investigated the prope… ▽ More Magnetic excitations are important quantum phenomena in magnetic systems and have been widely studied in individual magnetic atoms and molecules as well as their assembled structures over the past few decades. Using scanning tunneling microscopy/spectroscopy (STM/S) combined with density functional theory (DFT) and the state-of-the-art ab initio wavefunction calculations, we investigated the properties of a novel dual-core Cr2Br6 molecule, which consists of two Cr ions coupled via superexchange through a single near-90° Cr-Br-Cr scissors bond. Under zero magnetic field, we observed a Fano peak with multi-steps through STS. When an external magnetic field is applied, some steps exhibit additional splitting, while others change little. We find that the Cr2Br6, exhibits a spin-degenerate ground state, and the complex peak splitting arises from the coexistence of vibrational and magnetic excitations in the molecule. Our results reveal rich quantum spin behavior in a well-defined two-core magnetic trihalide complex at the atomic scale, offering not only a minimal model for superexchange-coupled multi-spin quantum excitations but also a possible foundational unit for future molecule-based quantum functionalities. △ Less

Submitted 11 May, 2025; originally announced May 2025.

Comments: 18 pages, 3 figues and 2 tables

Coexistence of distinct mobility edges in a 1D quasiperiodic mosaic model

Authors: Xu Xia, Weihao Huang, Ke Huang, Xiaolong Deng, Xiao Li

Abstract: We introduce a one-dimensional quasiperiodic mosaic model with analytically solvable mobility edges that exhibit different phase transitions depending on the system parameters. Specifically, by combining mosaic quasiperiodic next-nearest-neighbor hoppings and quasiperiodic on-site potentials, we rigorously demonstrate the existence of two distinct types of mobility edges: those separating extended… ▽ More We introduce a one-dimensional quasiperiodic mosaic model with analytically solvable mobility edges that exhibit different phase transitions depending on the system parameters. Specifically, by combining mosaic quasiperiodic next-nearest-neighbor hoppings and quasiperiodic on-site potentials, we rigorously demonstrate the existence of two distinct types of mobility edges: those separating extended and critical states, and those separating extended and localized states. Using Avila's global theory, we derive exact analytical expressions for these mobility edges and determine the parameter regimes where each type dominates. Our numerical calculations confirm these analytical results through fractal dimension analysis. Furthermore, we propose an experimentally feasible scheme to realize this model using Bose-Einstein condensates in optical lattices with engineered momentum-state transitions. We also investigate the effects of many-body interactions under mean-field approximation. Our work provides a fertile ground for studying the coexistence of different types of mobility edges in quasiperiodic systems and suggests a feasible experimental platform to observe and control these transitions. △ Less

Submitted 6 March, 2025; originally announced March 2025.

Comments: 5 pages, 3 figures. Comments are welcome

Signatures of Non-Abelian Kitaev quantum spin liquids in noise magnetormetry

Authors: Xiao Xiao, Masahiro O. Takahashi, Paul Stevenson, Satoshi Fujimoto, Arun Bansil

Abstract: Identification of isolated Majorana zero modes (MZMs) is a key step towards the realization of fault-tolerant topological quantum computation. Here we show how the $T_1$-based noise magnetormetry of a nitrogen-vacancy (NV) center qubit can reveal the unique signatures of Majorana fermions attached to vacancies in a non-Abelian Kitaev quantum spin liquid (KQSL). The $1/T_1$ of the NV center is foun… ▽ More Identification of isolated Majorana zero modes (MZMs) is a key step towards the realization of fault-tolerant topological quantum computation. Here we show how the $T_1$-based noise magnetormetry of a nitrogen-vacancy (NV) center qubit can reveal the unique signatures of Majorana fermions attached to vacancies in a non-Abelian Kitaev quantum spin liquid (KQSL). The $1/T_1$ of the NV center is found to be increased significantly when the working frequency of the NV center matches the energy difference between a MZM and a low-energy hybridized mode involving dangling Majorana fermions adjacent to vacancies. In experiments, this energy difference can be tuned by an external Zeeman field. Because of the large excitation gap of flipping a local $Z_2$ gauge field, the $1/T_1$ spectrum is robust against other fluctuations in KQSLs. Our study presents a promising pathway for identifying the non-Abelian phase in Kitaev materials. △ Less

Submitted 3 February, 2025; v1 submitted 31 January, 2025; originally announced January 2025.

Transmon qutrit-based simulation of spin-1 AKLT systems

Authors: Keerthi Kumaran, Faisal Alam, Norhan Eassa, Kaelyn Ferris, Xiao Xiao, Lukasz Cincio, Nicholas Bronn, Arnab Banerjee

Abstract: Qutrit-based quantum circuits could help reduce the overall circuit depths, and hence the effect of noise, when the system of interest has a local dimension of three. Accessing second excited states in superconducting transmons provides a straightforward hardware realization of qutrits useful for such ternary encoding. In this work, we successfully calibrate microwave pulse gates to a low error ra… ▽ More Qutrit-based quantum circuits could help reduce the overall circuit depths, and hence the effect of noise, when the system of interest has a local dimension of three. Accessing second excited states in superconducting transmons provides a straightforward hardware realization of qutrits useful for such ternary encoding. In this work, we successfully calibrate microwave pulse gates to a low error rate to operate transmon qutrits. We use these qutrits to simulate one-dimensional spin-1 AKLT states (Affleck, Kennedy, Lieb, and Tasaki), which exhibit a multitude of interesting phenomena, such as topologically protected ground states, string order, and the existence of a robust Berry phase. We demonstrate the efficacy of qutrit-based simulation by preparing high-fidelity ground states of the AKLT Hamiltonian with open boundaries for various chain lengths. We then use ground state preparations of the perturbed AKLT Hamiltonian with periodic boundaries to calculate the Berry phase and illustrate non-trivial ground state topology. To establish the advantage of qutrits over qubits in the presence of noise, we present scalable methods for preparing the AKLT state and computing its Berry phase using tensor network simulations. Our work provides a pathway toward more general spin-1 physics simulations using transmon qutrits, with applications in chemistry, magnetism, and topological phases of matter. △ Less

Submitted 14 October, 2025; v1 submitted 27 December, 2024; originally announced December 2024.

Report number: LA-UR-24-33344

Designed self-assembly of programmable colloidal atom-electron equivalents

Authors: Xiuyang Xia, Yuhan Peng, Ka Ki Li, Ran Ni

Abstract: To unlock the potential for assembling complex colloidal "molecules", we investigate a minimal binary system of programmable colloidal atom-electron equivalents (PAE-EE), where electron equivalents (EEs) are multivalent linkers with two distinct types of single-stranded DNA (ssDNA) ends complementary to those ssDNAs on binary programmable atom equivalents (PAEs). We derive a statistical mechanical… ▽ More To unlock the potential for assembling complex colloidal "molecules", we investigate a minimal binary system of programmable colloidal atom-electron equivalents (PAE-EE), where electron equivalents (EEs) are multivalent linkers with two distinct types of single-stranded DNA (ssDNA) ends complementary to those ssDNAs on binary programmable atom equivalents (PAEs). We derive a statistical mechanical framework for calculating the effective interaction between PAEs mediated by EEs with arbitrary valency, which quantitatively agrees with simulations that explicitly include EEs. Our analysis reveals an anomalous dependence of PAE-PAE interactions on the EE valency, showing that EE-mediated interactions converge at the large valency limit. Moreover, we identify an optimal EE valency that maximizes the interaction difference between targeted and non-targeted binding pairs of PAEs. These findings offer design principles for targeted self-assembly in PAE-EE systems. △ Less

Submitted 9 June, 2025; v1 submitted 31 October, 2024; originally announced October 2024.

Journal ref: Rep. Prog. Phys., 88, 078101 (2025)

Role of Wettability, Adhesion, and Instabilities in Transitions During Lubricated Sliding Friction

Authors: Hao Dong, Reshma Siddiquie, Xuemei Xiao, Michael Andrews, Brian Bergman, Chung-Yuen Hui, Anand Jagota

Abstract: Lubricated contacts in soft materials are important in various engineering systems and natural settings. Three major lubrication regimes are boundary (BL), mixed (ML), and elasto-hydrodynamic (EHL) lubrication, where the contact region is dry, partially wetted, or fully wetted, respectively. The transition between these regimes is insufficiently understood, especially for soft contacts, which impe… ▽ More Lubricated contacts in soft materials are important in various engineering systems and natural settings. Three major lubrication regimes are boundary (BL), mixed (ML), and elasto-hydrodynamic (EHL) lubrication, where the contact region is dry, partially wetted, or fully wetted, respectively. The transition between these regimes is insufficiently understood, especially for soft contacts, which impedes desired control of lubricated sliding friction. Here, we report on the role of solid wettability and adhesion on these transitions. Wettability of glycerol on polydimethylsiloxane (PDMS) surface, and adhesion between a glass indenter and PDMS, were varied by exposure of the PDMS to an ultraviolet light-ozone (UV-Ozone) cleaner. By combining friction tests and visualization, we demonstrate that the transition from ML to BL regime is dominated by the wettability of the lubricant; increasing wettability of glycerol makes removal of liquid from the contact region more difficult. Transition from EHL to ML is related to a series of events with increasing normal load, which are thinning of the lubricant layer, sudden jump to contact between the glass indenter and solid substrate across a gap of tens to a few hundreds of nanometers, and attendant elastic instabilities such as wrinkling and stick-slip. These results provide a deeper understanding of transitions in lubricated frictional behavior of soft materials which govern the maximum and minimum friction achievable. △ Less

Submitted 15 December, 2024; v1 submitted 11 October, 2024; originally announced October 2024.

From irregular to regular eutectic growth in the Al-Al3Ni system: in situ observations during directional solidification

Authors: Paul Chao, Shanmukha Kiran Aramanda, Xianghui Xiao, Sabine Bottin-Rousseau, Silvère Akamatsu, Ashwin J. Shahani

Abstract: We investigate the irregular eutectic growth dynamics of the Al-Al3Ni alloy, in which one of the solid phases (Al3Ni) grows faceted from the liquid. Leveraging in situ optical microscopy and synchrotron transmission x-ray microscopy, we address the question of the degree of coupling between Al and Al3Ni at the growth front and that of the shape of the microstructures left behind in the bulk solid… ▽ More We investigate the irregular eutectic growth dynamics of the Al-Al3Ni alloy, in which one of the solid phases (Al3Ni) grows faceted from the liquid. Leveraging in situ optical microscopy and synchrotron transmission x-ray microscopy, we address the question of the degree of coupling between Al and Al3Ni at the growth front and that of the shape of the microstructures left behind in the bulk solid during directional solidification. Real-time optical observations bring evidence for a morphological transition from a eutectic-grain dependent, irregular eutectic growth at low solidification velocity V (typically 1 $μms^{-1}$), to a weakly anisotropic, regular growth at higher V (reaching 10 $μms^{-1}$). Unprecedented x-ray nano-imaging of the solid-liquid interface, and 3D characterization of the growth patterns, were made possible by a new Directional Solidification (DS) setup at Brookhaven National Laboratory's NSLS-II. At low V, the leading tips of partly faceted Al3Ni crystals are observed to grow not far ahead of the Al growth front. Correlating in situ images and postmortem 3D tomographic reconstructions reveals that the presence of faceted and non-faceted regions of Al3Ni crystals in the solid is a direct consequence of coupling and decoupling during DS, respectively. Upon increasing V , the lead distance of Al3Ni vanishes, and the shape of Al3Ni ceases to be governed by faceted growth. These observations shed light on the basic mechanisms (faceted growth, diffusive coupling, and the dynamics of trijunctions) governing the transition from faceted to rod-like growth upon increasing V in the Al3Ni system, with broad implications for a large class of irregular eutectics. △ Less

Submitted 26 August, 2024; originally announced August 2024.

Comments: 56 pages, 20 figures, preprint accepted to Acta Materialia

Journal ref: Acta Materialia (2024) 120314

Polycatenated Architected Materials

Authors: Wenjie Zhou, Sujeeka Nadarajah, Liuchi Li, Anna G. Izard, Hujie Yan, Aashutosh K. Prachet, Payal Patel, Xiaoxing Xia, Chiara Daraio

Abstract: Architected materials derive their properties from the geometric arrangement of their internal structural elements. Their designs rely on continuous networks of members to control the global mechanical behavior of the bulk. Here, we introduce a class of materials that consist of discrete concatenated rings or cage particles interlocked in three-dimensional networks, forming polycatenated architect… ▽ More Architected materials derive their properties from the geometric arrangement of their internal structural elements. Their designs rely on continuous networks of members to control the global mechanical behavior of the bulk. Here, we introduce a class of materials that consist of discrete concatenated rings or cage particles interlocked in three-dimensional networks, forming polycatenated architected materials (PAMs). We propose a general design framework that translates arbitrary crystalline networks into particles' concatenations and geometries. In response to small external loads, PAMs behave like non-Newtonian fluids, showing both shear-thinning and shear-thickening responses. At larger strains, PAMs behave like lattices and foams, with a nonlinear stress-strain relation. At microscale, we demonstrate that PAMs can change their shapes in response to applied electrostatic charges. PAM's unique properties pave the path for developing stimuli-responsive materials, energy-absorbing systems and morphing architectures. △ Less

Submitted 9 October, 2024; v1 submitted 1 June, 2024; originally announced June 2024.

Surprising pressure-induced magnetic transformations from Helimagnetic order to Antiferromagnetic state in NiI2

Authors: Qiye Liu, Wenjie Su, Yue Gu, Xi Zhang, Xiuquan Xia, Le Wang, Ke Xiao, Xiaodong Cui, Xiaolong Zou, Bin Xi, Jia-Wei Mei, Jun-Feng Dai

Abstract: Interlayer magnetic interactions play a pivotal role in determining the magnetic arrangement within van der Waals (vdW) magnets, and the remarkable tunability of these interactions through applied pressure further enhances their significance. Here, we investigate NiI2 flakes, a representative vdW magnet, under hydrostatic pressures up to 11 GPa. We reveal a notable increase in magnetic transition… ▽ More Interlayer magnetic interactions play a pivotal role in determining the magnetic arrangement within van der Waals (vdW) magnets, and the remarkable tunability of these interactions through applied pressure further enhances their significance. Here, we investigate NiI2 flakes, a representative vdW magnet, under hydrostatic pressures up to 11 GPa. We reveal a notable increase in magnetic transition temperatures for both helimagnetic and antiferromagnetic states, and find that a reversible transition from helimagnetic to antiferromagnetic (AFM) phases at approximately 7 GPa challenges established theoretical and experimental expectations. While the increase in transition temperature aligns with pressure-enhanced overall exchange interaction strengths, we identify the significant role of the second-nearest neighbor interlayer interaction, which competes with intra-layer frustration and favors the AFM state as demonstrated in the Monte Carlo simulations. Experimental and simulated results converge on the existence of an intermediate helimagnetic ordered state in NiI2 before transitioning to the AFM state. These findings underscore the pivotal role of interlayer interactions in shaping the magnetic ground state, providing fresh perspectives for innovative applications in nanoscale magnetic device design. △ Less

Submitted 15 April, 2024; originally announced April 2024.

Anomalous quantum scattering and transport of electrons with Mexican-hat dispersion induced by electrical potential

Authors: Jiating Yao, Benliang Zhou, Xiaoying Zhou, Xianbo Xiao, Guanghui Zhou

Abstract: We theoretically study the quantum scattering and transport of electrons with Mexican-hat dispersion through both step and rectangular potential barriers by using the transfer matrix method. Owing to the torus-like iso-energy lines of the Mexican-hat dispersion, we observe the presence of double reflections and double transmissions in both two different barrier scenarios, i.e., the normal reflecti… ▽ More We theoretically study the quantum scattering and transport of electrons with Mexican-hat dispersion through both step and rectangular potential barriers by using the transfer matrix method. Owing to the torus-like iso-energy lines of the Mexican-hat dispersion, we observe the presence of double reflections and double transmissions in both two different barrier scenarios, i.e., the normal reflection (NR), retro-reflection (RR), normal transmission (NT), and specular transmission (ST).For the step potential with electrons incident from the large wavevector, the transmission is primarily governed by NT with nearly negligible ST, while the reflection is dominant by RR (NR) within (outside) the critical angle. Additionally, for electrons incident from the small wavevector, the NT can be reduced to zero by adjusting the barrier, resulting in a significant enhancement of ST and RR. For the rectangular barrier, the transmission and reflection spectra resemble those of the step barrier, but there are two kinds of resonant tunneling which can lead to perfect NT or ST. There exists a negative differential conductance (NDC) effect in the conductance spectrum. The conductance and the peak-to-valley ratio of the NDC effect can be effectively controlled by adjusting the height and width of the barrier as well as the incident energy. Our results provide a deeper understanding of the electron states governed by the Mexican-hat dispersion. △ Less

Submitted 14 March, 2024; originally announced March 2024.

Comments: 8 pages, 5 figures

Half-Metallic Ferromagnetic Weyl Fermions Related to Dynamic Correlations in the Zinc-blende Compound VAs

Authors: Xianyong Ding, Xin Jin, Haoran Wei, Ruixiang Zhu, Xiaoliang Xiao, Xiaozhi Wu, Fangyang Zhan, Rui Wang

Abstract: The realization of 100\% polarized topological Weyl fermions in half-metallic ferromagnets is of particular importance for fundamental research and spintronic applications. Here, we theoretically investigate the electronic and topological properties of the zinc-blende compound VAs, which was deemed as a half-metallic ferromagnet related to dynamic correlations. Based on the combination of density… ▽ More The realization of 100\% polarized topological Weyl fermions in half-metallic ferromagnets is of particular importance for fundamental research and spintronic applications. Here, we theoretically investigate the electronic and topological properties of the zinc-blende compound VAs, which was deemed as a half-metallic ferromagnet related to dynamic correlations. Based on the combination of density functional theory and dynamical mean field theory, we uncover that the half-metallic ferromagnet VAs exhibit attractive Weyl semimetallic behaviors with twelve pairs of Weyl points, which are very close to the Fermi level. Meanwhile, we also investigate the magnetization-dependent topological properties; the results show that the change of magnetization directions only slightly affects the positions of Weyl points, which is attributed to the weak spin-orbital coupling effects. The topological surface states of VAs projected on semi-infinite (001) and (111) surfaces are investigated. The Fermi arcs of all Weyl points are clearly visible on the projected Fermi surfaces. Our findings suggest that VAs is a fully spin-polarized Weyl semimetal with many-body correlated effects for spintronic applications. △ Less

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

Comments: 6figures

Majorana bound state induced drag current in capacitively coupled quantum dots

Authors: Xiao Xiao, Jian-Xin Zhu

Abstract: We show that nonzero drag current in a double quantum-dot system, consisting of a biased drive dot and an unbiased passive dot coupled capacitively, can be generated by a Majorana bound state located at one of the leads connected to the passive dot. Importantly, the drag current induced by Majorana bound states, either an isolated Majorana bound state or two weakly coupled but spatially separated… ▽ More We show that nonzero drag current in a double quantum-dot system, consisting of a biased drive dot and an unbiased passive dot coupled capacitively, can be generated by a Majorana bound state located at one of the leads connected to the passive dot. Importantly, the drag current induced by Majorana bound states, either an isolated Majorana bound state or two weakly coupled but spatially separated Majorana modes, shows qualitative differences from that induced by a near-zero-energy Andereev bound state. Thus, other than the tunneling spectroscopy, the proposed setup can serve as a complementary tool to detect Majorana fermions in proximitized Rashba nanowires. △ Less

Submitted 18 February, 2024; originally announced February 2024.

Comments: 7 pages, 4 figures

Report number: LA-UR-24-20166

Coherent Quantum Speed Limits

Authors: Xuhui Xiao, Hai Wang, Xingze Qiu

Abstract: We establish a comprehensive theoretical framework for coherent quantum speed limits (QSLs), deriving fundamental bounds on the rate of quantum evolution that explicitly isolate the contribution of quantum coherence. By applying Hölder's inequality for matrix norms to the Liouville-von Neumann equation, we construct two infinite families of QSLs for general unitary dynamics. These bounds are chara… ▽ More We establish a comprehensive theoretical framework for coherent quantum speed limits (QSLs), deriving fundamental bounds on the rate of quantum evolution that explicitly isolate the contribution of quantum coherence. By applying Hölder's inequality for matrix norms to the Liouville-von Neumann equation, we construct two infinite families of QSLs for general unitary dynamics. These bounds are characterized by coherence measures based on Schatten $p$-norms and Hellinger distance, respectively, defined with respect to the instantaneous energy eigenbasis. Unlike traditional Mandelstam-Tamm bounds, our approach disentangles the quantum state's coherence structure from the Hamiltonian's energy scale. Using the Landau-Zener model accelerated by shortcuts to adiabaticity, we demonstrate that coherence functions as a critical kinematic resource: achieving faster evolution entails maintaining a state with high coherence relative to the instantaneous basis. Our results provide a resource-theoretic perspective on time-energy uncertainty, offering insights into the fundamental limits of quantum control and information processing. △ Less

Submitted 24 March, 2026; v1 submitted 3 January, 2024; originally announced January 2024.

Comments: 5+1.5 pages, 1 table, 1 figure. We are grateful to the anonymous Referees for their constructive comments

Journal ref: Phys. Rev. A 113, 032438 (2026)

Anomalous exchange bias effect in ferromagnetic VI3 flakes

Authors: Xi Zhang, Xiuquan Xia, Qiye Liu, Yonggang He, Le Wang, Junhao Lin, Jia-Wei Mei, Yingchun Cheng, Jun-Feng Dai

Abstract: The exchange bias (EB) effect, pivotal in magnetic data storage and sensing devices, has been observed not only in interfacial regions but also in intrinsic ferromagnetic materials. Here, we've uncovered a robust and stable exchange bias effect within the layered van der Waals (vdW) ferromagnet VI3 employing magnetic circular dichroism microscopy. At 10 K, we observed a significant exchange field… ▽ More The exchange bias (EB) effect, pivotal in magnetic data storage and sensing devices, has been observed not only in interfacial regions but also in intrinsic ferromagnetic materials. Here, we've uncovered a robust and stable exchange bias effect within the layered van der Waals (vdW) ferromagnet VI3 employing magnetic circular dichroism microscopy. At 10 K, we observed a significant exchange field of approximately 0.1 T, accompanied by random shifts (positive or negative relative to zero magnetic field) after zero-field cooling. Notably, this effect is effectively controllable after field cooling, with shift direction opposing the applied magnetic field. The presence of strong magnetic anisotropic energy within VI3 results in larger coercivity-bound magnetic domains. These domains dictate the neighboring ferromagnetic alignment and induce shifts in the hysteresis loop. Our study not only contributes to comprehending fundamental nanoscale magnetic interactions but also sheds light on emergent phenomena within layered van der Waals magnets. △ Less

Submitted 28 December, 2023; originally announced December 2023.

Single pair of charge-two high-fold fermions with type-II van Hove singularities on the surface of ultralight chiral crystals

Authors: Xiaoliang Xiao, Yuanjun Jin, Da-Shuai Ma, Haoran Wei, Jing Fan, Rui Wang, Xiaozhi Wu

Abstract: The realization of single-pair chiral fermions in Weyl systems remains challenging in topology physics, especially for the systems with higher chiral charges $C$. In this work, based on the symmetry analysis, low-energy effective model, and first-principles calculations, we identify the single-pair high-fold fermions in chiral cubic lattices. We first derive the minimal lattice model that exhibits… ▽ More The realization of single-pair chiral fermions in Weyl systems remains challenging in topology physics, especially for the systems with higher chiral charges $C$. In this work, based on the symmetry analysis, low-energy effective model, and first-principles calculations, we identify the single-pair high-fold fermions in chiral cubic lattices. We first derive the minimal lattice model that exhibits a single pair of Weyl points with the opposite chiral charges of $C$ = $\pm{2}$. Furthermore, we show the ultralight chiral crystal P4$_3$32-type LiC$_2$ and its mirror enantiomer as high-quality candidate materials, which exhibit large energy windows to surmount the interruption of irrelevant bands. Since two enantiomers are connected by the mirror symmetry, we observe the opposite chiral charges $C$ and the reversal of the Fermi arc velocities, showing the correspondence of chirality in the momentum space and the real space. In addition, we also reveal type-II van Hove singularities on the helicoid surfaces, which may induce chirality-locked charge density waves on the crystal surface. Our work not only provides a promising platform for controlling the sign of topological charge through the structural chirality but also facilitates the exploration of electronic correlations on the surface of ultralight chiral crystals. △ Less

Submitted 12 December, 2023; v1 submitted 15 November, 2023; originally announced November 2023.

Comments: 8 pages, 5 figures

Fate of localization features in a one-dimensional non-Hermitian flat-band lattice with quasiperiodic modulations

Authors: Hui Liu, Zhanpeng Lu, Xu Xia, Zhihao Xu

Abstract: We investigate the influence of quasiperiodic modulations on one-dimensional non-Hermitian diamond lattices with an artificial magnetic flux $θ$ that possess flat bands. Our study shows that the symmetry of these modulations and the magnetic flux $θ$ play a pivotal role in shaping the localization properties of the system. When $θ=0$, the non-Hermitian lattice exhibits a single flat band in the cr… ▽ More We investigate the influence of quasiperiodic modulations on one-dimensional non-Hermitian diamond lattices with an artificial magnetic flux $θ$ that possess flat bands. Our study shows that the symmetry of these modulations and the magnetic flux $θ$ play a pivotal role in shaping the localization properties of the system. When $θ=0$, the non-Hermitian lattice exhibits a single flat band in the crystalline case, and symmetric as well as antisymmetric modulations can induce accurate mobility edges. In contrast, when $θ=π$, the clean diamond lattice manifests three dispersionless bands referred to as an "all-band-flat" (ABF) structure, irrespective of the non-Hermitian parameter. The ABF structure restricts the transition from delocalized to localized states, as all states remain localized for any finite symmetric modulation. Our numerical calculations further unveil that the ABF system subjected to antisymmetric modulations exhibits multifractal-to-localized edges. Multifractal states are predominantly concentrated in the internal region of the spectrum. Additionally, we explore the case where $θ$ lies within the range of $(0, π)$, revealing a diverse array of complex localization features. Finally, we propose a classical electrical circuit scheme to realize the non-Hermitian flat-band chain with quasiperiodic modulations. △ Less

Submitted 16 October, 2024; v1 submitted 6 November, 2023; originally announced November 2023.

Comments: 25 pages, 10 figures

Journal ref: New J. Phys. 26 (2024) 093007

A bond swap algorithm for simulating dynamically crosslinked polymers

Authors: Peilin Rao, Xiuyang Xia, Ran Ni

Abstract: Materials incorporating covalent adaptive networks (CAN), e.g., vitrimers, have received significant scientific attention due to their distinctive attributes of self-healing and stimuli-responsive properties. Different from direct crosslinked systems, bivalent and multivalent systems require a bond swap algorithm that respects detailed balance, considering the multiple equilibria in the system. He… ▽ More Materials incorporating covalent adaptive networks (CAN), e.g., vitrimers, have received significant scientific attention due to their distinctive attributes of self-healing and stimuli-responsive properties. Different from direct crosslinked systems, bivalent and multivalent systems require a bond swap algorithm that respects detailed balance, considering the multiple equilibria in the system. Here we propose a simple and robust algorithm to handle bond swap in multivalent and multi-species CAN systems. By including a bias term in the acceptance of Monte Carlo moves, we eliminate the imbalance from the bond swap site selection and multivalency effects, ensuring the detailed balance for all species in the system. △ Less

Submitted 12 January, 2024; v1 submitted 6 November, 2023; originally announced November 2023.

Journal ref: J. Chem. Phys. 160, 061102 (2024)

Designing superselectivity in linker-mediated multivalent nanoparticle adsorption

Authors: Xiuyang Xia, Ran Ni

Abstract: Using a statistical mechanical model and numerical simulations, we provide the design principle for the bridging strength ($ξ$) and linker density ($ρ$) dependent superselectivity in linker-mediated multivalent nanoparticle adsorption. When the bridges are insufficient, the formation of multiple bridges leads to both $ξ$- and $ρ$-dependent superselectivity. Whereas, when the bridges are excessive,… ▽ More Using a statistical mechanical model and numerical simulations, we provide the design principle for the bridging strength ($ξ$) and linker density ($ρ$) dependent superselectivity in linker-mediated multivalent nanoparticle adsorption. When the bridges are insufficient, the formation of multiple bridges leads to both $ξ$- and $ρ$-dependent superselectivity. Whereas, when the bridges are excessive, the system becomes insensitive to bridging strength due to entropy-induced self-saturation and shows a superselective desorption with respect to the linker density. Counterintuitively, lower linker density or stronger bridging strength enhances the superselectivity. These findings enhance understanding of relevant biological processes and open up opportunities for applications in biosensing, drug delivery, and programmable self-assembly. △ Less

Submitted 22 February, 2024; v1 submitted 24 October, 2023; originally announced October 2023.

Comments: Accepted in Physical Review Letters

Journal ref: Phys. Rev. Lett, 132, 118202 (2024)

Optical assembly of nanostructures mediated by surface roughness

Authors: Robert G. Felsted, Jaehun Chun, Gregory K. Schenter, Alexander B. Bard, Xiaojing Xia, Peter J. Pauzauskie

Abstract: Rigorous understanding of the self-assembly of colloidal nanocrystals is crucial to the development of tailored nanostructured materials. Despite extensive studies, a mechanistic understanding of self-assembly under non-equilibrium driven by an external field remains an ongoing challenge. We demonstrate self-assembly by optical tweezers imposing an external attractive field for cubic-phase sodium… ▽ More Rigorous understanding of the self-assembly of colloidal nanocrystals is crucial to the development of tailored nanostructured materials. Despite extensive studies, a mechanistic understanding of self-assembly under non-equilibrium driven by an external field remains an ongoing challenge. We demonstrate self-assembly by optical tweezers imposing an external attractive field for cubic-phase sodium yttrium fluoride nanocrystals. We show that surface roughness of the nanocrystals is a decisive factor for contact leading to assembly between the nanocrystals, manifested by the roughness-dependent hydrodynamic resistivity. This provides direct evidence that dynamics are equally important to energetics in understanding self-assembly. These results have implications in a wide variety of different fields, such as in understanding the factors that mediate oriented attachment-based crystal growth or in interpreting the structure of binding sites on viruses. △ Less

Submitted 10 October, 2023; originally announced October 2023.

Comments: 21 pages, 3 main figures, 8 supplemental figures, 2 supplemental videos. Submitted to Physical Review Letters

Journal ref: Phys. Rev. Lett. 134, (2025), 088201

Human Learning of Hierarchical Graphs

Authors: Xiaohuan Xia, Andrei A. Klishin, Jennifer Stiso, Christopher W. Lynn, Ari E. Kahn, Lorenzo Caciagli, Dani S. Bassett

Abstract: Humans are constantly exposed to sequences of events in the environment. Those sequences frequently evince statistical regularities, such as the probabilities with which one event transitions to another. Collectively, inter-event transition probabilities can be modeled as a graph or network. Many real-world networks are organized hierarchically and understanding how humans learn these networks is… ▽ More Humans are constantly exposed to sequences of events in the environment. Those sequences frequently evince statistical regularities, such as the probabilities with which one event transitions to another. Collectively, inter-event transition probabilities can be modeled as a graph or network. Many real-world networks are organized hierarchically and understanding how humans learn these networks is an ongoing aim of current investigations. While much is known about how humans learn basic transition graph topology, whether and to what degree humans can learn hierarchical structures in such graphs remains unknown. We investigate how humans learn hierarchical graphs of the Sierpiński family using computer simulations and behavioral laboratory experiments. We probe the mental estimates of transition probabilities via the surprisal effect: a phenomenon in which humans react more slowly to less expected transitions, such as those between communities or modules in the network. Using mean-field predictions and numerical simulations, we show that surprisal effects are stronger for finer-level than coarser-level hierarchical transitions. Surprisal effects at coarser levels of the hierarchy are difficult to detect for limited learning times or in small samples. Using a serial response experiment with human participants (n=$100$), we replicate our predictions by detecting a surprisal effect at the finer-level of the hierarchy but not at the coarser-level of the hierarchy. To further explain our findings, we evaluate the presence of a trade-off in learning, whereby humans who learned the finer-level of the hierarchy better tended to learn the coarser-level worse, and vice versa. Our study elucidates the processes by which humans learn hierarchical sequential events. Our work charts a road map for future investigation of the neural underpinnings and behavioral manifestations of graph learning. △ Less

Submitted 5 September, 2023; originally announced September 2023.

Comments: 22 pages, 10 figures, 1 table

Ultra-high mobility semiconducting epitaxial graphene on silicon carbide

Authors: Jian Zhao, Peixun Ji, Yaqi Li, Rui Li, Kaiming Zhang, Hao Tian, Kaichen Yu, Boyue Bian, Luzhen Hao, Xue Xiao, Will Griffin, Noel Dudeck, Ramiro Moro, Lei Ma, Walt A. de Heer

Abstract: Graphene nanoelectronics potential was limited by the lack of an intrinsic bandgap[1] and attempts to tailor a bandgap either by quantum confinement or by chemical functionalization failed to produce a semiconductor with a large enough band gap and a sufficient mobility. It is well known that by evaporating silicon from commercial electronics grade silicon carbide crystals an epitaxial graphene la… ▽ More Graphene nanoelectronics potential was limited by the lack of an intrinsic bandgap[1] and attempts to tailor a bandgap either by quantum confinement or by chemical functionalization failed to produce a semiconductor with a large enough band gap and a sufficient mobility. It is well known that by evaporating silicon from commercial electronics grade silicon carbide crystals an epitaxial graphene layer forms on the surfaces [2]. The first epigraphene layer to form on the silicon terminated face, known as the buffer layer, is insulating. It is chemically bonded to the SiC and spectroscopic measurements [3] have identified semiconducting signatures on the microscopic domains. However, the bonding to the SiC is disordered and the mobilities are small. Here we demonstrate a quasi-equilibrium annealing method that produces macroscopic atomically flat terraces covered with a well ordered epigraphene buffer layer that has a 0.6 eV bandgap. Room temperature mobilities exceed 5000 cm2/Vs which is much larger than silicon and 20 times larger than the phonon scattering imposed limit of current 2D semiconductors. Critical for nanotechnology, its lattice is aligned with the SiC substrate, it is chemically, mechanically, and thermally robust, and it can be conventionally patterned and seamlessly connected to semimetallic epigraphene making semiconducting epigraphene ideally suited for nanoelectronics. △ Less

Submitted 23 August, 2023; originally announced August 2023.

Comments: Text and figures + supplemental information

Controllable Weyl nodes and Fermi arcs in a light-irradiated carbon allotrope

Authors: Ruoning Ji, Xianyong Ding, Fangyang Zhan, Xiaoliang Xiao, Jing Fan, Zhen Ning, Rui Wang

Abstract: The precise control of Weyl physics in realistic materials oers a promising avenue to construct accessible topological quantum systems, and thus draw widespread attention in condensed-matter physics. Here, based on rst-principles calculations, maximally localized Wannier functions based tight-binding model, and Floquet theorem, we study the light-manipulated evolution of Weyl physics in a carbon a… ▽ More The precise control of Weyl physics in realistic materials oers a promising avenue to construct accessible topological quantum systems, and thus draw widespread attention in condensed-matter physics. Here, based on rst-principles calculations, maximally localized Wannier functions based tight-binding model, and Floquet theorem, we study the light-manipulated evolution of Weyl physics in a carbon allotrope C6 crystallizing a face-centered orthogonal structure (fco-C6), an ideal Weyl semimetal with two pairs of Weyl nodes, under the irradiation of a linearly polarized light (LPL). We show that the positions of Weyl nodes and Fermi arcs can be accurately controlled by changing light intensity. Moreover, we employ a low-energy eective k p model to understand light-controllable Weyl physics. The results indicate that the symmetry of light-irradiated fco-C6 can be selectively preserved, which guarantees that the light-manipulated Weyl nodes can only move in the highsymmetry plane in momentum space. Our work not only demonstrates the ecacy of employing periodic driving light elds as an ecient approach to manipulate Weyl physics, but also paves a reliable pathway for designing accessible topological states under light irradiation. △ Less

Submitted 21 August, 2023; originally announced August 2023.

High-density single-atom electrocatalytic centers on two-dimensional topological platinum tellurides with Te-vacancy superstructure

Authors: Xin Xu, Xuechun Wang, Shuming Yu, Chenhui Wang, Guowei Liu, Hao Li, Jiangang Yang, Jing Li, Tao Sun, Xiao Hai, Lei Li, Xue Liu, Ying Zhang, Weifeng Zhang, Quan Zhang, Kedong Wang, Nan Xu, Yaping Ma, Fangfei Ming, Ping Cui, Jiong Lu, Zhenyu Zhang, Xudong Xiao

Abstract: Chemical activation of the intrinsically inert basal planes of transition metal dichalcogenides (TMDs) is crucial for developing high-efficiency electrocatalysts for energy technology applications. Here we report the discovery of an efficient TMD-based topological catalyst for hydrogen evolution reaction (HER), containing high-density single-atom reactive centers on a few-layer (7x7)-PtTe2-x super… ▽ More Chemical activation of the intrinsically inert basal planes of transition metal dichalcogenides (TMDs) is crucial for developing high-efficiency electrocatalysts for energy technology applications. Here we report the discovery of an efficient TMD-based topological catalyst for hydrogen evolution reaction (HER), containing high-density single-atom reactive centers on a few-layer (7x7)-PtTe2-x superstructure with a Te-vacancy density of x. Compared with pristine Pt(111), PtTe2, and (2x2)-PtTe2-x, (7x7)-PtTe2-x exhibits superior HER performance owing to its substantially increased density of undercoordinated Pt sites, and displays exceptional catalytic stability when operating at high current densities. Our first-principles calculations confirm that multiple types of undercoordinated Pt sites on (7x7)-PtTe2-x exhibit favorable hydrogen adsorption Gibbs free energies, and that the reactive sites can further increase their population upon increasing hydrogen coverage. Both the (2x2)- and (7x7)-PtTe2-x are also shown to possess nontrivial band topology with robust edge states that may further facilitate HER. △ Less

Submitted 20 June, 2024; v1 submitted 20 July, 2023; originally announced July 2023.

Temperature-Dependent and Magnetism-Controlled Fermi Surface Changes in Magnetic Weyl Semimetals

Authors: Nan Zhang, Xianyong Ding, Fangyang Zhan, Houpu Li, Hongyu Li, Kaixin Tang, Yingcai Qian, Senyang Pan, Xiaoliang Xiao, Jinglei Zhang, Rui Wang, Ziji Xiang, Xianhui Chen

Abstract: The coupling between band structure and magnetism can lead to intricate Fermi surface modifications. Here we report on the comprehensive study of the Shubnikov-de Haas (SdH) effect in two rare-earth-based magnetic Weyl semimetals, NdAlSi and CeAlSi$_{0.8}$Ge$_{0.2}$. The results show that the temperature evolution of topologically nontrivial Fermi surfaces strongly depends on magnetic configuratio… ▽ More The coupling between band structure and magnetism can lead to intricate Fermi surface modifications. Here we report on the comprehensive study of the Shubnikov-de Haas (SdH) effect in two rare-earth-based magnetic Weyl semimetals, NdAlSi and CeAlSi$_{0.8}$Ge$_{0.2}$. The results show that the temperature evolution of topologically nontrivial Fermi surfaces strongly depends on magnetic configurations. In NdAlSi, the SdH frequencies vary with temperature in both the paramagnetic state and the magnetically ordered state with a chiral spin texture, but become temperature independent in the high-field fully polarized state. In CeAlSi$_{0.8}$Ge$_{0.2}$, SdH frequencies are temperature-dependent only in the ferromagnetic state with magnetic fields applied along the $c$ axis. First-principles calculations suggest that the notable temperature and magnetic-configuration dependence of Fermi surface morphology can be attributed to strong exchange coupling between the conduction electrons and local magnetic moments. △ Less

Submitted 30 April, 2023; originally announced May 2023.

Comments: 8 pages, 5 figures

Journal ref: Phys. Rev. Research 5, L022013 (2023)

Tuning multiple Landau Quantization in Transition-Metal Dichalcogenide with Strain

Authors: Zihao Huang, Guoyu Xian, Xiangbo Xiao, Xianghe Han, Guojian Qian, Chengmin Shen, Haitao Yang, Hui Chen, Banggui Liu, Ziqiang Wang, Hong-Jun Gao

Abstract: Landau quantization associated with the quantized cyclotron motion of electrons under magnetic field provides the effective way to investigate topologically protected quantum states with entangled degrees of freedom and multiple quantum numbers. Here we report the cascade of Landau quantization in a strained type-II Dirac semimetal NiTe2 with spectroscopic-imaging scanning tunneling microscopy. Th… ▽ More Landau quantization associated with the quantized cyclotron motion of electrons under magnetic field provides the effective way to investigate topologically protected quantum states with entangled degrees of freedom and multiple quantum numbers. Here we report the cascade of Landau quantization in a strained type-II Dirac semimetal NiTe2 with spectroscopic-imaging scanning tunneling microscopy. The uniform-height surfaces exhibit single-sequence Landau levels (LLs) at a magnetic field originating from the quantization of topological surface state (TSS) across the Fermi level. Strikingly, we reveal the multiple sequence of LLs in the strained surface regions where the rotation symmetry is broken. Firstprinciples calculations demonstrate that the multiple LLs attest to the remarkable lifting of the valley degeneracy of TSS by the in-plane uniaxial or shear strains. Our findings pave a pathway to tune multiple degrees of freedom and quantum numbers of TMDs via strain engineering for practical applications such as high-frequency rectifiers, Josephson diode and valleytronics. △ Less

Submitted 28 April, 2023; originally announced April 2023.

Comments: Nano Letters 23, 3274 (2023)

Correlating Chemical Reaction and Mass Transport in Hydrogen-based Direct Reduction of Iron Oxide

Authors: Xueli Zheng, Subhechchha Paul, Lauren Moghimi, Yifan Wang, Rafael A. Vilá, Fan Zhang, Xin Gao, Junjing Deng, Yi Jiang, Xin Xiao, Chaolumen Wu, Louisa C. Greenburg, Yufei Yang, Yi Cui, Arturas Vailionis, Ivan Kuzmenko, Jan llavsky, Yadong Yin, Yi Cui, Leora Dresselhaus-Marais

Abstract: Steelmaking contributes 8% to the total CO2 emissions globally, primarily due to coal-based iron ore reduction. Clean hydrogen-based ironmaking has variable performance because the dominant gas-solid reduction mechanism is set by the defects and pores inside the mm-nm sized oxide particles that change significantly as the reaction progresses. While these governing dynamics are essential to establi… ▽ More Steelmaking contributes 8% to the total CO2 emissions globally, primarily due to coal-based iron ore reduction. Clean hydrogen-based ironmaking has variable performance because the dominant gas-solid reduction mechanism is set by the defects and pores inside the mm-nm sized oxide particles that change significantly as the reaction progresses. While these governing dynamics are essential to establish continuous flow of iron and its ores through reactors, the direct link between agglomeration and chemistry is still contested due to missing measurements. In this work, we directly measure the connection between chemistry and agglomeration in the smallest iron oxides relevant to magnetite ores. Using synthesized spherical 10-nm magnetite particles reacting in H2, we resolve the formation and consumption of wüstite (FeO) - the step most commonly attributed to agglomeration. Using X-ray scattering and microscopy, we resolve crystallographic anisotropy in the rate of the initial reaction, which becomes isotropic as the material sinters. Complementing with imaging, we demonstrate how the particles self-assemble, subsequently react and sinter into ~100x oblong grains. Our insights into how morphologically uniform iron oxide particles react and agglomerate H2 reduction enable future size-dependent models to effectively describe the multiscale iron ore reduction. △ Less

Submitted 5 March, 2023; v1 submitted 27 February, 2023; originally announced February 2023.

Journal ref: PNAS, 120 (43) e2305097120 (2023)

How to predict critical state: Invariance of Lyapunov exponent in dual spaces

Authors: Tong Liu, Xu Xia

Abstract: The critical state in disordered systems, a fascinating and subtle eigenstate, has attracted a lot of research interest. However, the nature of the critical state is difficult to describe quantitatively. Most of the studies focus on numerical verification, and cannot predict the system in which the critical state exists. In this work, we propose an explicit and universal criterion that for the cri… ▽ More The critical state in disordered systems, a fascinating and subtle eigenstate, has attracted a lot of research interest. However, the nature of the critical state is difficult to describe quantitatively. Most of the studies focus on numerical verification, and cannot predict the system in which the critical state exists. In this work, we propose an explicit and universal criterion that for the critical state Lyapunov exponent should be 0 simultaneously in dual spaces, namely Lyapunov exponent remains invariant under Fourier transform. With this criterion, we exactly predict a specific system hosting a large number of critical states for the first time. Then, we perform numerical verification of the theoretical prediction, and display the self-similarity and scale invariance of the critical state. Finally, we conjecture that there exist some kind of connection between the invariance of the Lyapunov exponent and conformal invariance. △ Less

Submitted 20 September, 2023; v1 submitted 4 February, 2023; originally announced February 2023.

Comments: Comments are welcome

A driven quantum superconducting circuit with multiple tunable degeneracies

Authors: Jayameenakshi Venkatraman, Rodrigo G. Cortinas, Nicholas E. Frattini, Xu Xiao, Michel H. Devoret

Abstract: We present the experimental discovery of multiple simultaneous degeneracies in the spectrum of a Kerr oscillator subjected to a squeezing drive. This squeezing, in combination with the Kerr interaction creates an effective static two-well potential in the frame rotating at half the frequency of the sinusoidal driving force. Remarkably, these degeneracies can be turned on-and-off on demand, and the… ▽ More We present the experimental discovery of multiple simultaneous degeneracies in the spectrum of a Kerr oscillator subjected to a squeezing drive. This squeezing, in combination with the Kerr interaction creates an effective static two-well potential in the frame rotating at half the frequency of the sinusoidal driving force. Remarkably, these degeneracies can be turned on-and-off on demand, and their number is tunable. We find that when the detuning $Δ$ between the frequency of the oscillator and characteristic frequency of the drive equals an even multiple of the Kerr coefficient $K$, $Δ/K = 2m$, the oscillator displays $m + 1$ exact, parity-protected, spectral degeneracies, insensitive to the drive amplitude. The degeneracies stem from the unusual destructive interference of tunnel paths in the classically forbidden region of the double well static effective potential that models our experiment. Exploiting this interference, we measure a peaked enhancement of the incoherent well-switching lifetime creating a super-protected cat qubit in the ground state manifold of our oscillator. {Our results demonstrate the relationship between degeneracies and noise protection in quantum systems. △ Less

Submitted 10 May, 2023; v1 submitted 8 November, 2022; originally announced November 2022.

On the static effective Lindbladian of the squeezed Kerr oscillator

Authors: Jayameenakshi Venkatraman, Xu Xiao, Rodrigo G. Cortiñas, Michel H. Devoret

Abstract: We derive the static effective Lindbladian beyond the rotating wave approximation (RWA) for a driven nonlinear oscillator coupled to a bath of harmonic oscillators. The associated dissipative effects may explain orders of magnitude differences between the predictions of the ordinary RWA model and results from recent superconducting circuits experiments on the Kerr-cat qubit. The higher-order dissi… ▽ More We derive the static effective Lindbladian beyond the rotating wave approximation (RWA) for a driven nonlinear oscillator coupled to a bath of harmonic oscillators. The associated dissipative effects may explain orders of magnitude differences between the predictions of the ordinary RWA model and results from recent superconducting circuits experiments on the Kerr-cat qubit. The higher-order dissipators found in our calculations have important consequences for quantum error-correction protocols and parametric processses. △ Less

Submitted 23 September, 2022; v1 submitted 22 September, 2022; originally announced September 2022.

Observation of the Surface Layer of Lithium Metal using In Situ Spectroscopy

Authors: Ambrose Seo, Andrew Meyer, Sujan Shrestha, Ming Wang, Xingcheng Xiao, Yang-Tse Cheng

Abstract: We have investigated the surface of lithium metal using x-ray photoemission spectroscopy and optical spectroscopic ellipsometry. Even if we prepare the surface of lithium metal rigorously by chemical cleaning and mechanical polishing inside a glovebox, both spectroscopic investigations show the existence of a few tens of nanometer-thick surface layers, consisting of lithium oxides and lithium carb… ▽ More We have investigated the surface of lithium metal using x-ray photoemission spectroscopy and optical spectroscopic ellipsometry. Even if we prepare the surface of lithium metal rigorously by chemical cleaning and mechanical polishing inside a glovebox, both spectroscopic investigations show the existence of a few tens of nanometer-thick surface layers, consisting of lithium oxides and lithium carbonates. When lithium metal is exposed to room air (~50% moisture), in situ real-time monitoring of optical spectra indicates that the surface layer grows at a rate of approximately 24 nm/min, presumably driven by an interface-controlled process. Our results hint that surface-layer-free lithium metals are formidable to achieve by a simple cleaning/polishing method, suggesting that the initial interface between lithium metal electrodes and solid-state electrolytes in fabricated lithium metal batteries can differ from an ideal lithium/electrolyte contact. △ Less

Submitted 21 September, 2022; originally announced September 2022.

Comments: 4 pages, 3 figures

Journal ref: Appl. Phys. Lett. 120, 211602 (2022)

Strain tunability of perpendicular magnetic anisotropy in van der Waals ferromagnets VI3

Authors: Xi Zhang, Le Wang, Huimin Su, Xiuquan Xia, Cai Liu, Junhao Lin, Mingyuan Huang, Yingchun Cheng, Jia-Wei Mei, Jun-Feng Dai

Abstract: Layered ferromagnets with high coercivity have special applications in nanoscale memory elements in electronic circuits, such as data storage. Therefore, searching for new hard ferromagnets and effectively tuning or enhancing the coercivity are the hottest topics in layered magnets today. Here, we report a strain tunability of perpendicular magnetic anisotropy in van der Waals (vdW) ferromagnets V… ▽ More Layered ferromagnets with high coercivity have special applications in nanoscale memory elements in electronic circuits, such as data storage. Therefore, searching for new hard ferromagnets and effectively tuning or enhancing the coercivity are the hottest topics in layered magnets today. Here, we report a strain tunability of perpendicular magnetic anisotropy in van der Waals (vdW) ferromagnets VI3 using magnetic circular dichroism measurements. For an unstrained flake, the M-H curve shows a rectangular-shaped hysteresis loop with perpendicular magnetic anisotropy and a large coercivity (up to 1.775 T at 10 K). Furthermore, the coercivity can be enhanced to a maximum of 2.6 T at 10 K under a 2.9% in-plane tensile strain. Our DFT calculations show that the magnetic anisotropy energy (MAE) can be dramatically increased after applying an in-plain tensile strain, which contributes to the enhancement of coercivity in the VI3 flake. Meanwhile, the strain tunability on the coercivity of CrI3, with a similar crystal structure, is limited. The main reason is the strong spin-orbital coupling in V3+ in VI6 octahedra in comparison with that in Cr3+. The strain tunability of coercivity in VI3 flakes highlights its potential for integration into vdW heterostructures, paving the way toward nanoscale spintronic devices and applications in the future. △ Less

Submitted 7 August, 2022; originally announced August 2022.

The role of receptor uniformity in multivalent binding

Authors: Xiuyang Xia, Ge Zhang, Massimo Pica Ciamarra, Yang Jiao, Ran Ni

Abstract: Multivalency is prevalent in various biological systems and applications due to the superselectivity that arises from the cooperativity of multivalent binding. Traditionally, it was thought that weaker individual binding would improve the selectivity in multivalent targeting. Here using analytical mean field theory and Monte Carlo simulations, we discover that for receptors that are highly uniform… ▽ More Multivalency is prevalent in various biological systems and applications due to the superselectivity that arises from the cooperativity of multivalent binding. Traditionally, it was thought that weaker individual binding would improve the selectivity in multivalent targeting. Here using analytical mean field theory and Monte Carlo simulations, we discover that for receptors that are highly uniformly distributed, the highest selectivity occurs at an intermediate binding energy and can be significantly greater than the weak binding limit. This is caused by an exponential relationship between the bound fraction and receptor concentration, which is influenced by both the strength and combinatorial entropy of binding. Our findings not only provide new guidelines for the rational design of biosensors using multivalent nano-particles but also introduce a new perspective in understanding biological processes involving multivalency. △ Less

Submitted 11 April, 2023; v1 submitted 26 June, 2022; originally announced June 2022.

Comments: Accepted in JACS Au

Eigenvalues restricted by Lyapunov exponent of eigenstates

Authors: Tong Liu, Xu Xia

Abstract: We point out that the Lyapunov exponent of the eigenstate places restrictions on the eigenvalue. Consequently, with regard to non-Hermitian systems, even without any symmetry, the non-conservative Hamiltonians can exhibit real spectra as long as Lyapunov exponents of eigenstates inhibit imaginary parts of eigenvalues. Our findings open up a new route to study non-Hermitian physics. We point out that the Lyapunov exponent of the eigenstate places restrictions on the eigenvalue. Consequently, with regard to non-Hermitian systems, even without any symmetry, the non-conservative Hamiltonians can exhibit real spectra as long as Lyapunov exponents of eigenstates inhibit imaginary parts of eigenvalues. Our findings open up a new route to study non-Hermitian physics. △ Less

Submitted 20 June, 2022; originally announced June 2022.

Entropy driven thermo-gelling vitrimer

Authors: Xiuyang Xia, Peilin Rao, Juan Yang, Massimo Pica Ciamarra, Ran Ni

Abstract: Thermo-gelling polymers have been envisioned as promising smart biomaterials but limited to their weak mechanical and thermodynamic stabilities. Here we propose a new thermo-gelling vitrimer, which remains at a liquid state because of the addition of protector molecules preventing the crosslinking, and with increasing temperature, an entropy driven crosslinking occurs to induce the sol-gel transit… ▽ More Thermo-gelling polymers have been envisioned as promising smart biomaterials but limited to their weak mechanical and thermodynamic stabilities. Here we propose a new thermo-gelling vitrimer, which remains at a liquid state because of the addition of protector molecules preventing the crosslinking, and with increasing temperature, an entropy driven crosslinking occurs to induce the sol-gel transition. Moreover, we find that the activation barrier in the metathesis reaction of vitrimers plays an important role, and experimentally one can use catalysts to tune the activation barrier to drive the vitrimer to form an equilibrium gel at high temperature, which is not subject to any thermodynamic instability. We formulate a mean field theory to describe the entropy driven crosslinking of the vitrimer, which agrees quantitatively with computer simulations, and paves the way for design and fabrication of novel vitrimers for biomedical applications. △ Less

Submitted 21 September, 2022; v1 submitted 14 January, 2022; originally announced January 2022.

Comments: Accepted in JACS Au

Electric and magnetic fields tuned spin-polarized topological phases in two-dimensional ferromagnetic MnBi$_4$Te$_7$

Authors: Shi Xiao, Xiaoliang Xiao, Fangyang Zhan, Jing Fan, Xiaozhi Wu, Rui Wang

Abstract: Applying electric or magnetic fields is widely used to not only create and manipulate topological states but also facilitate their observations in experiments. In this work, we show by first-principles calculations and topological analysis that the time-reversal (TR) symmetry-broken quantum spin Hall (QSH) state emerges in a two-dimensional ferromagnetic MnBi$_4$Te$_7$ monolayer. This TR-symmetry… ▽ More Applying electric or magnetic fields is widely used to not only create and manipulate topological states but also facilitate their observations in experiments. In this work, we show by first-principles calculations and topological analysis that the time-reversal (TR) symmetry-broken quantum spin Hall (QSH) state emerges in a two-dimensional ferromagnetic MnBi$_4$Te$_7$ monolayer. This TR-symmetry broken QSH phase possesses a highly tunable nontrivial band gap under an external electric field (or tuning interlayer distance). Furthermore, based on the Wannier-function-based tight-binding approach, we reveal that a topological phase transition from the TR-symmetry broken QSH phase to the quantum anomalous Hall (QAH) phase occurs with the increase of magnetic fields. Besides, we also find that a reverse electric fields can facilitate the realization of QAH phase. Our work not only uncovers the ferromagnetic topological properties the MnBi$_4$Te$_7$ monolayer tuned by electric and magnetic fields, but also can stimulate further applications to spintronics and topological devices. △ Less

Submitted 28 December, 2021; originally announced December 2021.

Comments: 7 pages, 4 figures

Journal ref: PhysRevB 105, 125126 (2022)

Near unity Raman $β$-factor of surface enhanced Raman scattering in a waveguide

Authors: Ming Fu, Mónica P. dS. P. Mota, Xiaofei Xiao, Andrea Jacassi, Nicholas A. Güsken, Yi Li, Ahad Riaz, Stefan A. Maier, Rupert F. Oulton

Abstract: The Raman scattering of light by molecular vibrations offers a powerful technique to 'fingerprint' molecules via their internal bonds and symmetries. Since Raman scattering is weak, methods to enhance, direct and harness it are highly desirable, e.g. through the use of optical cavities, waveguides, and surface enhanced Raman scattering (SERS). While SERS offers dramatic enhancements by localizing… ▽ More The Raman scattering of light by molecular vibrations offers a powerful technique to 'fingerprint' molecules via their internal bonds and symmetries. Since Raman scattering is weak, methods to enhance, direct and harness it are highly desirable, e.g. through the use of optical cavities, waveguides, and surface enhanced Raman scattering (SERS). While SERS offers dramatic enhancements by localizing light within vanishingly small 'hot-spots' in metallic nanostructures, these tiny interaction volumes are only sensitive to few molecules, yielding weak signals that are difficult to detect. Here, we show that SERS from 4-Aminothiophenol (4-ATP) molecules bonded to a plasmonic gap waveguide is directed into a single mode with >99% efficiency. Although sacrificing a confinement dimension, we find 10$^4$ times SERS enhancement across a broad spectral range enabled by the waveguide's larger sensing volume and non-resonant mode. Remarkably, the waveguide-SERS (W-SERS) is bright enough to image Raman transport across the waveguides exposing the roles of nanofocusing and the Purcell effect. Emulating the $β$-factor from laser physics, the near unity Raman $β$-factor observed exposes the SERS technique in a new light and points to alternative routes to controlling Raman scattering. The ability of W-SERS to direct Raman scattering is relevant to Raman sensors based on integrated photonics with applications in gas and bio-sensing as well as healthcare. △ Less

Submitted 22 February, 2022; v1 submitted 22 December, 2021; originally announced December 2021.

Comments: 12 pages, 4 figures in the main article; 12 pages, 7 figures in the supplementary information

Journal ref: Nature Nanotechnology 2022

Low power continuous-wave all-optical magnetic switching in ferromagnetic nanoarrays

Authors: Kilian D. Stenning, Xiaofei Xiao, Holly H. Holder, Jack C. Gartside, Alex Vanstone, Oscar W. Kennedy, Rupert F. Oulton, Will R. Branford

Abstract: All-optical magnetic switching promises ultrafast, high-resolution magnetisation control with the technological attraction of requiring no magnetic field. Existing all-optical switching schemes are driven by ultrafast transient effects, typically requiring power-hungry femtosecond-pulsed lasers and complex magnetic materials. Here, we demonstrate deterministic, all-optical magnetic switching in si… ▽ More All-optical magnetic switching promises ultrafast, high-resolution magnetisation control with the technological attraction of requiring no magnetic field. Existing all-optical switching schemes are driven by ultrafast transient effects, typically requiring power-hungry femtosecond-pulsed lasers and complex magnetic materials. Here, we demonstrate deterministic, all-optical magnetic switching in simple ferromagnetic nanomagnets (Ni$_{81}$Fe$_{19}$, Ni$_{50}$Fe$_{50}$) with sub-diffraction limit dimensions using a focused low-power, linearly-polarised continuous-wave laser. Isolated nanomagnets are switched across a range of dimensions, laser wavelengths and powers. All square-geometry artificial spin ice vertex configurations are written, including ground-state and energetically-unfavourable `monopole-like' states at powers as low as 2.74 mW. Usually, magnetic switching with linearly polarised light is symmetry-forbidden; however, here the laser spot has a similar size to the nanomagnets, producing an absorption distribution dependent on the relative nanoisland-spot displacement. We attribute the observed deterministic switching to the transient dynamics of this asymmetric absorption. No switching is observed in Co samples, suggesting the multi-species nature of NiFe alloys plays a role in reversal. The results presented here usher in cheap, low-power optically-controlled devices with impact across data storage, neuromorphic computation and reconfigurable magnonics. △ Less

Submitted 12 April, 2022; v1 submitted 1 December, 2021; originally announced December 2021.

Reversible Structural Transition of Two-Dimensional Copper Selenide on Cu(111)

Authors: Yuan Zhuang, Yande Que, Chaoqiang Xu, Bin Liu, Xudong Xiao

Abstract: Structural engineering opens a door to manipulating the structures and thus tuning the properties of two-dimensional materials. Here, we report a reversible structural transition in honeycomb CuSe monolayer on Cu(111) through scanning tunneling microscopy (STM) and Auger electron spectroscopy (AES). Direct selenization of Cu(111) gives rise to the formation of honeycomb CuSe monolayers with 1D moi… ▽ More Structural engineering opens a door to manipulating the structures and thus tuning the properties of two-dimensional materials. Here, we report a reversible structural transition in honeycomb CuSe monolayer on Cu(111) through scanning tunneling microscopy (STM) and Auger electron spectroscopy (AES). Direct selenization of Cu(111) gives rise to the formation of honeycomb CuSe monolayers with 1D moiré structures (stripe-CuSe), due to the asymmetric lattice distortions in CuSe induced by the lattice mismatch. Additional deposition of Se combined with post annealing results in the formation of honeycomb CuSe with quasi-ordered arrays of triangular holes (hole-CuSe), namely, the structural transition from stripe-CuSe to hole-CuSe. Further, annealing the hole-CuSe at higher temperature leads to the reverse structural transition, namely from hole-CuSe to stripe-CuSe. AES measurement unravels the Se content change in the reversible structural transition. Therefore, both the Se coverage and annealing temperature play significant roles in the reversible structural transition in CuSe on Cu(111). Our work provides insights in understanding of the structural transitions in 2D materials. △ Less

Submitted 1 October, 2021; originally announced October 2021.

Comments: 17 pages, 5 figures

Magnetic field induced Valley-Polarized Quantum Anomalous Hall Effects in Ferromagnetic van der Waals Heterostructures

Authors: Fangyang Zhan, Baobing Zheng, Xiaoliang Xiao, Jing Fan, Xiaozhi Wu, Rui Wang

Abstract: The valley-polarized quantum anomalous Hall effect (VQAHE) attracts intensive interest since it uniquely combines valleytronics and spintronics with nontrivial band topology. Here, based on first-principles calculations and Wannier-function-based tight-binding (WFTB) model, we reveal that valley-based Hall effects and especially the VQAHE induced by external magnetic fields can occur in two-dimens… ▽ More The valley-polarized quantum anomalous Hall effect (VQAHE) attracts intensive interest since it uniquely combines valleytronics and spintronics with nontrivial band topology. Here, based on first-principles calculations and Wannier-function-based tight-binding (WFTB) model, we reveal that valley-based Hall effects and especially the VQAHE induced by external magnetic fields can occur in two-dimensional (2D) ferromagnetic van der Waals heterostructures (vdWHs). The results show that considerable valley-splitting derived from the Zeeman exchange energy drives these vdWHs generating the valley anomalous Hall effect and then the VQAHE. The chiral edge states and quantized Hall conductance are utilized to confirm the presence of VQAHE. Besides, it is also found that external electric fields (or tuning interlayer distances) can facilitate the realization of VQAHE, and thus we present a phase diagram in a broad parameter regime of magnetic fields and electric fields (or interlayer distances). Our work not only offers a class of ferromagnetic vdWHs to realize various valley-based Hall phases, but also can guide advancements for designing topological devices with spin-valley filtering effects based on the VQAHE. △ Less

Submitted 24 September, 2021; v1 submitted 22 September, 2021; originally announced September 2021.

Comments: 6 pages, 4 figures in the main text

Non-Hermitian Aubry-André model with Power-Law Hopping

Authors: Zhihao Xu, Xu Xia, Shu Chen

Abstract: We study a non-Hermitian AA model with long-range hopping, $1/r^a$, and different choices of quasiperiodic parameters $β$ to be a member of the metallic mean family. We find that when the power-law exponent is in the $a<1$ regime, the system displays a delocalized-to-multifractal (DM) edge in its eigenstate spectrum. For the $a>1$ case, a delocalized-to-localized (DL) edge exists, also called the… ▽ More We study a non-Hermitian AA model with long-range hopping, $1/r^a$, and different choices of quasiperiodic parameters $β$ to be a member of the metallic mean family. We find that when the power-law exponent is in the $a<1$ regime, the system displays a delocalized-to-multifractal (DM) edge in its eigenstate spectrum. For the $a>1$ case, a delocalized-to-localized (DL) edge exists, also called the mobility edge. While a striking feature of the Hermitian AA model with long-range hopping is that the fraction of delocalized states can be obtained from a general sequence manifesting a mathematical feature of the metallic mean family, we find that the DM or DL edge for the non-Hermitian cases is independent of the mathematical feature of the metallic mean family. To understand this difference, we consider a specific case of the non-Hermitian long-range AA model with $a=2$, for which we can apply the Sarnak method to analytically derive its localization transition points and the exact expression of the DL edge. Our analytical result clearly demonstrates that the mobility edge is independent of the quasi-periodic parameter $β$, which confirms our numerical result. Finally, an optical setup is proposed to realize the non-Hermitian long-range AA model. △ Less

Submitted 22 December, 2021; v1 submitted 5 September, 2021; originally announced September 2021.

Comments: 9 pages, 5 figures

Journal ref: Phys. Rev. B 104, 224204 (2021)

Real-complex transition driven by quasiperiodicity: a new universality class beyond $\mathcal{PT}$ symmetric one

Authors: Tong Liu, Xu Xia

Abstract: We study a one-dimensional lattice model subject to non-Hermitian quasiperiodic potentials. Firstly, we strictly demonstrate that there exists an interesting dual mapping relation between $|a|<1$ and $|a|>1$ with regard to the potential tuning parameter $a$. The localization property of $|a|<1$ can be directly mapping to that of $|a|>1$, the analytical expression of the mobility edge of $|a|>1$ is… ▽ More We study a one-dimensional lattice model subject to non-Hermitian quasiperiodic potentials. Firstly, we strictly demonstrate that there exists an interesting dual mapping relation between $|a|<1$ and $|a|>1$ with regard to the potential tuning parameter $a$. The localization property of $|a|<1$ can be directly mapping to that of $|a|>1$, the analytical expression of the mobility edge of $|a|>1$ is therefore obtained through spectral properties of $|a|<1$. More impressive, we prove rigorously that even if the phase $θ\neq 0$ in quasiperiodic potentials, the model becomes non-$\mathcal{PT}$ symmetric, however, there still exists a new type of real-complex transition driven by non-Hermitian disorder, which is a new universality class beyond $\mathcal{PT}$ symmetric class. △ Less

Submitted 25 August, 2021; v1 submitted 22 August, 2021; originally announced August 2021.

On the static effective Hamiltonian of a rapidly driven nonlinear system

Authors: Jayameenakshi Venkatraman, Xu Xiao, Rodrigo G. Cortiñas, Alec Eickbusch, Michel H. Devoret

Abstract: We present a recursive formula for the computation of the static effective Hamiltonian of a system under a fast-oscillating drive. Our analytical result is well-suited to symbolic calculations performed by a computer and can be implemented to arbitrary order, thus overcoming limitations of existing time-dependent perturbation methods and allowing computations that were impossible before. We also p… ▽ More We present a recursive formula for the computation of the static effective Hamiltonian of a system under a fast-oscillating drive. Our analytical result is well-suited to symbolic calculations performed by a computer and can be implemented to arbitrary order, thus overcoming limitations of existing time-dependent perturbation methods and allowing computations that were impossible before. We also provide a simple diagrammatic tool for calculation and treat illustrative examples. By construction, our method applies directly to both quantum and classical systems; the difference is left to a low-level subroutine. This aspect sheds light on the relationship between seemingly disconnected independently developed methods in the literature and has direct applications in quantum engineering. △ Less

Submitted 12 January, 2022; v1 submitted 5 August, 2021; originally announced August 2021.

Comments: 5 pages, 3 figures