Showing 1–50 of 104 results for author: Tian, J

Dimensional crossover and local strain induced deflection of the spin spiral state in multiferroic NiI2

Authors: Tianxing Jiang, Lianchuang Li, Haiyan Zhu, Hongyu Wang, Junchao Tian, Wenzhao Wang, Weiyi Pan, Haitao Wang, Changlin Zheng, Hongjun Xiang, Changsong Xu, Donglai Feng, Tong Zhang

Abstract: Low-dimensional multiferroics hold great promise for integrated magnetoelectric devices. Spin spiral state has recently been shown to induce ferroelectricity in single-layer van der Waals (vdW) material NiI2. However, how this state evolves and can be tuned towards the two-dimensional limit remain unclear. Here, we combine spin-polarized scanning tunneling microscopy, layer-by-layer film growth, a… ▽ More Low-dimensional multiferroics hold great promise for integrated magnetoelectric devices. Spin spiral state has recently been shown to induce ferroelectricity in single-layer van der Waals (vdW) material NiI2. However, how this state evolves and can be tuned towards the two-dimensional limit remain unclear. Here, we combine spin-polarized scanning tunneling microscopy, layer-by-layer film growth, and multi-scale theoretical modeling to investigate the spin spirals in NiI2 thin films. As the film thickness increases from 1 to 7 monolayers, we observed a continuous increase of spin-spiral wavelength and a rotation of wavevector from near [110] to [1-10] direction, which evidences a dimensional crossover primarily driven by enhanced interlayer exchange energy. Moreover, we find that the film wrinkles can cause deflection of the spin spiral wavevector, which is caused by local curvature induced modification of exchange interactions. Our findings establish thickness and local strain as two tuning methods for engineering non-collinear helical magnetism and accompanied electric polarization in vdW multiferroics. △ Less

Submitted 12 August, 2026; originally announced August 2026.

Journal ref: Science Bulletin 2026

A d-Electron Heavy-Fermion-Like Superconductor with Frustration-Induced Flat Bands

Authors: Chaoguo Wang, Hyeonhu Bae, Jiaqi Tian, Qing-Ping Ding, Yongbin Lee, Yuji Furukawa, Shannon Gould, Brianna R. Billingsley, Si Athena Chen, Matthias Frontzek, Tai Kong, Binghai Yan, Sheng Ran, Xin Gui

Abstract: Heavy-fermion superconductors are mostly associated with f-electron materials with Kondo lattices, while known d-electron heavy-fermion-like systems are often linked to orbital-selective local moments, Hund-metal physics, or a charge-density-wave mechanism. Here we report Mo4PtGa17, a noncentrosymmetric itinerant d-electron superconductor with a geometrically frustrated breathing-pyrochlore Mo lat… ▽ More Heavy-fermion superconductors are mostly associated with f-electron materials with Kondo lattices, while known d-electron heavy-fermion-like systems are often linked to orbital-selective local moments, Hund-metal physics, or a charge-density-wave mechanism. Here we report Mo4PtGa17, a noncentrosymmetric itinerant d-electron superconductor with a geometrically frustrated breathing-pyrochlore Mo lattice. Thermodynamic, transport and NMR measurements reveal heavy-fermion-like behavior superconductivity and dominant ferromagnetic spin fluctuations near a ferromagnetic instability. Theoretical calculations identify nearly flat bands, van Hove singularities and Kramers nodal lines near the Fermi energy, derived intrinsically from Mo-4d states and are robust against on-site electronic correlations. These results suggest that the geometrically frustrated lattice in Mo4PtGa17 generates an intriguing electronic structure that enhances the density of states, spin susceptibility and quasiparticle mass. Mo4PtGa17 therefore identifies a unique route to heavy-fermion-like superconductivity in d-electron materials through geometrical frustration, different from the previously reported systems. △ Less

Submitted 15 July, 2026; v1 submitted 10 July, 2026; originally announced July 2026.

Comments: 39 pages, 13 figures, 3 tables

Extracting effective scaling exponents in finite-size hyperuniform systems

Authors: Yuan Liu, Xurui Li, Jianxiang Tian, Xunwang Yan, Ge Zhang

Abstract: Hyperuniform systems strongly suppress long-wavelength density fluctuations, which is quantitatively characterized by the small-wavenumber scaling. In finite samples, however, accurately estimating the hyperuniformity exponent α can be challenging. The inferred value depends strongly on the range of length scales accessible in the measurement, finite-size effects, and the specific characterization… ▽ More Hyperuniform systems strongly suppress long-wavelength density fluctuations, which is quantitatively characterized by the small-wavenumber scaling. In finite samples, however, accurately estimating the hyperuniformity exponent α can be challenging. The inferred value depends strongly on the range of length scales accessible in the measurement, finite-size effects, and the specific characterization method employed, whether based on Fourier-space structure factors, real-space density fluctuations, or dynamical probes such as diffusion spreadability. In particular, the structure-factor method provides the most direct estimate of α, but is sensitive to empirical low-k fitting cutoffs. The number-variance method offers a real-space Class-like diagnosis, but contributes a numerical exponent only when the finite-size data retain Class III-like scaling information. The spreadability method provides a smoother dynamic estimate and reduces configuration-level fluctuations, but requires a physically admissible long-time fitting window. Here, we develop a practical method-aware protocol for robust estimation of the effective scaling exponent α in finite-size hyperuniform point configurations, combining three complementary methods with distinct roles. Our protocol summarizes the method-specific estimates through a joint empirical estimator and reports the internal dispersion among the participating methods to determine the optimal estimate. △ Less

Submitted 17 June, 2026; originally announced June 2026.

Comments: 48 pages, 7 figures

Layer-mediated tuning of spin and valley physics in stacked tetragonal altermagnetic bilayers

Authors: Jianke Tian, Xiaowen Zhou, Gui-Bin Liu

Abstract: As an emerging magnetic phase, altermagnets (AMs) with collinear compensated magnetism in real space and alternating spin splitting in the band structure have attracted widespread attention. Here, based on first-principles calculations, we demonstrate that the layer stacking imposes symmetry constraints on the spin and valley degrees of freedom (DOFs) in an AM bilayer composed of two tetragonal al… ▽ More As an emerging magnetic phase, altermagnets (AMs) with collinear compensated magnetism in real space and alternating spin splitting in the band structure have attracted widespread attention. Here, based on first-principles calculations, we demonstrate that the layer stacking imposes symmetry constraints on the spin and valley degrees of freedom (DOFs) in an AM bilayer composed of two tetragonal altermagnetic monolayers, thereby enabling the tuning of these DOFs through interlayer sliding as well as by an external electric field. Using several representative AM bilayers, we reveal that the [C2||P] and [C2||Mz] symmetries intrinsically enforce spin degeneracy, while the coupling between spin and layer DOFs establishes a general framework for achieving electric field control of spin states. Appropriate interlayer sliding breaks the [C2||Md] symmetry of AM bilayers, thereby giving rise to a spontaneous valley splitting and driving a transition to a fully compensated ferrimagnetic state. Furthermore, owing to the tunable valley splitting induced by interlayer sliding, enhanced tunneling magnetoresistance (TMR) can be realized by AM bilayers. This work highlights the intrinsic correlation among spin, valley, and layer DOFs, offering symmetry-based design principles for layer-based spintronic and valleytronic devices. △ Less

Submitted 22 April, 2026; originally announced April 2026.

Comments: 19 pages, 6figures

Microscopic evidence of spin-driven multiferroicity and topological spin textures in monolayer NiI2

Authors: Haitao Wang, Tianxing Jiang, Weiyi Pan, Xu Wang, Hongyu Wang, Junchao Tian, Lianchuang Li, Dongming Zhao, Qingle Zhang, Chenxi Wang, Ying Yang, Hongjun Xiang, Changsong Xu, Donglai Feng, Tong Zhang

Abstract: In type II multiferroics, noncollinear spin textures are expected to induce electric polarization directly, leading to strong magnetoelectric coupling. Realizing such spin driven multiferroicity in two-dimensional systems, and elucidating the interplay between local spins and electric polarization, are of both fundamental and technological importance. Here, using vectorial spin polarized scanning… ▽ More In type II multiferroics, noncollinear spin textures are expected to induce electric polarization directly, leading to strong magnetoelectric coupling. Realizing such spin driven multiferroicity in two-dimensional systems, and elucidating the interplay between local spins and electric polarization, are of both fundamental and technological importance. Here, using vectorial spin polarized scanning tunneling microscopy, we investigated the spin-driven multiferroicity in monolayer NiI2 at atomic scale. We identify a canted spin-spiral state with fully determined spin rotation plane, accompanied by a 2Q charge modulation. At spin spiral domain walls, we discover topological spin textures that composed of meron/antimeron pairs. These textures are associated with distinct charge pattern and notable band shifts, indicating local bound charges induced by variations of ferroelectricity at domain wall. Our observations are well captured by a realistic spin model incorporating Kitaev interactions and generalized spin-current model of type II multiferroicity. The findings provide microscopic evidence of spin-driven multiferroicity in an extreme 2D system and establish a platform for low-dissipation, electric-field control of topological spin textures. △ Less

Submitted 8 April, 2026; originally announced April 2026.

Comments: 26 pages, 20 figures, supplementary materials included

Journal ref: Phys. Rev. Lett. 136, 026402 (2026)

Benchmarking of Massively Parallel Phase-Field Codes for Directional Solidification

Authors: Jiefu Tian, David Montiel, Kaihua Ji, Trevor Lyons, Jason Landini, Katsuyo Thornton, Alain Karma

Abstract: We present a detailed benchmark comparing two state-of-the-art phase-field implementations for simulating alloy solidification under experimentally relevant conditions. The study investigates the directional solidification of Al-3wt%Cu under high-velocity solidification conditions and SCN-0.46wt% camphor under microgravity conditions from National Aeronautics and Space Administration (NASA) DECLIC… ▽ More We present a detailed benchmark comparing two state-of-the-art phase-field implementations for simulating alloy solidification under experimentally relevant conditions. The study investigates the directional solidification of Al-3wt%Cu under high-velocity solidification conditions and SCN-0.46wt% camphor under microgravity conditions from National Aeronautics and Space Administration (NASA) DECLIC-DSI-R experiments. Both codes, one employing finite-difference discretization with uniform mesh and GPU-acceleration (GPU-PF) and the other one employing finite-element discretization with adaptive-mesh and CPU-parallelization (PRISMS-PF), solve the same quantitative phase-field formulation that incorporates an anti-trapping current for the solidification of dilute alloys. We evaluate the predictions of each code for dendritic morphology, primary spacing, and tip dynamics in both 2D and 3D, as well as their numerical convergence and computational performance. While existing benchmark problems have primarily focused on simplified or small-scale simulations, they do not reflect the computational and modeling challenges posed by employing experimentally relevant time and length scales. Our results provide a practical framework for assessing phase-field code performance as well as validating and facilitating their application in integrated computational materials engineering (ICME) workflows that require integration with realistic experimental data. △ Less

Submitted 29 April, 2026; v1 submitted 10 February, 2026; originally announced February 2026.

Journal ref: Computational Materials Science 269, 114720 (2026)

Mechanisms of Spatiotemporal Damage Evolution in Double Polymer Networks

Authors: Magali Le Goff, Laureano Ortellado, Jiting Tian, Mehdi Bouzid, Jean-Louis Barrat, Kirsten Martens

Abstract: Double polymer networks exhibit a striking enhancement of toughness compared to single networks, yet the microscopic mechanisms governing stress redistribution, damage evolution, and fracture remain incompletely understood. Using large-scale coarse-grained molecular dynamics simulations under uniaxial deformation, we resolve bond scission statistics, local stress redistribution following individua… ▽ More Double polymer networks exhibit a striking enhancement of toughness compared to single networks, yet the microscopic mechanisms governing stress redistribution, damage evolution, and fracture remain incompletely understood. Using large-scale coarse-grained molecular dynamics simulations under uniaxial deformation, we resolve bond scission statistics, local stress redistribution following individual bond-breaking events, and the spatiotemporal evolution of damage in single- and double-network architectures. We show that while the early mechanical response is dominated by the pre-stretched sacrificial network, damage evolution in double networks follows a qualitatively distinct pathway. In contrast to single networks, where anisotropic stress redistribution promotes rapid localization and catastrophic fracture, the presence of a soft matrix in double networks induces a screening of stress redistribution generated by sacrificial bond scission. This screening suppresses correlated rupture events and stabilizes multiple damage zones, leading to a strongly delocalized damage landscape over a broad deformation range. At larger strains, when the matrix becomes load-bearing, damage progressively localizes, ultimately triggering fracture. By isolating the dynamics of individual damage zones, we further demonstrate that matrix-mediated stress screening stabilizes defects and delays localization. Together, these results identify stress-screening-induced damage delocalization as a central microscopic mechanism underlying toughness enhancement in multiple-network elastomers. △ Less

Submitted 8 January, 2026; originally announced January 2026.

Comments: 22 pages, 22 figures

Equation of state for the hard sphere fluids

Authors: Can Cui, Jianxiang Tian

Abstract: Based on the survey of the literatures on the new improvements on the equation of state (EOS) for the hard sphere fluids, we here compare lots of different EOSs and present a very accurate equation of state for this kind of fluids. The new equation is built up on the basis of (1) the best estimated virial coefficients B5-B11 by Tian et al. [ Phys. Chem. Chem. Phys., 2019, 21, 13070] and (2) the ne… ▽ More Based on the survey of the literatures on the new improvements on the equation of state (EOS) for the hard sphere fluids, we here compare lots of different EOSs and present a very accurate equation of state for this kind of fluids. The new equation is built up on the basis of (1) the best estimated virial coefficients B5-B11 by Tian et al. [ Phys. Chem. Chem. Phys., 2019, 21, 13070] and (2) the newest numerical simulation data of the compressibility factor versus the density by Pieprzyk et al. [Phys. Chem. Chem. Phys., 2019, 21, 6886]. Our results show that this equation is accurate in not only the stable density range but also the metastable density range with the proper closest packing fraction pole, and well derives the predictive values of the high order virial coefficients B13-B16. △ Less

Submitted 5 December, 2025; originally announced December 2025.

Comments: 21 pages, 1 figure, 3 tables

Impact of Random Spatial Truncation and Reciprocal-Space Binning on the Detection of Hyperuniformity in Disordered Systems

Authors: Yuan Liu, Xurui Li, Jianxiang Tian, Xunwang Yan, Ge Zhang

Abstract: We study how finite-window sampling (random spatial truncation) and reciprocal-space radial binning influence the detection of hyperuniformity in disordered systems. Using thirteen representative two-dimensional simulation systems (two stealthy hyperuniform systems with distinct constraint parameters and ; hyperuniform Gaussian pair statistics system; six hyperuniform targeted systems with distinc… ▽ More We study how finite-window sampling (random spatial truncation) and reciprocal-space radial binning influence the detection of hyperuniformity in disordered systems. Using thirteen representative two-dimensional simulation systems (two stealthy hyperuniform systems with distinct constraint parameters and ; hyperuniform Gaussian pair statistics system; six hyperuniform targeted systems with distinct alpha=0.5, 0.7, 1.0, 1.3, 1.5, 3.0, random sequential addition system; Poisson points distribution system; Lennard-Jones fluid system and Yukawa fluid system) and two real biological systems (avian photoreceptor patterns and looped leaf vein networks) We find that moderate random spatial truncation (i.e., randomly extracting a smaller subwindow from the original full-field configuration) does not change qualitatively the hyperuniformity classification of the systems. Specifically, disordered hyperuniform systems retain their respective hyperuniformity classes despite a modest reduction in measured hyperuniformity exponent alpha (i.e., reduction in small-k suppression). Moreover, spatial truncation commonly induces configuration-dependent fluctuations of small-k values of S(k). We show that modest reciprocal-space radial pooling (controlled by a binning parameter m) effectively smooths such spurious wiggles without changing the hyperuniformity class. Practical guidelines for choosing m, cross-checking spectral fits with the local number variance scaling, and increasing effective sampling are provided. These results provide concrete, low-cost and effective methodology for robust spectral detection of hyperuniformity in finite and truncated datasets which abound in experimental systems. △ Less

Submitted 6 March, 2026; v1 submitted 18 November, 2025; originally announced November 2025.

Comments: 16 pages, 6 figures, 5 tables

Journal ref: J. Chem. Phys. 164, 094102 (2026)

Spatiotemporal Raman Probing of Molecular Transport in sub-2-nm Plasmonic Quasi-2D Nanochannels

Authors: Haoran Liu, Zihe Jiang, Zhiwei Hu, Banghuan Zhang, Tao He, Xiaohui Dong, Chaowei Sun, Jun Tian, Wei Jiang, Huatian Hu, Wen Chen, Hongxing Xu

Abstract: Capturing molecular dynamics in nanoconfined channels with high spatiotemporal resolution is a key challenge in nanoscience, crucial for advancing catalysis, energy conversion, and molecular sensing. Bottom-up ultrathin plasmonic nanogaps, such as nanoparticle-on-mirror (NPoM) structures, are ideal for ultrasensitive probing due to their extreme light confinement, but their perceived sealed geomet… ▽ More Capturing molecular dynamics in nanoconfined channels with high spatiotemporal resolution is a key challenge in nanoscience, crucial for advancing catalysis, energy conversion, and molecular sensing. Bottom-up ultrathin plasmonic nanogaps, such as nanoparticle-on-mirror (NPoM) structures, are ideal for ultrasensitive probing due to their extreme light confinement, but their perceived sealed geometry has cast doubt on the existence of accessible transport pathways. Here, counterintuitively, we demonstrate that ubiquitous ligand-capped NPoM-type nanogaps can form a natural quasi-two-dimensional nanochannel, supporting molecular transport over unprecedented length scales ($\gtrsim5$ $μ$m) with an extreme aspect ratio ($>10^3$). Using wavelength-multiplexed Raman spectroscopy, we resolve the underlying centripetal infiltration pathway with a spatial resolving power of $\sim$20 nm. This redefines the NPoM architecture as a sensitive, \textit{in-situ}, all-in-one "transport-and-probe" platform, enabling real-time, reusable monitoring of analyte with $\sim$10$^{-11}$ M. This work establishes a versatile new platform for advancing super-resolved \textit{in-situ} molecular sensing, nanoscale physicochemical studies, and on-chip nanophotofluidics. △ Less

Submitted 30 September, 2025; originally announced September 2025.

Phase field simulation of dendrite growth in solid-state lithium batteries based on mechanical-thermo-electrochemical coupling

Authors: Pengyang Hou, Jiamiao Xie, Jingyang Li, Peng Zhang, Zhaokai Li, Wenqian Hao, Jia Tian, Zhe Wang, Fuzheng Li

Abstract: Solid-state lithium batteries possess numerous advantages, such as high energy density, excellent cycle stability, superior mechanical strength, non-flammability, enhanced safety, and extended service life. These characteristics make them highly suitable for applications in aerospace, new energy vehicles, and portable electronic devices. However, the growth of lithium dendrite at the electrode/ele… ▽ More Solid-state lithium batteries possess numerous advantages, such as high energy density, excellent cycle stability, superior mechanical strength, non-flammability, enhanced safety, and extended service life. These characteristics make them highly suitable for applications in aerospace, new energy vehicles, and portable electronic devices. However, the growth of lithium dendrite at the electrode/electrolyte interface remains a critical challenge, limiting both performance and safety. The growth of lithium dendrites in the electrolyte not only reduces the Coulombic efficiency of the battery but also poses a risk of puncturing the electrolyte, leading to internal short circuits between the anode and cathode. This study is to solve the problem of lithium dendrite growth in solid-state lithium batteries by employing phase-field theory for numerical simulations. A phase-field model is developed by coupling the mechanical stress field, thermal field, and electrochemical field, to investigate the morphology and evolution of lithium dendrites under the condition of different ambient temperatures, external pressures, and their combined effects. The results indicate that higher temperature and greater external pressure significantly suppress lithium dendrite growth, leading to fewer side branches, smoother surfaces, and more uniform electrochemical deposition. Increased external pressure inhibits longitudinal dendrite growth, resulting in a compressed morphology with higher compactness, but at the cost of increased mechanical instability. The combined effect of temperature and pressure exhibits a pronounced inhibitory influence on dendrite growth, with stress concentrating at the dendrite roots. This stress distribution promotes lateral growth, facilitating the formation of flatter and denser lithium deposits. △ Less

Submitted 2 September, 2025; originally announced September 2025.

Comments: 28 pages,9 figures

Journal ref: Acta Phys. Sin., 2025, 74(7): 070201

Graph Learning Metallic Glass Discovery from Wikipedia

Authors: K. -C. Ouyang, S. -Y. Zhang, S. -L. Liu, J. Tian, Y. -H. Li, H. Tong, H. -Y. Bai, W. -H. Wang, Y. -C. Hu

Abstract: Synthesizing new materials efficiently is highly demanded in various research fields. However, this process is usually slow and expensive, especially for metallic glasses, whose formation strongly depends on the optimal combinations of multiple elements to resist crystallization. This constraint renders only several thousands of candidates explored in the vast material space since 1960. Recently,… ▽ More Synthesizing new materials efficiently is highly demanded in various research fields. However, this process is usually slow and expensive, especially for metallic glasses, whose formation strongly depends on the optimal combinations of multiple elements to resist crystallization. This constraint renders only several thousands of candidates explored in the vast material space since 1960. Recently, data-driven approaches armed by advanced machine learning techniques provided alternative routes for intelligent materials design. Due to data scarcity and immature material encoding, the conventional tabular data is usually mined by statistical learning algorithms, giving limited model predictability and generalizability. Here, we propose sophisticated data learning from material network representations. The node elements are encoded from the Wikipedia by a language model. Graph neural networks with versatile architectures are designed to serve as recommendation systems to explore hidden relationships among materials. By employing Wikipedia embeddings from different languages, we assess the capability of natural languages in materials design. Our study proposes a new paradigm to harvesting new amorphous materials and beyond with artificial intelligence. △ Less

Submitted 22 July, 2025; originally announced July 2025.

Comments: 7 figures

Journal ref: AI for Science 1, 025004 (2025)

Constructing material network representations for intelligent amorphous alloys design

Authors: S. -Y. Zhang, J. Tian, S. -L. Liu, H. -M. Zhang, H. -Y. Bai, Y. -C. Hu, W. -H. Wang

Abstract: Designing high-performance amorphous alloys is demanding for various applications. But this process intensively relies on empirical laws and unlimited attempts. The high-cost and low-efficiency nature of the traditional strategies prevents effective sampling in the enormous material space. Here, we propose material networks to accelerate the discovery of binary and ternary amorphous alloys. The ne… ▽ More Designing high-performance amorphous alloys is demanding for various applications. But this process intensively relies on empirical laws and unlimited attempts. The high-cost and low-efficiency nature of the traditional strategies prevents effective sampling in the enormous material space. Here, we propose material networks to accelerate the discovery of binary and ternary amorphous alloys. The network topologies reveal hidden material candidates that were obscured by traditional tabular data representations. By scrutinizing the amorphous alloys synthesized in different years, we construct dynamical material networks to track the history of the alloy discovery. We find that some innovative materials designed in the past were encoded in the networks, demonstrating their predictive power in guiding new alloy design. These material networks show physical similarities with several real-world networks in our daily lives. Our findings pave a new way for intelligent materials design, especially for complex alloys. △ Less

Submitted 22 July, 2025; originally announced July 2025.

Comments: 5 figures

Journal ref: National Science Review 12, nwaf398 (2025)

Probing universal phase diagram of dimensional crossover with an atomic quantum simulator

Authors: Jinyuan Tian, Zhongcheng Yu, Jing Liu, Chi-Kin Lai, Lorenzo Pizzino, Chengyang Wu, Hongmian Shui, Thierry Giamarchi, Hepeng Yao, Xiaoji Zhou

Abstract: Dimensionality is a fundamental concept in physics, which plays a hidden but crucial role in various domains, including condensed matter physics, relativity and string theory, statistical physics, etc. In quantum physics, reducing dimensionality usually enhances fluctuations and leads to novel properties. Owing to these effects, quantum simulators in which dimensionality can be controlled have eme… ▽ More Dimensionality is a fundamental concept in physics, which plays a hidden but crucial role in various domains, including condensed matter physics, relativity and string theory, statistical physics, etc. In quantum physics, reducing dimensionality usually enhances fluctuations and leads to novel properties. Owing to these effects, quantum simulators in which dimensionality can be controlled have emerged as a new area of interest. However, such a platform has only been studied in specific regimes and a universal phase diagram is lacking. Here, we produce an interacting atomic quantum simulator with continuous tunability of anisotropy and temperature, and probe the universal phase diagram of dimensional crossover. At low temperatures, we identify the regimes from quantum three to zero dimensions. By increasing temperature, we observe the non-trivial emergence of a thermal regime situated between the quantum zero and integer dimensions. We show that the quantum-to thermal transition falls into four different universality classes depending on the dimensionality. Surprisingly, we also detect a fifth type where the high-dimensional quantum system can reach the thermal phase by crossing a low-dimensional quantum regime. Our results provide a crucial foundation for understanding the projective condensed matter structures in unconventional dimensions. △ Less

Submitted 23 June, 2025; originally announced June 2025.

Electric Field Control of Spin Orbit Coupling and Circular Photogalvanic Effect in a True Ferrielectric Crystal

Authors: Yunlin Lei, Xinyu Yang, Shouyu Wang, Daliang Zhang, Zitao Wang, Jiayou Zhang, Yihao Yang, Chuanshou Wang, Tianqi Xiao, Yinxin Bai, Junjiang Tian, Congcong Chen, Yu Han, Shuai Dong, Junling Wang

Abstract: Materials possessing long range ordering of magnetic spins or electric dipoles have been the focus of condensed matter research. Among them, ferri-systems with two sublattices of unequal/noncollinear spins or electric dipoles are expected to combine the properties of ferro- and antiferro-systems, but lack experimental observations in single phase materials. This is particularly true for the ferrie… ▽ More Materials possessing long range ordering of magnetic spins or electric dipoles have been the focus of condensed matter research. Among them, ferri-systems with two sublattices of unequal/noncollinear spins or electric dipoles are expected to combine the properties of ferro- and antiferro-systems, but lack experimental observations in single phase materials. This is particularly true for the ferrielectric system, since the electric dipoles usually can be redefined to incorporate the two sublattices into one, making it indistinguishable from ferroelectric. This raises doubts about whether or not ferrielectricity can be considered as an independent ferroic order. Here we report the observation of true ferrielectric behaviors in a hybrid single crystal (MV)[SbBr5] (MV2+ = N,N'-dimethyl-4,4'-bipyridinium or methylviologen), where the two electric dipole sublattices switch asynchronously, thus cannot be reduced to ferroelectric by redefining the unit cell. Furthermore, the complex dipole configuration imparts circularly polarized light sensitivity to the system. An electric field can modulate the non-collinear dipole sublattices and even induce a transition from ferrielectric to ferroelectric state, thereby tuning the helicity-dependent photocurrent. This study opens a new paradigm for the study of true irreducible ferrielectricity (a new class of polar system) and provides an effective approach to the electric field control of spin-orbit coupling and circular photogalvanic effect. △ Less

Submitted 15 June, 2025; originally announced June 2025.

Journal ref: National Science Review, 2025;, nwaf320

Influence of preparation and architecture on the elastic modulus of polymer networks

Authors: Jiting Tian, Jean-Louis Barrat, Walter Kob

Abstract: The elastic modulus $G$ of a polymer network depends notably on parameters such as the initial concentration of the monomers before the synthesis ($ρ_0$), the density of the cross-linker, or the topology of the network. Understanding how these factors influence the stiffness of the sample is hampered by the fact that in experiments it is difficult to tune them individually. Here we use coarse-grai… ▽ More The elastic modulus $G$ of a polymer network depends notably on parameters such as the initial concentration of the monomers before the synthesis ($ρ_0$), the density of the cross-linker, or the topology of the network. Understanding how these factors influence the stiffness of the sample is hampered by the fact that in experiments it is difficult to tune them individually. Here we use coarse-grained molecular dynamics simulations to study how these quantities, as well as excluded volume interactions, affect the elastic modulus of the network. We find that for a regular diamond network, $G$ is independent of the initial monomer concentration, while for disordered networks (monodisperse or polydisperse) the modulus increases with $ρ_0$, at odds with the classical predictions for rubber elasticity. Analysis of the network structure reveals that, for the disordered networks, defects contribute only weakly to the observed increase, and that instead the $ρ_0$-dependence of $G$ can be rationalized by the presence of a pre-strain in the sample. This pre-strain can be quantified by the topological factor introduced in the affine network theory (ANT). Comparison of the disordered networks with their phantom counterparts reveals that weakly crosslinked systems show a stronger $ρ_0$-dependence of $G$ due to an increase in entanglements at higher $ρ_0$, and that the polydisperse networks contain more entanglements than the monodisperse ones with the same average strand length. Finally we discuss the quantitative application of ANT to the simulated real networks and their phantom counterparts and conclude that the presence of excluded volume effects must be comprehensively taken into account for reaching a qualitative understanding of the mechanical modulus of the disordered networks. △ Less

Submitted 11 June, 2025; originally announced June 2025.

A Multi-Agent Framework Integrating Large Language Models and Generative AI for Accelerated Metamaterial Design

Authors: Jie Tian, Martin Taylor Sobczak, Dhanush Patil, Jixin Hou, Lin Pang, Arunachalam Ramanathan, Libin Yang, Xianyan Chen, Yuval Golan, Xiaoming Zhai, Hongyue Sun, Kenan Song, Xianqiao Wang

Abstract: Metamaterials, renowned for their exceptional mechanical, electromagnetic, and thermal properties, hold transformative potential across diverse applications, yet their design remains constrained by labor-intensive trial-and-error methods and limited data interoperability. Here, we introduce CrossMatAgent -- a novel multi-agent framework that synergistically integrates large language models with st… ▽ More Metamaterials, renowned for their exceptional mechanical, electromagnetic, and thermal properties, hold transformative potential across diverse applications, yet their design remains constrained by labor-intensive trial-and-error methods and limited data interoperability. Here, we introduce CrossMatAgent -- a novel multi-agent framework that synergistically integrates large language models with state-of-the-art generative AI to revolutionize metamaterial design. By orchestrating a hierarchical team of agents -- each specializing in tasks such as pattern analysis, architectural synthesis, prompt engineering, and supervisory feedback -- our system leverages the multimodal reasoning of GPT-4o alongside the generative precision of DALL-E 3 and a fine-tuned Stable Diffusion XL model. This integrated approach automates data augmentation, enhances design fidelity, and produces simulation- and 3D printing-ready metamaterial patterns. Comprehensive evaluations, including CLIP-based alignment, SHAP interpretability analyses, and mechanical simulations under varied load conditions, demonstrate the framework's ability to generate diverse, reproducible, and application-ready designs. CrossMatAgent thus establishes a scalable, AI-driven paradigm that bridges the gap between conceptual innovation and practical realization, paving the way for accelerated metamaterial development. △ Less

Submitted 6 April, 2025; v1 submitted 25 March, 2025; originally announced March 2025.

Inefficiency of orbital Hall effect on the spin torque in transition metal/ferromagnet bilayers

Authors: Yizhuo Song, Jialin Tian, Fanxing Zheng, Jianting Dong, Meng Zhu, Jia Zhang

Abstract: Current induced spin torque is essential and crucial in spintronics. In this work, we systematically investigate the spin torque in transition metal(TM)/ferromagnet(FM) bilayers by using first-principles calculations and taking into account the phonon scattering at room temperature. To examine the spin and orbital Hall contribution, the studied transition metals include 5d heavy metals Pt, W, Au a… ▽ More Current induced spin torque is essential and crucial in spintronics. In this work, we systematically investigate the spin torque in transition metal(TM)/ferromagnet(FM) bilayers by using first-principles calculations and taking into account the phonon scattering at room temperature. To examine the spin and orbital Hall contribution, the studied transition metals include 5d heavy metals Pt, W, Au as well as 3d light metals Ti, V, Cr, Cu etc. We found that in TM/CoFe bilayers with typical 3d and 5d transition metals, the spin torque on CoFe mainly originates from spin Hall mechanism with the magnitude and sign of damping like torque efficiency consistent with the spin Hall conductivity. In TM/Ni bilayers, the spin torque is contributed by three mechanisms including spin and orbital Hall current in TM, as well as self-torque in Ni. The orbital Hall contribution in TM is accompanied by noteworthy opposite self spin torque in Ni, which leads to inapparent torque efficiency in Ti/Ni and V/Ni bilayers. For TM(5d heavy metal)/Ni bilayers, the spin torque induced by orbital Hall and self-torque in Ni nearly cancel each other, which makes the spin torque on Ni still align with that of the spin Hall effect in TM. Our work reveals much less efficient contribution of orbital Hall than spin Hall effect on the spin torque in transition metal/ferromagnet bilayers. △ Less

Submitted 28 March, 2025; v1 submitted 2 March, 2025; originally announced March 2025.

Dimension Effect of Nanocarbon Precursors on Diamond Synthesis and Transformation Mechanism under Extreme Conditions

Authors: Jiaxin Ming, Jingyi Tian, Liming Zhao, Jiayin Li, Guoshuai Du, Lixing Kang, Zheng Hu, Yabin Chen

Abstract: Diamond holds significant promise for a wide range of applications due to its exceptional physicochemical properties. Investigating the controlled diamond preparation from nanocarbon precursors with varying dimensions is crucial to optimize the transition conditions and even elucidate the daunting transformation mechanism, however, this remains outstanding challenge despite considerable effort. He… ▽ More Diamond holds significant promise for a wide range of applications due to its exceptional physicochemical properties. Investigating the controlled diamond preparation from nanocarbon precursors with varying dimensions is crucial to optimize the transition conditions and even elucidate the daunting transformation mechanism, however, this remains outstanding challenge despite considerable effort. Herein, we report the imperative dimension effect of nanocarbon precursors on diamond synthesis and physical mechanism under high temperature and high pressure, by comparing the distinct transition processes of zero-dimensional (0D) carbon nanocages (CNCs) and one-dimensional (1D) carbon nanotubes (CNTs) from conventional graphite. The optical and structural characterizations evidently demonstrated that both 0D CNCs and 1D CNTs first undergo collapse and graphitization, followed by the formation of mixed amorphous carbon with embedded diamond clusters, eventually leading to cubic diamond. The plotted pressure-temperature diagram exhibits the unique dimension effect of carbon nanomaterials to diamond transformation. These results provide valuable insights into the phase transition mechanisms of diamond synthesis and its derivatives under extreme conditions. △ Less

Submitted 16 February, 2025; originally announced February 2025.

Comments: 20 pages, 5 figures

Ferroelectricity in layered bismuth oxide down to 1 nanometer

Authors: Qianqian Yang, Jingcong Hu, Yue-Wen Fang, Yueyang Jia, Rui Yang, Shiqing Deng, Yue Lu, Oswaldo Dieguez, Longlong Fan, Dongxing Zheng, Xixiang Zhang, Yongqi Dong, Zhenlin Luo, Zhen Wang, Huanhua Wang, Manling Sui, Xianran Xing, Jun Chen, Jianjun Tian, Linxing Zhang

Abstract: Atomic-scale ferroelectrics are of great interest for high-density electronics, particularly field-effect transistors, low-power logic, and nonvolatile memories. We devised a film with a layered structure of bismuth oxide that can stabilize the ferroelectric state down to 1 nanometer through samarium bondage. This film can be grown on a variety of substrates with a cost-effective chemical solution… ▽ More Atomic-scale ferroelectrics are of great interest for high-density electronics, particularly field-effect transistors, low-power logic, and nonvolatile memories. We devised a film with a layered structure of bismuth oxide that can stabilize the ferroelectric state down to 1 nanometer through samarium bondage. This film can be grown on a variety of substrates with a cost-effective chemical solution deposition. We observed a standard ferroelectric hysteresis loop down to a thickness of ~1 nanometer. The thin films with thicknesses that range from 1 to 4.56 nanometers possess a relatively large remanent polarization from 17 to 50 microcoulombs per square centimeter. We verified the structure with first-principles calculations, which also pointed to the material being a lone pair-driven ferroelectric material. The structure design of the ultrathin ferroelectric films has great potential for the manufacturing of atomic-scale electronic devices. △ Less

Submitted 16 January, 2025; originally announced January 2025.

Comments: preprint, 27 pages, 5 figures

Journal ref: Science 379(6638): 1218-1224 (2023)

Anisotropic Raman scattering and lattice orientation identification of 2M-WS2

Authors: Sabin Gautam, Sougata Mardanya, Joseph McBride, A K M Manjur Hossain, Qian Yang, Wenyong Wang, John Ackerman, Brian M. Leonard, Sugata Chowdhury, Jifa Tian

Abstract: Anisotropic materials with low symmetries hold significant promise for next-generation electronic and quantum devices. 2M-WS2, a candidate for topological superconductivity, has garnered considerable interest. However, a comprehensive understanding of how its anisotropic features contribute to unconventional superconductivity, along with a simple, reliable method to identify its crystal orientatio… ▽ More Anisotropic materials with low symmetries hold significant promise for next-generation electronic and quantum devices. 2M-WS2, a candidate for topological superconductivity, has garnered considerable interest. However, a comprehensive understanding of how its anisotropic features contribute to unconventional superconductivity, along with a simple, reliable method to identify its crystal orientation, remains elusive. Here, we combine theoretical and experimental approaches to investigate angle- and polarization-dependent anisotropic Raman modes of 2M-WS2. Through first-principles calculations, we predict and analyze phonon dispersion and lattice vibrations of all Raman modes in 2M-WS2. We establish a direct correlation between their anisotropic Raman spectra and high-resolution transmission electron microscopy images. Finally, we demonstrate that anisotropic Raman spectroscopy can accurately determine the crystal orientation and twist angle between two stacked 2M-WS2 layers. Our findings provide insights into the electron-phonon coupling and anisotropic properties of 2M-WS2, paving the way for the use of anisotropic materials in advanced electronic and quantum devices. △ Less

Submitted 1 January, 2025; originally announced January 2025.

Comments: 19 pages, 5 figures

Crossover from Conventional to Unconventional Superconductivity in 2M-WS2

Authors: Piumi Samarawickrama, Joseph McBride, Sabin Gautam, ZhuangEn Fu, Kenji Watanabe, Takashi Taniguchi, Wenyong Wang, Jinke Tang, John Ackerman, Brian M. Leonard, Jifa Tian

Abstract: Leveraging reciprocal-space proximity effect between superconducting bulk and topological surface states (TSSs) offers a promising way to topological superconductivity. However, elucidating the mutual influence of bulk and TSSs on topological superconductivity remains a challenge. Here, we report pioneering transport evidence of a thickness-dependent transition from conventional to unconventional… ▽ More Leveraging reciprocal-space proximity effect between superconducting bulk and topological surface states (TSSs) offers a promising way to topological superconductivity. However, elucidating the mutual influence of bulk and TSSs on topological superconductivity remains a challenge. Here, we report pioneering transport evidence of a thickness-dependent transition from conventional to unconventional superconductivity in 2M-phase WS2 (2M-WS2). As the sample thickness reduces, we see clear changes in key superconducting metrics, including critical temperature, critical current, and carrier density. Notably, while thick 2M-WS2 samples show conventional superconductivity, with an in-plane (IP) upper critical field constrained by the Pauli limit, samples under 20 nm exhibit a pronounced IP critical field enhancement, inversely correlated with 2D carrier density. This marks a distinct crossover to unconventional superconductivity with strong spin-orbit-parity coupling. Our findings underscore the crucial role of sample thickness in accessing topological states in 2D topological superconductors, offering pivotal insights into future studies of topological superconductivity. △ Less

Submitted 9 December, 2024; originally announced December 2024.

Comments: 15 pages, 4 figures

PyMatterSim: a Python Data Analysis Library for Computer Simulations of Materials Science, Physics, Chemistry, and Beyond

Authors: Y. -C. Hu, J. Tian

Abstract: Computer simulation has become one of the most important tools in scientific research in many disciplines. Benefiting from the dynamical trajectories regulated by versatile interatomic interactions, various material properties can be quantitatively characterized at the atomic scale. This greatly deepens our understanding of Nature and provides incredible insights supplementing experimental observa… ▽ More Computer simulation has become one of the most important tools in scientific research in many disciplines. Benefiting from the dynamical trajectories regulated by versatile interatomic interactions, various material properties can be quantitatively characterized at the atomic scale. This greatly deepens our understanding of Nature and provides incredible insights supplementing experimental observations. Hitherto, a plethora of literature discusses the computational discoveries in studying glasses in which positional disorder is inherent in their configurations. Motivated by active research and knowledge sharing, we developed a data analysis library in Python for computational materials science research. We hope to help promote scientific progress and narrow some technical gaps for the wide communities. The toolkit mainly focuses on physical analyses of glassy properties from the open-source simulator LAMMPS. Nevertheless, the code design renders high flexibility, with functionalities extendable to other computational tools. The library provides data-driven insights for different subjects and can be incorporated into advanced machine-learning workflows. The scope of the data analysis methodologies applies not only to materials science but also to physics, chemistry, and beyond. △ Less

Submitted 26 November, 2024; originally announced November 2024.

Comments: 29 pages, 2 figures

Conjugate-dual clusters

Authors: Silei Wang, Jing Tian, Jiayu Li, Tian Gong, Xing Yan, Jijun Zhao, Xin-Gao Gong, Xiao Gu

Abstract: Discovery of clusters with high symmetrical geometry, such as C60 fullerene, always attract lots of interest because of their diverse nature. However, most of such interesting cluster were sporadically discovered, is there any systematic method to explore all possible high symmetrical clusters? Herein, we propose an idea to systematically construct high symmetrical structures based on novel conjug… ▽ More Discovery of clusters with high symmetrical geometry, such as C60 fullerene, always attract lots of interest because of their diverse nature. However, most of such interesting cluster were sporadically discovered, is there any systematic method to explore all possible high symmetrical clusters? Herein, we propose an idea to systematically construct high symmetrical structures based on novel conjugate-dual (co-dual) concept. A co-dual structure would be constructed by conjugately combining one high symmetrical polyhedron and its corresponding dual. In such co-dual structures, the symmetry of the original polyhedron could be kept or even promoted in some special cases. This provides a way to explore high symmetrical polyhedra and leads to a new cluster family, co-dual clusters. In this paper, we have systematically studied the spherical co-dual clusters with one- or two-element shells, and found a series of stable cage-like and core-shell clusters. The co-dual structures would be a new treasure box for hunting novel clusters. △ Less

Submitted 3 November, 2024; originally announced November 2024.

Tunneling current-controlled spin states in few-layer van der Waals magnets

Authors: ZhuangEn Fu, Piumi I. Samarawickrama, John Ackerman, Yanglin Zhu, Zhiqiang Mao, Kenji Watanabe, Takashi Taniguchi, Wenyong Wang, Yuri Dahnovsky, Mingzhong Wu, TeYu Chien, Jinke Tang, Allan H. MacDonald, Hua Chen, Jifa Tian

Abstract: Effective control of magnetic phases in two-dimensional magnets would constitute crucial progress in spintronics, holding great potential for future computing technologies. Here, we report a new approach of leveraging tunneling current as a tool for controlling spin states in CrI3. We reveal that a tunneling current can deterministically switch between spin-parallel and spin-antiparallel states in… ▽ More Effective control of magnetic phases in two-dimensional magnets would constitute crucial progress in spintronics, holding great potential for future computing technologies. Here, we report a new approach of leveraging tunneling current as a tool for controlling spin states in CrI3. We reveal that a tunneling current can deterministically switch between spin-parallel and spin-antiparallel states in few-layer CrI3, depending on the polarity and amplitude of the current. We propose a mechanism involving nonequilibrium spin accumulation in the graphene electrodes in contact with the CrI3 layers. We further demonstrate tunneling current-tunable stochastic switching between multiple spin states of the CrI3 tunnel devices, which goes beyond conventional bi-stable stochastic magnetic tunnel junctions and has not been documented in two-dimensional magnets. Our findings not only address the existing knowledge gap concerning the influence of tunneling currents in controlling the magnetism in two-dimensional magnets, but also unlock possibilities for energy-efficient probabilistic and neuromorphic computing. △ Less

Submitted 24 October, 2024; originally announced October 2024.

Comments: 23 pages, 4 figures

Journal ref: Nature Communications 15 (2024) 3630

Anomalous Tunneling Magnetoresistance Oscillation and Electrically Tunable Tunneling Anisotropic Magnetoresistance in Few-layer CrPS4

Authors: ZhuangEn Fu, Hong-Fei Huang, Piumi Samarawickrama, Kenji Watanabe, Takashi Taniguchi, Wenyong Wang, John Ackerman, Jiadong Zang, Jie-Xiang Yu, Jifa Tian

Abstract: Two-dimensional (2D) van der Waals (vdW) magnets with layer-dependent magnetic states and/or diverse magnetic interactions and anisotropies have attracted extensive research interest. Despite the advances, a notable challenge persists in effectively manipulating the tunneling anisotropic magnetoresistance (TAMR) of 2D vdW magnet-based magnetic tunnel junctions (MTJs). Here, we report the novel and… ▽ More Two-dimensional (2D) van der Waals (vdW) magnets with layer-dependent magnetic states and/or diverse magnetic interactions and anisotropies have attracted extensive research interest. Despite the advances, a notable challenge persists in effectively manipulating the tunneling anisotropic magnetoresistance (TAMR) of 2D vdW magnet-based magnetic tunnel junctions (MTJs). Here, we report the novel and anomalous tunneling magnetoresistance (TMR) oscillations and pioneering demonstration of bias and gate voltage controllable TAMR in 2D vdw MTJs, utilizing few-layer CrPS4. This material, inherently an antiferromagnet, transitions to a canted magnetic order upon application of external magnetic fields. Through TMR measurements, we unveil the novel, layer-dependent oscillations in the tunneling resistance for few-layer CrPS4 devices under both out-of-plane and in-plane magnetic fields, with a pronounced controllability via gate voltage. Intriguingly, we demonstrate that both the polarity and magnitude of TAMR in CrPS4 can be effectively tuned through either a bias or gate voltage. We further elucidate the mechanism behind this electrically tunable TAMR through first-principles calculations. The implications of our findings are far-reaching, providing new insights into 2D magnetism and opening avenues for the development of innovative spintronic devices based on 2D vdW magnets. △ Less

Submitted 24 October, 2024; originally announced October 2024.

Comments: 22 pages, 4 figures

Journal ref: Adv. Physics Res. 2024, 3, 2400052

Possible way to achieve anomalous valley Hall effect by tunable intrinsic piezoelectric polarization in FeO$_2$SiGeN$_2$ monolayer

Authors: Jianke Tian, Jia Li, Hengbo Liu, Yan Li, Ze Liu, Linyang Li, Jun Li, Guodong Liu, Junjie Shi

Abstract: Valley-related multiple Hall effect and piezoelectric response are novel transport characteristics in low-dimensional system, however few studies have reported their coexistence in a single system as well as their coupling relationships. By first-principles calculations, we propose a multifunctional Janus semiconductor, i.e. FeO$_2$SiGeN$_2$ monolayer with large valley polarization of about 120 me… ▽ More Valley-related multiple Hall effect and piezoelectric response are novel transport characteristics in low-dimensional system, however few studies have reported their coexistence in a single system as well as their coupling relationships. By first-principles calculations, we propose a multifunctional Janus semiconductor, i.e. FeO$_2$SiGeN$_2$ monolayer with large valley polarization of about 120 meV and in-plane piezoelectric polarization with d11 of -0.714.03 pm/V. The magnetic anisotropy energy can be significantly regulated by electronic correlation strength and strain, which can be attributed to the change of competition relationship about Fe-3d-resolved magnetic anisotropy energy brought about by external regulatory means. Electronic correlation strength can induce phase transitions in Janus FeO$_2$SiGeN$_2$ monolayer from ferrovalley to quantum anomalous Hall phase, while the half-valley metallic state as the boundary of the phase transition can gererate 100% spin- and valley polarization. The related phase transition mechanism is analyzed based on the two-band strained kp model. The presence of piezoelectric strain coefficients d11 in valleytronic material makes the coupling between charge degrees of freedom and valley degrees of freedom possible, and the intrinsic electric field caused by the in-plane piezoelectric response provide the way to realize piezoelectric anomalous valley Hall effect. This work may pave a way to find a new member of materials with valley-related multiple Hall effect and stimulate further experimental works related to valleytronics and piezotronics. △ Less

Submitted 21 October, 2024; originally announced October 2024.

Piezoelectric Manipulation and Engineering for Layertronics in Two-Dimensional Materials

Authors: Jianke Tian, Jia Li, Hengbo Liu, Yan Li, Ze Liu, Linyang Li, Jun Li, Guodong Liu, Junjie Shi

Abstract: The electronic transport characteristics of two-dimensional (2D) systems have widespread application prospects in the fabrication of multifunctional nanodevices. However, the current research for basic transport phenomena, such as anomalous valley Hall effect (AVHE) and piezoelectric response, is limited to discrete discussion. Here, we theoretically propose a valley-piezoelectricity coupling stra… ▽ More The electronic transport characteristics of two-dimensional (2D) systems have widespread application prospects in the fabrication of multifunctional nanodevices. However, the current research for basic transport phenomena, such as anomalous valley Hall effect (AVHE) and piezoelectric response, is limited to discrete discussion. Here, we theoretically propose a valley-piezoelectricity coupling strategy beyond the existing paradigm to realize AVHE and layer Hall effect (LHE) in ferrovalley (FV) systems, and its essential principle can be extended to general valleytronic materials. Through first-principles calculations, we demonstrate that the large polarized electric field of 2.8*106 (1.67*107) V/m can be induced by 0.1% uniaxial strain in FV 2H-LaHF (1T-LaHF) monolayers. In addition, the microscopic mechanism of interlayer antiferromagnetic (AFM) state of 2H-LaHF bilayer is uncovered by the spin Hamiltonian and super-superexchange (SSE) interaction. Our findings pave the way for new explorations of valley Hall-related effect involving piezoelectricity. △ Less

Submitted 21 October, 2024; originally announced October 2024.

Spin-layer coupling in altermagnets multilayer: a design principle for spintronics

Authors: Jianke Tian, Jia Li, Hengbo Liu, Yan Li, Ze Liu, Linyang Li, Jun Li, Guodong Liu, Junjie Shi

Abstract: The discovery of collinear symmetric-compensated altermagnets (AM) with intrinsic spin splitting provides a route towards energy-efficient and ultrafast device applications. Here, using first-principles calculations and symmetry analysis, we propose a series of AM Cr2SX (X=O, S, Se) monolayer and explore the spin splitting in Cr2SX multilayer. A general design principle for realizing the spin-laye… ▽ More The discovery of collinear symmetric-compensated altermagnets (AM) with intrinsic spin splitting provides a route towards energy-efficient and ultrafast device applications. Here, using first-principles calculations and symmetry analysis, we propose a series of AM Cr2SX (X=O, S, Se) monolayer and explore the spin splitting in Cr2SX multilayer. A general design principle for realizing the spin-layer coupling in odd/even-layer is mapped out based on the comprehensive analysis of spin group symmetry. The spin splitting behavior related with the MzUt, Mz and ML symmetries in AM multilayer can be significantly modulated by magnetic orders, crystal symmetry and external perpendicular gate field (Ez). Due to the spin-compensated bands of sublayers linked by overall Mz and interlayers ML symmetries, the Cr2S2 odd-layer exhibits the unique coexistence of spin splitting and spin degeneracy at high symmetric paths and X/Y valley, respectively. Furthermore, owing to the higher priority of overall ML symmetry compared to interlayers ML symmetry in AM even-layer, the spin-layer coupling of AM multilayer shows strong odd/even-layer dependence. Our work not only offer a new direction for manipulating spin splitting, but also greatly enrich the research on AM monolayer and multilayer. △ Less

Submitted 21 October, 2024; originally announced October 2024.

Precise Control of Process Parameters for >23% Efficiency Perovskite Solar Cells in Ambient Air Using an Automated Device Acceleration Platform

Authors: Jiyun Zhang, Anastasia Barabash, Tian Du, Jianchang Wu, Vincent M. Le Corre, Yicheng Zhao, Shudi Qiu, Kaicheng Zhang, Frederik Schmitt, Zijian Peng, Jingjing Tian, Chaohui Li, Chao Liu, Thomas Heumueller, Larry Lüer, Jens A. Hauch, Christoph J. Brabec

Abstract: Achieving high-performance perovskite photovoltaics, especially in ambient air relies heavily on optimizing process parameters. However, traditional manual methods often struggle to effectively control the key variables. This inherent challenge requires a paradigm shift toward automated platforms capable of precise and reproducible experiments. Herein, we use a fully automated device acceleration… ▽ More Achieving high-performance perovskite photovoltaics, especially in ambient air relies heavily on optimizing process parameters. However, traditional manual methods often struggle to effectively control the key variables. This inherent challenge requires a paradigm shift toward automated platforms capable of precise and reproducible experiments. Herein, we use a fully automated device acceleration platform (DAP) to optimize the process parameters for preparing full perovskite devices using a two-step method in ambient air. Eight process parameters that have the potential to significantly influence device performance are systematically optimized. Specifically, we delve into the impact of the dispense speed of organic ammonium halide, a parameter that is difficult to control manually, on both perovskite film and device performance. Through the targeted design of experiments, we reveal that the dispense speed significantly affects device performance primarily by adjusting the residual PbI2 content in the films. We find that moderate dispense speeds, e.g., 50 μl/s, contribute to top-performance devices. Conversely, too fast or too slow speeds result in devices with relatively poorer performance and lower reproducibility. The optimized parameter set enables us to establish a Standard Operation Procedure (SOP) for additive-free perovskite processing under ambient conditions, which yield devices with efficiencies surpassing 23%, satisfactory reproducibility, and state-of-the-art photo-thermal stability. This research underscores the importance of understanding the causality of process parameters in enhancing perovskite photovoltaic performance. Furthermore, our study highlights the pivotal role of automated platforms in discovering innovative workflows and accelerating the development of high-performing perovskite photovoltaic technologies. △ Less

Submitted 29 March, 2024; originally announced April 2024.

Room-temperature correlated states in twisted bilayer MoS$_2$

Authors: Fanfan Wu, Qiaoling Xu, Qinqin Wang, Yanbang Chu, Lu Li, Jian Tang, Jieying Liu, Jinpeng Tian, Yiru Ji, Le Liu, Yalong Yuan, Zhiheng Huang, Jiaojiao Zhao, Xiaozhou Zan, Kenji Watanabe, Takashi Taniguchi, Dongxia Shi, Gangxu Gu, Yang Xu, Lede Xian, Wei Yang, Luojun Du, Guangyu Zhang

Abstract: Moiré superlattices have emerged as an exciting condensed-matter quantum simulator for exploring the exotic physics of strong electronic correlations. Notable progress has been witnessed, but such correlated states are achievable usually at low temperatures. Here, we report the transport evidences of room-temperature correlated electronic states and layer-hybridized SU(4) Hubbard model simulator i… ▽ More Moiré superlattices have emerged as an exciting condensed-matter quantum simulator for exploring the exotic physics of strong electronic correlations. Notable progress has been witnessed, but such correlated states are achievable usually at low temperatures. Here, we report the transport evidences of room-temperature correlated electronic states and layer-hybridized SU(4) Hubbard model simulator in AB-stacked MoS$_2$ homo-bilayer moiré superlattices. Correlated insulating states at moiré band filling factors v = 1, 2, 3 are unambiguously established in twisted bilayer MoS$_2$. Remarkably, the correlated electronic states can persist up to a record-high critical temperature of over 285 K. The realization of room-temperature correlated states in twisted bilayer MoS$_2$ can be understood as the cooperation effects of the stacking-specific atomic reconstruction and the resonantly enhanced interlayer hybridization, which largely amplify the moiré superlattice effects on electronic correlations. Furthermore, extreme large non-linear Hall responses up to room-temperature are uncovered near correlated insulating states, demonstrating the quantum geometry of moiré flat conduction band. △ Less

Submitted 28 November, 2023; originally announced November 2023.

Comments: 13 pages, 3 figures

Universal Hyperuniform Organization in Looped Leaf Vein Networks

Authors: Yuan Liu, Duyu Chen, Jianxiang Tian, Wenxiang Xu, Yang Jiao

Abstract: Leaf vein network is a hierarchical vascular system that transports water and nutrients to the leaf cells. The thick primary veins form a branched network, while the secondary veins develop closed circuits forming a well-defined cellular structure. Through extensive analysis of a variety of distinct leaf species, we discover that the apparently disordered cellular structures of the secondary vein… ▽ More Leaf vein network is a hierarchical vascular system that transports water and nutrients to the leaf cells. The thick primary veins form a branched network, while the secondary veins develop closed circuits forming a well-defined cellular structure. Through extensive analysis of a variety of distinct leaf species, we discover that the apparently disordered cellular structures of the secondary vein networks exhibit a universal hyperuniform organization and possess a hidden order on large scales. Disorder hyperuniform (DHU) systems lack conventional long-range order, yet they completely suppress normalized large-scale density fluctuations like crystals. Specifically, we find that the distributions of the geometric centers associated with the vein network loops possess a vanishing static structure factor in the zero-wavenumber limit, i.e., $S(k) \sim k^α$, where $α\approx 0.64$, providing an example of class III hyperuniformity in biology. This hyperuniform organization leads to superior efficiency of diffusive transport, as evidenced by the much faster convergence of the time-dependent spreadability $\mathcal{S}(t)$ to its long-time asymptotic limit, compared to that of other uncorrelated or correlated disordered but non-hyperuniform organizations. Our results also have implications for the discovery and design of novel disordered network materials with optimal transport properties. △ Less

Submitted 28 June, 2024; v1 submitted 15 November, 2023; originally announced November 2023.

Comments: 6 pages; 4 figures

Time bound of atomic adiabatic evolution in the accelerated optical lattice

Authors: Guoling Yin, Lingchii Kong, Zhongcheng Yu, Jinyuan Tian, Xuzong Chen, Xiaoji Zhou

Abstract: The accelerated optical lattice has emerged as a valuable technique for the investigation of quantum transport physics and has found widespread application in quantum sensing, including atomic gravimeters and atomic gyroscopes. In our study, we focus on the adiabatic evolution of ultra-cold atoms within an accelerated optical lattice. Specifically, we derive a time bound that delimits the duration… ▽ More The accelerated optical lattice has emerged as a valuable technique for the investigation of quantum transport physics and has found widespread application in quantum sensing, including atomic gravimeters and atomic gyroscopes. In our study, we focus on the adiabatic evolution of ultra-cold atoms within an accelerated optical lattice. Specifically, we derive a time bound that delimits the duration of atomic adiabatic evolution in the oscillating system under consideration. To experimentally substantiate the theoretical predictions, precise measurements to instantaneous band populations were conducted within a one-dimensional accelerated optical lattice, encompassing systematic variations in both lattice's depths and accelerations. The obtained experimental results demonstrate a quantitatively consistent correspondence with the anticipated theoretical expressions. Afterwards, the atomic velocity distributions are also measured to compare with the time bound. This research offers a quantitative framework for the selection of parameters that ensure atom trapped throughout the acceleration process. Moreover, it contributes an experimental criterion by which to assess the adequacy of adiabatic conditions in an oscillating system, thereby augmenting the current understanding of these systems from a theoretical perspective. △ Less

Submitted 13 September, 2023; v1 submitted 20 August, 2023; originally announced August 2023.

Journal ref: Phys. Rev. A 108,033310(2023)

Optimal lattice depth on lifetime of D-band ultracold atoms in a triangular optical lattice

Authors: Hongmian Shui, Chi-Kin Lai, Zhongcheng Yu, Jinyuan Tian, Chengyang Wu, Xuzong Chen, Xiaoji Zhou

Abstract: Ultracold atoms in optical lattices are a flexible and effective platform for quantum precision measurement, and the lifetime of high-band atoms is an essential parameter for the performance of quantum sensors. In this work, we investigate the relationship between the lattice depth and the lifetime of D-band atoms in a triangular optical lattice and show that there is an optimal lattice depth for… ▽ More Ultracold atoms in optical lattices are a flexible and effective platform for quantum precision measurement, and the lifetime of high-band atoms is an essential parameter for the performance of quantum sensors. In this work, we investigate the relationship between the lattice depth and the lifetime of D-band atoms in a triangular optical lattice and show that there is an optimal lattice depth for the maximum lifetime. After loading the Bose Einstein condensate into D-band of optical lattice by shortcut method, we observe the atomic distribution in quasi-momentum space for the different evolution time, and measure the atomic lifetime at D-band with different lattice depths. The lifetime is maximized at an optimal lattice depth, where the overlaps between the wave function of D-band and other bands (mainly S-band) are minimized. Additionally, we discuss the influence of atomic temperature on lifetime. These experimental results are in agreement with our numerical simulations. This work paves the way to improve coherence properties of optical lattices, and contributes to the implications for the development of quantum precision measurement, quantum communication, and quantum computing. △ Less

Submitted 3 August, 2023; originally announced August 2023.

Comments: 11 pages, 7 figures, 2 tables

Journal ref: Opt. Express 31, 26599-26609 (2023)

Observation of first-order quantum phase transitions and ferromagnetism in twisted double bilayer graphene

Authors: Le Liu, Xin Lu, Yanbang Chu, Guang Yang, Yalong Yuan, Fanfan Wu, Yiru Ji, Jinpeng Tian, Kenji Watanabe, Takashi Taniguchi, Luojun Du, Dongxia Shi, Jianpeng Liu, Jie Shen, Li Lu, Wei Yang, Guangyu Zhang

Abstract: Twisted graphene multilayers are highly tunable flatband systems for developing new phases of matter. Thus far, while orbital ferromagnetism has been observed in valley polarized phases, the long-range orders of other correlated phases as well as the quantum phase transitions between different orders mostly remain unknown. Here, we report an observation of Coulomb interaction driven first-order qu… ▽ More Twisted graphene multilayers are highly tunable flatband systems for developing new phases of matter. Thus far, while orbital ferromagnetism has been observed in valley polarized phases, the long-range orders of other correlated phases as well as the quantum phase transitions between different orders mostly remain unknown. Here, we report an observation of Coulomb interaction driven first-order quantum phase transitions and ferromagnetism in twisted double bilayer graphene (TDBG). At zero magnetic field, the transitions are revealed in a series of step-like abrupt resistance jumps with prominent hysteresis loop when either the displacement field (D) or the carrier density (n) is tuned across symmetry-breaking boundary near half filling, indicating a formation of ordered domains. It is worth noting that the good turnability and switching of these states gives a rise to a memory performance with a large on/off ratio. Moreover, when both spin and valley play the roles at finite magnetic field, we observe abundant first-order quantum phase transitions among normal metallic states from charge neutral point, orbital ferromagnetic states from quarter filling, and spin-polarized states from half filling. We interpret these first-order phase transitions in the picture of phase separations and spin domain percolations driven by multi-field tunable Coulomb interactions, in agreement with Lifshitz transition from Hartree-Fock calculations. The observed multi-filed tunable domain structure and its hysteresis resembles the characteristics of multiferroics, revealing intriguing magnetoelectric properties. Our result enriches the correlated phase diagram in TDBG for discovering novel exotic phases and quantum phase transitions, and it would benefit other twisted moiré systems as well. △ Less

Submitted 24 June, 2023; originally announced June 2023.

Journal ref: Phys. Rev. X 13, 031015 (2023)

Atomic Ramsey interferometry with S- and D-band in a triangular optical lattice

Authors: Xiangyu Dong, Chengyang Wu, Zhongcheng Yu, Jinyuan Tian, Zhongkai Wang, Xuzong Chen, Shengjie Jin, Xiaoji Zhou

Abstract: Ramsey interferometers have wide applications in science and engineering. Compared with the traditional interferometer based on internal states, the interferometer with external quantum states has advantages in some applications for quantum simulation and precision measurement. Here, we develop a Ramsey interferometry with Bloch states in S- and D-band of a triangular optical lattice for the first… ▽ More Ramsey interferometers have wide applications in science and engineering. Compared with the traditional interferometer based on internal states, the interferometer with external quantum states has advantages in some applications for quantum simulation and precision measurement. Here, we develop a Ramsey interferometry with Bloch states in S- and D-band of a triangular optical lattice for the first time. The key to realizing this interferometer in two-dimensionally coupled lattice is that we use the shortcut method to construct $π/2$ pulse. We observe clear Ramsey fringes and analyze the decoherence mechanism of fringes. Further, we design an echo $π$ pulse between S- and D-band, which significantly improves the coherence time. This Ramsey interferometer in the dimensionally coupled lattice has potential applications in the quantum simulations of topological physics, frustrated effects, and motional qubits manipulation. △ Less

Submitted 8 November, 2022; originally announced November 2022.

Journal ref: Opt. Express 30, 41437-41446 (2022)

Transport of ultracold atoms in superpositions of S- and D-band states in a moving optical lattice

Authors: Zhongcheng Yu, Jinyuan Tian, Peng Peng, Dekai Mao, Xuzong Chen, Xiaoji Zhou

Abstract: The ultracold atoms in a moving optical lattice with its high controllability is a feasible platform to research the transport phenomenon. Here, we study the transport process of ultracold atoms at the D band in a one-dimensional optical lattice, and perform the manipulation of atomic transport by modulating the population proportion of S band and D band. In the experiment, we first load ultracold… ▽ More The ultracold atoms in a moving optical lattice with its high controllability is a feasible platform to research the transport phenomenon. Here, we study the transport process of ultracold atoms at the D band in a one-dimensional optical lattice, and perform the manipulation of atomic transport by modulating the population proportion of S band and D band. In the experiment, we first load ultracold atoms into the optical lattice using shortcut method, and then accelerate the optical lattice by scanning the phase of lattice beams. The atomic transport in D band and S band is demonstrated respectively. We find the group velocity of atoms in D band is opposite to that in S band. By doping atoms of D band into that of S band, group velocity of the atomic superposition state is modulated, and the manipulation of atomic group velocity from positive to negative is realized. Furthermore, the influence of lattice depth and acceleration on the transport distance are studied. The calculations with multi-orbital simulation are coincident with the experimental results. Our work provides a useful method to manipulate the atomic transport in higher or mixed orbits. △ Less

Submitted 3 February, 2023; v1 submitted 15 October, 2022; originally announced October 2022.

Comments: 9 pages, 6 figures

Journal ref: Phys. Rev. A, 107, 023303 (2023)

Frustrated ferromagnetic transition in AB-stacked honeycomb bilayer

Authors: S. Y. Wang, Y. Wang, S. H. Yan, C. Wang, B. K. Xiang, K. Y. Liang, Q. S. He, K. Watanabe, T. Taniguchi, S. J. Tian, H. C. Lei, W. Ji, Y. Qi, Y. H. Wang

Abstract: In two-dimensional (2D) ferromagnets, anisotropy is essential for the magnetic ordering as dictated by the Mermin-Wagner theorem. But when competing anisotropies are present, the phase transition becomes nontrivial. Here, utilizing highly sensitive susceptometry of scanning superconducting quantum interference device microscopy, we probe the spin correlations of ABC-stacked CrBr3 under zero magnet… ▽ More In two-dimensional (2D) ferromagnets, anisotropy is essential for the magnetic ordering as dictated by the Mermin-Wagner theorem. But when competing anisotropies are present, the phase transition becomes nontrivial. Here, utilizing highly sensitive susceptometry of scanning superconducting quantum interference device microscopy, we probe the spin correlations of ABC-stacked CrBr3 under zero magnetic field. We identify a plateau feature in susceptibility above the critical temperature (Tc) in thick samples. It signifies a crossover regime induced by the competition between easy-plane intralayer exchange anisotropy versus uniaxial interlayer anisotropy. The evolution of the critical behavior from the bulk to 2D shows that the competition between the anisotropies is magnified in the reduced dimension. It leads to a strongly frustrated ferromagnetic transition in the bilayer with fluctuation on the order of Tc, which is distinct from both the monolayer and the bulk. Our observation potentially offers a 2D localized spin system on honeycomb lattice to explore magnetic frustration. △ Less

Submitted 22 July, 2022; originally announced July 2022.

Quantum oscillations in field-induced correlated insulators of a moiré superlattice

Authors: Le Liu, Yanbang Chu, Guang Yang, Yalong Yuan, Fanfan Wu, Yiru Ji, Jinpeng Tian, Rong Yang, Kenji Watanabe, Takashi Taniguchi, Gen Long, Dongxia Shi, Jianpeng Liu, Jie Shen, Li Lu, Wei Yang, Guangyu Zhang

Abstract: We report an observation of quantum oscillations (QOs) in the correlated insulators with valley anisotropy of twisted double bilayer graphene (TDBG). The anomalous QOs are best captured in the magneto resistivity oscillations of the insulators at v = -2, with a period of 1/B and an oscillation amplitude as high as 150 kΩ. The QOs can survive up to ~10 K, and above 12 K, the insulating behaviors ar… ▽ More We report an observation of quantum oscillations (QOs) in the correlated insulators with valley anisotropy of twisted double bilayer graphene (TDBG). The anomalous QOs are best captured in the magneto resistivity oscillations of the insulators at v = -2, with a period of 1/B and an oscillation amplitude as high as 150 kΩ. The QOs can survive up to ~10 K, and above 12 K, the insulating behaviors are dominant. The QOs of the insulator are strongly D dependent: the carrier density extracted from the 1/B periodicity decreases almost linearly with D from -0.7 to -1.1 V/nm, suggesting a reduced Fermi surface; the effective mass from Lifshitz-Kosevich analysis depends nonlinearly on D, reaching a minimal value of 0.1 me at D = ~ -1.0 V/nm. Similar observations of QOs are also found at v = 2, as well as in other devices without graphite gate. We interpret the D sensitive QOs of the correlated insulators in the picture of band inversion. By reconstructing an inverted band model with the measured effective mass and Fermi surface, the density of state at the gap, calculated from thermal broadened Landau levels, agrees qualitatively with the observed QOs in the insulators. While more theoretical understandings are needed in the future to fully account for the anomalous QOs in this moiré system, our study suggests that TDBG is an excellent platform to discover exotic phases where correlation and topology are at play. △ Less

Submitted 14 May, 2023; v1 submitted 20 May, 2022; originally announced May 2022.

A disorder-sensitive emergent vortex phase identified in high-Tc superconductor (Li,Fe)OHFeSe

Authors: Dong Li, Peipei Shen, Jinpeng Tian, Ge He, Shunli Ni, Zhaosheng Wang, Chuanying Xi, Li Pi, Hua Zhang, Jie Yuan, Kui Jin, Evgeny F. Talantsev, Li Yu, Fang Zhou, Jens Hänisch, Xiaoli Dong, Zhongxian Zhao

Abstract: The magneto-transport properties are systematically measured under c-direction fields up to 33 T for a series of single-crystal films of intercalated iron-selenide superconductor (Li,Fe)OHFeSe. The film samples with varying degree of disorder are grown hydrothermally. We observe a magnetic-field-enhanced shoulder-like feature in the mixed state of the high-Tc (Li,Fe)OHFeSe films with weak disorder… ▽ More The magneto-transport properties are systematically measured under c-direction fields up to 33 T for a series of single-crystal films of intercalated iron-selenide superconductor (Li,Fe)OHFeSe. The film samples with varying degree of disorder are grown hydrothermally. We observe a magnetic-field-enhanced shoulder-like feature in the mixed state of the high-Tc (Li,Fe)OHFeSe films with weak disorder, while the feature fades away in the films with enhanced disorder. The irreversibility field is significantly suppressed to lower temperatures with the appearance of the shoulder feature. Based on the experiment and model analysis, we establish a new vortex phase diagram for the weakly disordered high-Tc (Li,Fe)OHFeSe, which features an emergent dissipative vortex phase intermediate between the common vortex glass and liquid phases. The reason for the emergence of this intermediate vortex state is further discussed based on related experiments and models. △ Less

Submitted 6 May, 2022; originally announced May 2022.

Comments: 15 pages, 4 figures

Journal ref: Supercond. Sci. Technol. 35 (2022) 064007

Layer-by-Layer Epitaxy of Multilayer MoS2 Wafers

Authors: Qinqin Wang, Jian Tang, Xiaomei Li, Jinpeng Tian, Jing Liang, Na Li, Depeng Ji, Lede Xian, Yutuo Guo, Lu Li, Qinghua Zhang, Yanbang Chu, Zheng Wei, Yanchong Zhao, Luojun Du, Hua Yu Xuedong Bai, Lin Gu, Kaihui Liu, Wei Yang, Rong Yang, Dongxia Shi, Guangyu Zhang

Abstract: Two-dimensional (2D) semiconductor of MoS2 has great potential for advanced electronics technologies beyond silicon1-9. So far, high-quality monolayer MoS2 wafers10-12 are already available and various demonstrations from individual transistors to integrated circuits have also been shown13-15. In addition to the monolayer, multilayers have narrower band gaps but improved carrier mobilities and cur… ▽ More Two-dimensional (2D) semiconductor of MoS2 has great potential for advanced electronics technologies beyond silicon1-9. So far, high-quality monolayer MoS2 wafers10-12 are already available and various demonstrations from individual transistors to integrated circuits have also been shown13-15. In addition to the monolayer, multilayers have narrower band gaps but improved carrier mobilities and current capacities over the monolayer5,16-18. However, achieving high-quality multilayer MoS2 wafers remains a challenge. Here we report the growth of high quality multilayer MoS2 4-inch wafers via the layer-by-layer epitaxy process. The epitaxy leads to well-defined stacking orders between adjacent epitaxial layers and offers a delicate control of layer numbers up to 6. Systematic evaluations on the atomic structures and electronic properties were carried out for achieved wafers with different layer numbers. Significant improvements on device performances were found in thicker-layer field effect transistors (FETs), as expected. For example, the average field-effect mobility (μFE) at room temperature (RT) can increase from ~80 cm2V-1s-1 for monolayer to ~110/145 cm2V-1s-1 for bilayer/trilayer devices. The highest RT μFE=234.7 cm2V-1s-1 and a record-high on-current densities of 1.704 mAμm-1 at Vds=2 V were also achieved in trilayer MoS2 FETs with a high on/off ratio exceeding 107. Our work hence moves a step closer to practical applications of 2D MoS2 in electronics. △ Less

Submitted 17 March, 2022; originally announced March 2022.

Comments: 13 pages,4 Figures

Journal ref: Natl. Sci. Rev. 9, nwac077 (2022)

Are strongly confined colloids good models for two dimensional liquids?

Authors: Jiting Tian, Walter Kob, Jean-Louis Barrat

Abstract: Quasi-two-dimensional (quasi-2D) colloidal hard-sphere suspensions confined in a slit geometry are widely used as two dimensional (2D) model systems in experiments that probe the glassy relaxation dynamics of 2D systems. However, the question to what extent these quasi-2D systems indeed represent 2D systems is rarely brought up. Here, we use computer simulations that take into account hydrodynamic… ▽ More Quasi-two-dimensional (quasi-2D) colloidal hard-sphere suspensions confined in a slit geometry are widely used as two dimensional (2D) model systems in experiments that probe the glassy relaxation dynamics of 2D systems. However, the question to what extent these quasi-2D systems indeed represent 2D systems is rarely brought up. Here, we use computer simulations that take into account hydrodynamic interactions to show that dense quasi-2D colloidal bi-disperse hard-sphere suspensions exhibit much more rapid diffusion and relaxation than their 2D counterparts at the same area fraction. This difference is induced by the additional vertical space in the quasi-2D samples in which the small colloids can move out of the 2D plane, therefore allowing overlap between particles in the projected trajectories. Surprisingly, this difference in the dynamics can be accounted for if, instead of using the surface density, one characterizes the systems by means of a suitable structural quantity related to the radial distribution function. This implies that in the two geometries the relevant physics for glass-formation is essentially identical. Our results provide not only practical implications on 2D colloidal experiments but also interesting insights into the 3D-to-2D crossover in glass-forming systems. △ Less

Submitted 1 February, 2022; originally announced February 2022.

Journal ref: J. Chem. Phys. 156, 164903 (2022)

Designs for programmable quantum resistance standards based on epitaxial graphene p-n junctions

Authors: Jiuning Hu, Albert F. Rigosi, Mattias Kruskopf, Yanfei Yang, Bi-Yi Wu, Jifa Tian, Alireza R. Panna, Hsin-Yen Lee, Shamith U. Payagala, George R. Jones, Marlin E. Kraft, Dean G. Jarrett, Kenji Watanabe, Takashi Taniguchi, Randolph E. Elmquist, David B. Newell

Abstract: We report the fabrication and measurement of top gated epitaxial graphene p-n junctions where exfoliated hexagonal boron nitride (h-BN) is used as the gate dielectric. The four-terminal longitudinal resistance across a single junction is well quantized at the von Klitzing constant R_K with a relative uncertainty of 10-7. After the exploration of numerous parameter spaces, we summarize the conditio… ▽ More We report the fabrication and measurement of top gated epitaxial graphene p-n junctions where exfoliated hexagonal boron nitride (h-BN) is used as the gate dielectric. The four-terminal longitudinal resistance across a single junction is well quantized at the von Klitzing constant R_K with a relative uncertainty of 10-7. After the exploration of numerous parameter spaces, we summarize the conditions upon which these devices could function as potential resistance standards. Furthermore, we offer designs of programmable electrical resistance standards over six orders of magnitude by using external gating. △ Less

Submitted 10 January, 2022; originally announced January 2022.

Isospin competitions and valley polarized correlated insulators in twisted double bilayer graphene

Authors: Le Liu, Shihao Zhang, Yanbang Chu, Cheng Shen, Yuan Huang, Yalong Yuan, Jinpeng Tian, Jian Tang, Yiru Ji, Rong Yang, Kenji Watanabe, Takashi Taniguchi, Dongxia Shi, Jianpeng Liu, Wei Yang, Guangyu Zhang

Abstract: New phase of matter usually emerges when a given symmetry breaks spontaneously, which can involve charge, spin, and valley degree of freedoms. Here, we report an observation of new correlated insulators evolved from spin polarized states to valley polarized states in AB-BA stacked twisted double bilayer graphene (TDBG). The transition of the isospin polarization is a result of the competition betw… ▽ More New phase of matter usually emerges when a given symmetry breaks spontaneously, which can involve charge, spin, and valley degree of freedoms. Here, we report an observation of new correlated insulators evolved from spin polarized states to valley polarized states in AB-BA stacked twisted double bilayer graphene (TDBG). The transition of the isospin polarization is a result of the competition between spin and valley, driven by the displacement field (D). At a high field |D| > 0.7 V/nm, we observe valley polarized correlated insulators with a big Zeeman g factor of ~10, both at v = 2 in the moiré conduction band and more surprisingly at v = -2 in the moiré valence band. At a medium field |D| < 0.6 V/nm, by contrast, it is a conventional spin polarized correlated insulator at v = 2 and a featureless metal at v = -2. Moreover, we observe a valley polarized Chern insulator with C = 2 emanating at v = 2 in the electron side and a valley polarized Fermi surface around v = -2 in the hole side. The valley Chern insulator with C = 2 is evident from a well quantized Hall conductance plateau at 2e^2/h and correspondingly a vanishing longitudinal component. The valley polarized Fermi surface is topologically trivial with C = 0, and it shows a series of quantized Landau levels with v_LL = 0, 1, 2, 3, 4 and others. These observations are in good agreements with our band and topology calculations. Our results demonstrate a feasible way to realize isospin control and to obtain new phases of matter in TDBG by the displacement field, and might benefit other twisted or non-twisted multilayer systems. △ Less

Submitted 15 December, 2021; originally announced December 2021.

Comments: 13 pages, 4 figures

Journal ref: Nature Communications 13, 3292(2022)

Atomistic Evidence of Nucleation Mechanism for the Direct Graphite-to-Diamond Transformation

Authors: Duan Luo, Liuxiang Yang, Hongxian Xie, Srilok Srinivasan, Jinshou Tian, Subramanian Sankaranarayanan, Ilke Arslan, Wenge Yang, Ho-kwang Mao, Jianguo Wen

Abstract: The direct graphite-to-diamond transformation mechanism has been a subject of intense study and remains debated concerning the initial stages of the conversion, the intermediate phases, and their transformation pathways. Here, we successfully recover samples at early conversion stage by tuning high-pressure/high-temperature conditions and reveal direct evidence supporting the nucleation-growth mec… ▽ More The direct graphite-to-diamond transformation mechanism has been a subject of intense study and remains debated concerning the initial stages of the conversion, the intermediate phases, and their transformation pathways. Here, we successfully recover samples at early conversion stage by tuning high-pressure/high-temperature conditions and reveal direct evidence supporting the nucleation-growth mechanism. Atomistic observations show that intermediate orthorhombic graphite phase mediates the growth of diamond nuclei. Furthermore, we observe that quenchable orthorhombic and rhombohedra graphite are stabilized in buckled graphite at lower temperatures. These intermediate phases are further converted into hexagonal and cubic diamond at higher temperatures following energetically favorable pathways in the order: graphite -> orthorhombic graphite -> hexagonal diamond, graphite -> orthorhombic graphite -> cubic diamond, graphite -> rhombohedra graphite -> cubic diamond. These results significantly improve our understanding of the transformation mechanism, enabling the synthesis of different high-quality forms of diamond from graphite. △ Less

Submitted 26 November, 2021; originally announced November 2021.

Acoustic topological Anderson insulators

Authors: Hui Liu, Boyang Xie, Haonan Wang, Wenwei Liu, Zhancheng Li, Hua Cheng, Jianguo Tian, Zhengyou Liu, Shuqi Chen

Abstract: Recent breakthrough on topological Anderson insulators revealed the breakdown of the traditional perception that sufficiently strong disorder may induce the appearance of topological protected transport states instead of destruction. Although topological Anderson insulators have been observed in various time-reversal symmetry breaking systems, the observation of topological Anderson insulators pro… ▽ More Recent breakthrough on topological Anderson insulators revealed the breakdown of the traditional perception that sufficiently strong disorder may induce the appearance of topological protected transport states instead of destruction. Although topological Anderson insulators have been observed in various time-reversal symmetry breaking systems, the observation of topological Anderson insulators protected by time-reversal symmetry remains scarce, which are considered to be more promising in applications such as the integrated devices. Here, we report the experimental observation of topological Anderson insulator in a two-dimensional bilayer phononic crystal. The robust spin-dependent edge states, as evidence of topological Anderson insulating phase, are observed by introducing on-site disorder. In addition, spin Bott index was computed to identify the topological invariants of the system with disorder, which confirmed the occurrence of disorder-induced topological state. Our results reveal that the impurities and defects introduced in the processing of integrated devices may induce the formation of topological transport states, which are promising for the exploration of new routes for the integration devices design. △ Less

Submitted 11 November, 2021; originally announced November 2021.

Spatially indirect intervalley excitons in bilayer WSe2

Authors: Zhiheng Huang, Yanchong Zhao, Tao Bo, Yanbang Chu, Jinpeng Tian, Le Liu, Yalong Yuan, Fanfan Wu, Jiaojiao Zhao, Lede Xian, Kenji Watanabe, Takashi Taniguchi, Rong Yang, Dongxia Shi, Luojun Du, Zhipei Sun, Sheng Meng, Wei Yang, Guangyu Zhang

Abstract: Spatially indirect excitons with displaced wavefunctions of electrons and holes play a pivotal role in a large portfolio of fascinating physical phenomena and emerging optoelectronic applications, such as valleytronics, exciton spin Hall effect, excitonic integrated circuit and high-temperature superfluidity. Here, we uncover three types of spatially indirect excitons (including their phonon repli… ▽ More Spatially indirect excitons with displaced wavefunctions of electrons and holes play a pivotal role in a large portfolio of fascinating physical phenomena and emerging optoelectronic applications, such as valleytronics, exciton spin Hall effect, excitonic integrated circuit and high-temperature superfluidity. Here, we uncover three types of spatially indirect excitons (including their phonon replicas) and their quantum-confined Stark effects in hexagonal boron nitride encapsulated bilayer WSe2, by performing electric field-tunable photoluminescence measurements. Because of different out-of-plane electric dipole moments, the energy order between the three types of spatially indirect excitons can be switched by a vertical electric field. Remarkably, we demonstrate, assisted by first-principles calculations, that the observed spatially indirect excitons in bilayer WSe2 are also momentum-indirect, involving electrons and holes from Q and K/Γ valleys in the Brillouin zone, respectively. This is in contrast to the previously reported spatially indirect excitons with electrons and holes localized in the same valley. Furthermore, we find that the spatially indirect intervalley excitons in bilayer WSe2 can exhibit considerable, doping-sensitive circular polarization. The spatially indirect excitons with momentum-dark nature and highly tunable circular polarization open new avenues for exotic valley physics and technological innovations in photonics and optoelectronics. △ Less

Submitted 20 August, 2021; originally announced August 2021.

Comments: 14 pages, 4 figures

Large Magneto-Electric Resistance in the Topological Dirac Semimetal alpha Sn

Authors: Yuejie Zhang, Vijaysankar Kalappattil, Chuanpu Liu, Steven S. -L. Zhang, Jinjun Ding, Uppalaiah Erugu, Jifa Tian, Jinke Tang, Mingzhong Wu

Abstract: The spin-momentum locking of surface states in topological quantum materials can produce a resistance that scales linearly with magnetic and electric fields. Such a bilinear magneto-electric resistance (BMER) effect offers a completely new approach for magnetic storage and magnetic field sensing applications. The effects demonstrated so far, however, are relatively weak or for low temperatures. St… ▽ More The spin-momentum locking of surface states in topological quantum materials can produce a resistance that scales linearly with magnetic and electric fields. Such a bilinear magneto-electric resistance (BMER) effect offers a completely new approach for magnetic storage and magnetic field sensing applications. The effects demonstrated so far, however, are relatively weak or for low temperatures. Strong room-temperature BMER effects have now been found in topological Dirac semimetal alpha-Sn thin films. The epitaxial alpha-Sn films were grown by sputtering on silicon substrates. They showed BMER responses that are 10^6 times larger than previously reported at room temperature and also larger than that previously reported at low temperatures. These results represent a major advance toward realistic BMER applications. The data also made possible the first characterization of the three-dimensional, Fermi-level spin texture of topological surface states in alpha-Sn. △ Less

Submitted 7 July, 2021; originally announced July 2021.

Comments: 15 pages, 8 Figures

Optically and magnetically addressable valley pseudospin of interlayer excitons in bilayer MoS2

Authors: Yanchong Zhao, Luojun Du, Shiqi Yang, Jinpeng Tian, Xiaomei Li, Cheng Shen, Jian Tang, Yanbang Chu, Kenji Watanabe, Takashi Taniguchi, Rong Yang, Dongxia Shi, Zhipei Sun, Yu Ye, Wei Yang, Guangyu Zhang

Abstract: Interlayer valley excitons in bilayer MoS2 feature concurrently large oscillator strength and long lifetime, and hence represent an advantageous scenario for valleytronic applications. However, control of valley pseudospin of interlayer excitons in pristine bilayer MoS2, which lies at the heart of valleytronics, has remained elusive. Here we report the observation of highly circularly polarized ph… ▽ More Interlayer valley excitons in bilayer MoS2 feature concurrently large oscillator strength and long lifetime, and hence represent an advantageous scenario for valleytronic applications. However, control of valley pseudospin of interlayer excitons in pristine bilayer MoS2, which lies at the heart of valleytronics, has remained elusive. Here we report the observation of highly circularly polarized photoluminescence from interlayer excitons of bilayer MoS2 with both optical and magnetic addressability. Under excitation of circularly polarized light near exciton resonance, interlayer excitons of bilayer MoS2 show a near-unity, but negative circular polarization. Significantly, by breaking time-reversal symmetry with an out-of-plane magnetic field, a record level of spontaneous valley polarization (7.7%/Tesla) is identified for interlayer excitons in bilayer MoS2. The giant valley polarization of the interlayer excitons in bilayer MoS2, together with the feasibility of electrical/optical/magnetic control and strong oscillator strength, provides a firm basis for the development of next-generation electronic and optoelectronic applications. △ Less

Submitted 1 June, 2021; originally announced June 2021.

Comments: 31 pages, 15 figures

Temperature-linear Resistivity in Twisted Double Bilayer Graphene

Authors: Yanbang Chu, Le Liu, Cheng Shen, Jinpeng Tian, Jian Tang, Yanchong Zhao, Jieying Liu, Yalong Yuan, Yiru Ji, Rong Yang, Kenji Watanabe, Takashi Taniguchi, Dongxia Shi, Fengcheng Wu, Wei Yang, Guangyu Zhang

Abstract: We report an experimental study of carrier density (n), displacement field (D) and twist angle (θ) dependence of temperature (T)-linear resistivity in twisted double bilayer graphene (TDBG). For a large twist angle (θ>1.5°) where correlated insulating states are absent, we observe a T-linear resistivity (with the slope of the order ~10Ω/K) over a wide range of carrier density and its slope decreas… ▽ More We report an experimental study of carrier density (n), displacement field (D) and twist angle (θ) dependence of temperature (T)-linear resistivity in twisted double bilayer graphene (TDBG). For a large twist angle (θ>1.5°) where correlated insulating states are absent, we observe a T-linear resistivity (with the slope of the order ~10Ω/K) over a wide range of carrier density and its slope decreases with increasing of n, in agreement with acoustic phonon scattering model semi-quantitatively. The slope of T-linear resistivity is non-monotonically dependent on the displacement field with a single peak structure. For device with θ~1.23° at which correlated states emerge, the slope of T-linear resistivity is found maximum (~100Ω/K) at the boundary of the halo structure where phase transition occurs, with signatures of continuous phase transition, Planckian dissipation, and the diverging effective mass; these observations are in line with quantum critical behaviors, which might be due to the symmetry-breaking instability at the critical points. Our results shed new light on correlated physics in TDBG and other twisted moiré systems. △ Less

Submitted 27 June, 2022; v1 submitted 12 April, 2021; originally announced April 2021.

Comments: Accepted by Phys. Rev. B, with 20 pages and 15 figures

Journal ref: Phys. Rev. B 106, 035107 (2022)