arXiv subjects

Jiayong Zhang

Publications and source records attributed to Jiayong Zhang.

18 recordsLinked to original sources

arXiv2026

Altermagnetic Proximity Effect

Proximity effects not only complement the conventional methods of designing materials, but also enable realizing properties that are not present in any constituent region of the considered heterostructure. Here we reveal an unexplored altermagnetic proximity effect (AMPE), distinct from its ferromagnetic and antiferromagnetic counterparts. Using first-principles and model analyses of van der Waals heterostructures based on the prototypical altermagnet V$_2$Se$_2$O, we show that its hallmark momentum-alternating spin splitting can be directly imprinted onto adjacent nonmagnetic layers -- a process we term altermagnetization. This is demonstrated in a monolayer PbO through characteristic band splitting and real-space spin densities, with systematic dependence on interlayer spacing and magnetic configuration. We further predict broader AMPE manifestations: Valley-selective spin splitting in a monolayer PbS and a topological superconducting phase in monolayer NbSe$_2$, both inheriting the alternating $k$-space spin texture of the altermagnet. These results establish AMPE not only as a distinct proximity mechanism, but also as a powerful method of using altermagnetism in designing emergent phenomena and versatile applications.

cond-mat.mtrl-sci↗

arXiv2026

Altermagnets Enable Gate-Switchable Helical and Chiral Topological Transport with Spin-Valley-Momentum-Locked Dual Protection

We establish a unified, symmetry-driven framework that combines the alternating spin splitting of altermagnets with valley topology to realize and electrically interconvert helical and chiral topological phases within a single material platform. We first demonstrate a magnetic analogue of the quantum spin Hall effect in altermagnets, hosting helical spin-valley-momentum-locked (SVML) edge states characterized by a composite spin-valley Chern number Csv = 2. Large-scale quantum transport simulations show these SVML edge states exhibit fully quantized spin conductance robust against nonmagnetic and long-range magnetic disorder, reflecting their dual topological protection, while remaining vulnerable to short-range magnetic disorder. Exploiting that the counterpropagating SVML modes are linked by crystal rotation symmetry, we introduce a gate-tunable sublattice-staggered potential that selectively gaps one valley and converts the helical state into a chiral quantum anomalous Hall phase with Csv = 1, robust against all disorder types. Reversing the potential switches the transmitted spin-valley polarization. Our first-principles calculations identify monolayer V2STeO and VO families as realistic platforms supporting both helical and chiral topological phases and their electrical switching. These results establish altermagnets as electrically programmable platforms for robust topological devices across charge, spin, and valley.

cond-mat.mes-hall↗

arXiv2026

Symmetry-Driven Unconventional Magnetoelectric Coupling in Perovskite Altermagnets: From Bulk to the Two-Dimensional Limit

The emergence of altermagnets establishes a new paradigm for multiferroics. Unlike conventional multiferroics relying on direct magnetoelectric coupling, multiferroic altermagnets host a crystal-symmetry-mediated magnetoelectric interaction that is intrinsically more efficient and robust. Among candidate material platforms, layered perovskites are particularly appealing owing to their structural diversity and synthetic versatility. However, magnetoelectric properties at the two-dimensional scale remain largely unexplored, hindering their applicability in miniaturized, highly integrated devices. Here, we systematically investigate the dimensional evolution of ferroelectric polarization and magnetism in perovskite systems through symmetry analysis. We demonstrate that altermagnetism can persist in the two-dimensional limit, yet is strongly constrained by the magnetic configuration-with only C-type antiferromagnetic order supporting it. Based on mode-decomposition calculations, we further reveal that symmetry-restricted multimode couplings simultaneously govern ferroelectric polarization and altermagnetic spin splitting. Finally, combined with first-principles calculations, we propose several strategies to lift the magnetic-configuration constraint, extending the range of viable altermagnetic systems. These results underscore the critical role of dimensionality in symmetry-driven magnetoelectric coupling in perovskite altermagnets and pave the way toward next-generation electrically controlled spintronic and multiferroic devices.

cond-mat.mtrl-sci↗

arXiv2025

Altermagnetoelectric Spin Field Effect Transistor

Spin field-effect transistors (SFETs) are promising candidates for low-power spin-based electronics, yet existing realizations that rely on spin-orbit coupling are constrained by limited material choices and short spin-coherence lengths. Here we propose a different operating principle based on multiferroic altermagnets, in which spin splitting is tuned by an electric field through symmetry control rather than conventional spin-orbit physics. Using an effective model combined with quantum transport simulations, we show that the conductance is determined by the degree of matching between the electrically controlled spin texture of the channel and the fixed spin polarization of ferromagnetic contacts, enabling clear ON and OFF states. Remarkably, we also address a long-standing challenge in multiferroic device design: spintronic channels require metallic carriers, whereas ferroelectricity is usually suppressed in metals. We resolve this conflict by imprinting multiferroic altermagnetism into highly conductive materials via the proximity effect. First-principles calculations for graphene on multiferroic vanadium sulfide halides confirm that graphene acquires a ferroelectrically switchable spin splitting while retaining its metallic character. These results establish a practical route to SFET implementation and identify multiferroic altermagnets as a versatile platform for next-generation spintronic devices.

cond-mat.mtrl-sci↗

arXiv2025

Emergent Multiferroic Altermagnets and Spin Control via Noncollinear Molecular Polarization

Altermagnets, with spin splitting and vanishing magnetization, have been attributed to many fascinating phenomena and potential applications. In particular, integrating ferroelectricity with altermagnetism to enable magnetoelectric coupling and electric control of spin has drawn significant attention. However, its experimental realization and precise spin manipulation remain elusive. Here, by focusing on molecular ferroelectrics, the first discovered ferroelectrics renowned for their highly controllable molecular polarizations and structural flexibility, we reveal that these obstacles can be removed by an emergent multiferroic altermagnets with tunable spin polarization in a large class of fabricated organic materials. Using a symmetry-based design and a tight-binding model, we uncover the underlying mechanism of such molecular ferroelectric altermagnets and demonstrate how noncollinear molecular polarization can switch the spin polarization on and off and even reverse its sign. From the first-principles calculations, we verify the feasibility of these materials in a series of well-established hybrid organic-inorganic perovskites and metal-organic frameworks. Our findings bridge molecular ferroelectrics and altermagnetic spintronics, highlighting an unexplored potential of multifunctional organic multiferroics.

cond-mat.mtrl-sci↗

arXiv2025

Two-Dimensional Ferroelectric Altermagnets: From Model to Material Realization

Multiferroic altermagnets offer new opportunities for magnetoelectric coupling and electrically tunable spintronics. However, due to intrinsic symmetry conflicts between altermagnetism and ferroelectricity, achieving their coexistence, known as ferroelectric altermagnets (FEAM), remains an outstanding challenge, especially in two-dimensional (2D) systems. Here, we propose a universal, symmetry-based design principle for 2D FEAM, supported by tight-binding models and first-principles calculations. We show that lattice distortions can break spin equivalence and introduce the necessary rotation-related symmetry, enabling altermagnetism with electrically reversible spin splitting. Guided by this framework, we identify a family of 2D vanadium oxyhalides and sulfide halides as promising FEAM candidates. In these compounds, pseudo Jahn-Teller distortions and Peierls-like dimerization cooperatively establish the required symmetry conditions. We further propose the magneto-optical Kerr effect as an experimental probe to confirm FEAM and its electric spin reversal. Our findings provide a practical framework for 2D FEAM and advancing electrically controlled spintronic devices.

cond-mat.mtrl-sci↗

arXiv2025

Antiferroelectric Altermagnets: Antiferroelectricity Alters Magnets

Magnetoelectric coupling is crucial for uncovering fundamental phenomena and advancing technologies in high-density data storage and energy-efficient devices. The emergence of altermagnets, which unify the advantages of ferromagnets and antiferromagnets, offers unprecedented opportunities for magnetoelectric coupling. However, electrically tuning altermagnets remains an outstanding challenge. Here, we demonstrate how this challenge can be overcome by using antiferroelectricity and ferroelectricity to modulate the spin splitting in altermagnets, employing a universal, symmetry-based design principle supported by an effective model. We introduce an unexplored class of multiferroics: antiferroelectric altermagnets (AFEAM), where antiferroelectricity and altermagnetism coexist in a single material. From first-principles calculations, we validate the feasibility of AFEAM in well-established van der Waals metal thio(seleno)phosphates and perovskite oxides. We reveal the design of AFEAM ranging from two-dimensional monolayers to three-dimensional bulk structures. Remarkably, even a weak electric field can effectively toggle spin polarization in the AFEAM by switching between antiferroelectric and ferroelectric states. Our findings not only enrich the understanding of magnetoelectric coupling but also pave the way for electrically controlled spintronic and multiferroic devices.

cond-mat.mtrl-sci↗

arXiv2023

Dynamic crystallography reveals spontaneous anisotropy in cubic GeTe

Cubic energy materials such as thermoelectrics or hybrid perovskite materials are often understood to be highly disordered. In GeTe and related IV-VI compounds, this is thought to provide the low thermal conductivities needed for thermoelectric applications. Since conventional crystallography cannot distinguish between static disorder and atomic motions, we develop the energy-resolved variable-shutter pair distribution function technique. This collects structural snapshots with varying exposure times, on timescales relevant for atomic motions. In disagreement with previous interpretations, we find the time-averaged structure of GeTe to be crystalline at all temperatures, but with anisotropic anharmonic dynamics at higher temperatures that resemble static disorder at fast shutter speeds, with correlated ferroelectric fluctuations along the $<$100$>$c direction. We show that this anisotropy naturally emerges from a Ginzburg-Landau model that couples polarization fluctuations through long-range elastic interactions. By accessing time-dependent atomic correlations in energy materials, we resolve the long-standing disagreement between local and average structure probes, and show that spontaneous anisotropy is ubiquitous in cubic IV-VI materials.

cond-mat.mtrl-sci↗

arXiv2022

Metal-insulator phase separation in KTaO3-based two-dimensional electron gas

Electronic phase separation (EPS) originates from an incomplete transformation between electronic phases, causing the inhomogeneous spatial distribution of electronic properties. In the system of two-dimensional electron gas (2DEG), the EPS is usually identified based on a percolative metal-to-superconductor transition. Here, we report a metal-insulator transition (MIT) in KTaO3-based 2DEG with the width of conductive channel decreasing into micrometer scale. Hysteretic resistance-temperature relations are observed due to the competition between metallic and insulating phases, which is tunable by magnetic field. Such a size-dependent MIT effect is attributed to the coexistence and separation of metallic and insulating phases. Combining density functional theory calculation, we propose a theoretical model to simulate the dynamic process of the EPS using the percolation theory, demonstrating the mechanism of size-dependent MIT. Our work suggests a clear and simple 2DEG platform to achieve the spatial coexistence of metallic and insulating phases.

cond-mat.mtrl-sci↗

arXiv2022

Observation of room-temperature ferroelectricity in elemental Te nanowires

Ferroelectrics are essential in low-dimensional memory devices for multi-bit storage and high-density integration. A polar structure is a necessary premise for ferroelectricity, mainly existing in compounds. However, it is usually rare in elemental materials, causing a lack of spontaneous electric polarization. Here, we report an unexpected room-temperature ferroelectricity in few-chain Te nanowires. Out-of-plane ferroelectric loops and domain reversal are observed by piezoresponse force microscopy. Through density functional theory, we attribute the ferroelectricity to the ion-displacement created by the interlayer interaction between lone pair electrons. Ferroelectric polarization can induce a strong field effect on the transport along the Te chain, supporting a self-gated field-effect transistor. It enables a nonvolatile memory with high in-plane mobility, zero supply voltage, multilevel resistive states, and a high on/off ratio. Our work provides new opportunities for elemental ferroelectrics with polar structures and paves a way towards applications such as low-power dissipation electronics and computing-in-memory devices.

cond-mat.mtrl-sci↗

arXiv2020

Study of anharmonicity in Zirconium Hydrides using inelastic neutron scattering and ab-initio computer modeling

The anharmonic phenomena in Zirconium Hydrides and Deuterides, including ε-ZrH2, γ-ZrH, and γ-ZrD, have been investigated from aspects of inelastic neutron scattering (INS) and lattice dynamics calculations within the framework of density functional theory (DFT). The observed multiple sharp peaks below harmonic multi-phonon bands in the experimental spectra of all three materials did not show up in the simulated INS spectra based on the harmonic approximation, indicating the existence of strong anharmonicity in those materials and the necessity of further explanations. We present a detailed study on the anharmonicity of zirconium hydrides/deuterides by exploring the 2D potential energy surface of hydrogen/deuterium atoms, and solving the corresponding 2D single-particle Schrodinger equation to get the eigenfrequencies. The obtained results well describe the experimental INS spectra and show harmonic behavior in the fundamental modes and strong anharmonicity at higher energies.

cond-mat.mtrl-sci↗

arXiv2019

Novel Chern insulators with half-metallic edge states

The central target of spintronics research is to achieve flexible control of highly efficient and spin-polarized electronic currents. Based on first-principles calculations and k-p models, we demonstrate that Cu2S/MnSe heterostructures are a novel type of Chern insulators with half-metallic chiral edge states and a very high Fermi velocity (0.87 * 10^6 m/s). The full spin-polarization of the edge states is found to be robust against the tuning of the chemical potential. Unlike the mechanisms reported previously, this heterostructure has quadratic bands with a normal band order, that is, the p/d-like band is below the s-like band. Charge transfer between the Cu2S moiety and the substrate results in variation in the occupied bands, which together with spin-orbit coupling, triggers the appearance of the topological state in the system. These results imply that numerous ordinary semiconductors with normal band order may convert into Chern insulators with half-metallic chiral edge states through this mechanism, providing a strategy to find a rich variety of materials for dissipationless, 100% spin-polarized and high-speed spintronic devices.

cond-mat.mtrl-sci↗

arXiv2018

Origin of topologically trivial states and topological phase transitions in low-buckled plumbene

Combining tight-binding (TB) models with first-principles calculations, we investigate electronic and topological properties of plumbene. Different from the other two-dimensional (2D) topologically nontrivial insulators in group IVA (from graphene to stanene), low-buckled plumbene is a topologically trivial insulator. The plumbene without spin-orbit coupling exhibits simultaneously two kinds of degeneracies, i.e., quadratic non-Dirac and linear Dirac band dispersions around the Γand K/K' points, respectively. Our TB model calculations show that it is the coupling between the two topological states around the Γand K/K' points that triggers the global topologically trivial property of plumbene. Quantum anomalous Hall effects with Chern numbers of 2 or -2 can be, however, achieved after an exchange field is introduced. When the plumbene is functionalized with ethynyl (PbC2H), quantum spin Hall effects appear due to the breaking of the coupling effect of the local topological states.

cond-mat.mtrl-sci↗

arXiv2018

Giant Spin-valley Polarization and Multiple Hall Effect in Functionalized Bi Monolayers

Valleytronic materials, characterized by local extrema (valley) in their bands, and topological insulators have separately attracted great interest recently. However, the interplay between valleytronic and topological properties in one single system, likely to enable important unexplored phenomena and applications, has been largely overlooked so far. Here, by combining a tight-binding model with first-principles calculations, we find the large-band-gap quantum spin Hall effects (QSHEs) and valley Hall effects (VHEs) appear simultaneously in the Bi monolayers decorated with halogen elements, denoted as Bi2XY (X, Y = H, F, Cl, Br, or I). A staggered exchange field is introduced into the Bi2XY monolayers by transition metal atom (Cr, Mo, or W) doping or LaFeO3 magnetic substrates, which together with the strong SOC of Bi atoms generates a time-reversal-symmetry-broken QSHE and a huge valley splitting (up to 513 meV) in the system. With gate control, QSHE and anomalous charge, spin, valley Hall effects can be observed in the single system. These predicted multiple and exotic Hall effects, associated with various degrees of freedom of electrons, could enable applications of the functionalized Bi monolayers in electronics, spintronics, and valleytronics.

cond-mat.mes-hall↗

arXiv2018

Coupling effect of topological states and Chern insulators in two-dimensional triangular lattices

We investigate topological states of two-dimensional (2D) triangular lattices with multi-orbitals. Tight-binding model calculations of a 2D triangular lattice based on $\emph{p}_{x}$ and \emph{p}_{y} orbitals exhibit very interesting doubly degenerate energy points at different positions ($Γ$ and K/K$^{\prime}$) in momentum space, with quadratic non-Dirac and linear Dirac band dispersions, respectively. Counterintuitively, the system shows a global topologically trivial rather than nontrivial state with consideration of spin-orbit coupling due to the "destructive interference effect" between the topological states at the $Γ$ and K/K$^{\prime}$ points. The topologically nontrivial state can emerge by introducing another set of triangular lattices to the system (bitriangular lattices) due to the breakdown of the interference effect. With first-principles calculations, we predict an intrinsic Chern insulating behavior (quantum anomalous Hall effect) in a family of 2D triangular lattice metal-organic framework of Co(C$_{21}$N$_{3}$H$_{15}$) (TPyB-Co) from this scheme. Our results provide a different path and theoretical guidance for the search for and design of new 2D topological quantum materials.

cond-mat.mes-hall↗

arXiv2017

Strong magnetization and Chern insulators in compressed graphene/CrI$_{3}$ van der Waals heterostructures

Graphene-based heterostructures are a promising material system for designing the topologically nontrivial Chern insulating devices. Recently, a two-dimensional (2D) monolayer ferromagnetic insulator CrI$_{3}$ was successfully synthesized in experiments [Huang et al., Nature 546, 270 (2017)]. Here, these two interesting materials are proposed to build a heterostructure (Gr/CrI$_{3}$). Our first-principles calculations show that the system forms a van der Waals (vdW) heterostructure, relatively facilely fabricated in experiments. A Chern insulating state is acquired in the Gr/CrI$_{3}$ heterostructure if the vdW gap is compressed to certain extents by applying an external pressure. Amazingly, very strong magnetization (about 150 meV) is found in graphene, induced by the substrate CrI$_{3}$, despite the vdW interactions between them. A low-energy effective model is employed to understand the mechanism. The work functions, contact types, and band alignments of the Gr/CrI$_{3}$ heterostructure system are also studied. Our work demonstrates that the Gr/CrI$_{3}$ heterostructure is a promising system to observe the quantum anomalous Hall effect at high temperatures (up to 45 K) in experiments.

cond-mat.mtrl-sci↗

arXiv2017

Chern insulators without band inversion in MoS2 monolayers with 3d adatoms

Electronic and topological properties of MoS2 monolayers endowed with 3d transition metal (TM) adatoms (V-Fe) are explored by using ab initio methods and k.p models. Without the consideration of the Hubbard U interaction, the V, Cr, and Fe adatoms tend to locate on the top of the Mo atoms, while the most stable site for the Mn atom is at the hollow position of the Mo-S hexagon. After the Hubbard U is applied, the most stable sites of all the systems become the top of the Mo atoms. Chern insulators without band inversion are achieved in these systems. The V and Fe adsorption systems are the best candidates to produce the topological states. The k.p model calculations indicate that these topological states are determined by the TM magnetism, the C3v crystal field from the MoS2 substrate, and the TM atomic spin-orbit coupling (SOC). The special two-meron pseudospin texture is found to contribute to the topology. The apparent difference between the Berry curvatures for the V and Fe adsorption systems are also explored. Our results widen the understanding to the Chern insulators and are helpful for the applications of the MoS2 monolayers in the future electronics and spintronics.

cond-mat.mtrl-sci↗

arXiv2016

Quantum Spin-quantum Anomalous Hall Effect with Tunable Edge States in Sb Monolayer-based Heterostructures

A novel topological insulator with tunable edge states, called quantum spin-quantum anomalous Hall (QSQAH) insulator, is predicted in a heterostructure of a hydrogenated Sb (SbH) monolayer on a LaFeO3 substrate by using ab initio methods. The substrate induces a drastic staggered exchange field in the SbH film, which plays an important role to generate the QSQAH effect. A topologically nontrivial band gap (up to 35 meV) is opened by Rashba spin-orbit coupling, which can be enlarged by strain and electric field. To understand the underlying physical mechanism of the QSQAH effect, a tight-binding model based on px and py orbitals is constructed. With the model, the exotic behaviors of the edge states in the heterostructure are investigated. Dissipationless chiral charge edge states related to one valley are found to emerge along the both sides of the sample, while low-dissipation spin edge states related to the other valley flow only along one side of the sample. These edge states can be tuned flexibly by polarization-sensitive photoluminescence controls and/or chemical edge modifications. Such flexible manipulations of the charge, spin, and valley degrees of freedom provide a promising route towards applications in electronics, spintronics, and valleytronics.

cond-mat.mtrl-sci↗