arXiv subjects

X. C. Xie

Publications and source records attributed to X. C. Xie.

At least 55 records · Page 3Linked to original sources

arXiv2025

Nonlinear Néel Spin-Orbit Torque in Centrosymmetric Antiferromagnets

Electric control of Néel vector is a central task of antiferromagnetic (AFM) spintronics. The major scheme so far relies on the linear Néel torque, which however is restricted to AFMs with broken inversion symmetry. Here, we propose a nonlinear Néel spin-orbit torque, uniquely enabling electric control in the vast class of centrosymmetric AFMs, where the existing scheme fails. Importantly, its intrinsic component, rooted in sublattice-resolved band quantum geometry, offers two additional advantages: It operates also in $\mathcal{PT}$-symmetric AFM insulators, where linear torque is forbidden; and it has anti-damping character, making it more efficient in driving magnetic dynamics. Combined with first-principles calculations, we predict large effect in MnRh and MnBi$_{2}$Te$_{4}$, which can be readily detected in experiment. Our work unveils a new fundamental effect, offers a new strategy of electric control in AFM systems beyond the existing paradigm, and opens the door to the field of nonlinear AFM spintronics.

cond-mat.mtrl-sci↗

arXiv2025

Unified model for non-Abelian braiding of Majorana and Dirac fermion zero modes

Majorana zero modes (MZMs) are the most intensively studied non-Abelian anyons. The Dirac fermion zero modes in topological insulators, which are symmetry-protected doubling of MZMs under fermion number conservation, offer an alternative approach to explore non-Abelian anyons. However, a unified model that elucidates the braiding statistics of these types of topological zero modes remains absent. We show that the minimal Kitaev chain model beyond fine-tuning regime provides a unified characterization of the non-Abelian statistics of both MZMs and Dirac fermion zero modes in different parameter regimes. In particular, we introduce a minimal tri-junction setting based on the minimal Kitaev chain model and show it facilitates the unified scheme of braiding Dirac fermion zero modes, as well as the MZMs in the assistance of a Dirac mode. This unified minimal model provides deeper insights into non-Abelian statistics, demonstrating that the non-Abelian braiding of MZMs can be continuously extended to encompass Dirac fermion zero modes. The minimal Kitaev chain has been realized in coupled quantum dots [Nature 614, 445 (2023)]. Our extension, which demonstrates novel nontrivial phases with non-Abelian MZM pairs and Dirac zero modes emerging in the broader parameter regimes without fine-tuning, expands the accessible experimental parameter space and enhances the feasibility of observing non-Abelian statistics in the minimal Kitaev chain model.

cond-mat.mes-hall↗

arXiv2025

Electrical switching of altermagnetism

Switching magnetism using only electricity is of great significance for industrial applications but remains challenging. We find that, altermagnetism, as a newly discovered unconventional magnetism, may open an avenue along this effort. Specifically, to have deterministic switching, i.e., reversing current direction must reverse magnetic structure, parity symmetry has to be broken. We discover that, due to their symmetry which depends on chemical environments, altermagnet devices may naturally carry the parity symmetry breaking required for deterministic electrical switching of magnetism. More importantly, we identify MnTe bilayers (Te-Mn-Te-Mn-Te) as candidate devices, with the help of symmetry analysis, first-principles calculations, and magnetic dynamics simulations. This scheme will inspire further explorations on unconventional magnetism.

cond-mat.mes-hall↗

arXiv2025

Unconventional Orbital Magnetism in Graphene-based Fractional Chern Insulators

Orbital magnetism in graphene originates from correlation-driven spontaneous valley symmetry breaking1-7. It can lead to various anomalous transport phenomena such as integer and fractional quantum anomalous Hall effects8-11. In general, the in-plane magnetic field B|| primarily couples to the spin degrees of freedom in graphene and has long been presumed to have a negligible effect on orbital magnetism due to the ultra-weak spin-orbit coupling12-18. In this work, we report multiple unconventional orbital magnetic phenomena that are highly sensitive to the B|| field in graphene/hBN superlattices hosting both integer and fractional Chern insulators (FCIs). We observed chirality-switching behaviors of the Chern insulator at moiré filling factor ν = 1 under a finite B_par, demonstrating that both the C = +-1 states are permissible ground states at zero perpendicular magnetic field B_per. For the FCI at ν = 2/3, we observed topological phase transitions between two states characterized by Hall resistivity \r{ho}xy = +-3h/2e2 under both B_per and B_par fields. In-plane B|| field can effectively suppress the FCI state at zero B_per field and enhance the FCI state with the opposite chirality, as resolved in Landau fan diagrams. Moreover, we observed rich phase transitions at 1 < ν < 2, accompanied by intervalley coherence and anomalous Hall effects (AHE) that can be triggered by sweeping either B_per or B_par. Our work has unveiled new properties of orbital magnetism, providing a new knob for engineering various AHE in graphene.

cond-mat.mes-hall↗

arXiv2025

Transdimensional anomalous Hall effect in rhombohedral thin graphite

Anomalous Hall effect (AHE), occurring in materials with broken time-reversal symmetry, epitomizes the intricate interplay between magnetic order and orbital motions of electrons[1-4]. In two dimensional (2D) systems, AHE is always coupled with out-of-plane orbital magnetization associated in-plane chiral orbital motions. In three dimensional (3D) systems, carriers can tunnel or scatter along the third dimension within the vertical mean free path lz. When sample thickness far exceeds lz, scattering disrupts coherent out-of-plane motion, making 3D AHE effectively a thickness-averaged 2D counterpart[4] -- still governed by out-of-plane orbital magnetization arising from in-plane orbital motions. Here, we explore an uncharted regime where the sample thickness is much larger than the atomic layer thickness yet smaller than or comparable to lz. In such "transdimensional" regime, carriers can sustain coherent orbital motions both within and out of the 2D plane, leading to a fundamentally new type of AHE that couples both out-of-plane and in-plane orbital magnetizations. We report the first observation of such phenomenon -- transdimensional AHE (TDAHE) -- in electrostatically gated rhombohedral ennealayer graphene. This state emerges from a peculiar metallic phase that spontaneously breaks time-reversal, mirror and rotational symmetries driven by electron-electron interactions. Such TDAHE manifests as concurrent out-of-plane and in-plane Hall resistance hysteresis, controlled by external magnetic fields along either direction. Our findings unveils a new class of AHE, opening an unexplored paradigm for correlated and topological physics in transdimensional systems.

cond-mat.mes-hall↗

arXiv2025

Rare-Event-Induced Ergodicity Breaking in Logarithmic Aging Systems

Ergodicity breaking and aging effects are fundamental challenges in out-of-equilibrium systems. Various mechanisms have been proposed to understand the non-ergodic and aging phenomena, possibly related to observations in systems ranging from structural glass and Anderson glasses to biological systems and mechanical systems. While anomalous diffusion described by Levy statistics efficiently captures ergodicity breaking, the origin of aging and ergodicity breaking in systems with ultraslow dynamics remain unclear. Here, we report a novel mechanism of ergodicity breaking in systems exhibiting log-aging diffusion. This mechanism, characterized by increasingly infrequent rare events with aging, yields statistics deviating significantly from Levy distribution, breaking ergodicity as shown by unequal time- and ensemble-averaged mean squared displacements and two distinct asymptotic probability distribution functions. Notably, although these rare events contribute negligibly to statistical averages, they dramatically change the system's characteristic time. This work lays the groundwork for microscopic understanding of out-of-equilibrium systems and provides new perspectives on glasses and Griffiths-McCoy singularities.

cond-mat.dis-nn↗

arXiv2025

Coulomb Drag in Altermagnets

An altermagnet is a newly discovered antiferromagnet, characterized by unique anisotropic spin-split energy bands. It has attracted tremendous interest, because of its promising potential in information storage and processing. However, measuring the distinctive spin-split energy bands arising from altermagnetism remains a challenge. Here, we propose to employ the Coulomb drag to probe altermagnetism. In the Coulomb drag, an electric current in an active layer of electron gases can induce currents in a close but well-isolated passive layer, due to interlayer Coulomb interactions. We find that the Coulomb drag effects in altermagnets are highly sensitive to the orientation of the spin-split Fermi surfaces. As a result, transverse currents can be dragged in the passive layer, leading to Hall drag effects even in absence of spin-orbit coupling, a feature quite different from all previous systems. More importantly, all the drag effects of altermagnets have unique angle dependence, which can be measured in a multi-terminal setup to serve as signatures for altermagnetism. This proposal will inspire increasing explorations on emergent magnetism.

cond-mat.mes-hall↗

arXiv2025

Coherent detection of the oscillating acoustoelectric effect in graphene

In recent years, surface acoustic waves (SAWs) have emerged as a novel technique for generating quasiparticle transport and band modulation in condensed matter systems. SAWs interact with adjacent materials through piezoelectric and strain fields, dragging carriers in the direction of wave propagation. Most studies on the acoustoelectric effect have focused on the collective directional motion of carriers, which generates a steady electric potential difference, while the oscillating component from dynamic spatial charge modulation has remained challenging to probe. In this work, we report the coherent detection of oscillating acoustoelectric effect in graphene. This is achieved through the coherent rectification of spatial-temporal charge oscillation with electromagnetic waves emitted by interdigital transducers. We systematically investigate the frequency and gate dependence of rectified signals and quantitatively probe the carrier redistribution dynamics driven by SAWs. The observation of oscillating acoustoelectric effect provides direct access to the dynamic spatial charge modulation induced by SAWs through transport experiments.

cond-mat.mes-hall↗

arXiv2025

Tunable Fractional Chern Insulators in Rhombohedral Graphene Superlattices

Fractional Chern insulators (FCIs) showing a transport effect with fractionally quantized Hall plateaus emerging under zero magnetic field, provide a radically new opportunity to engineer topological quantum electronics. By construction of topological flat band with moire engineering, intrinsic FCIs have been observed in twisted MoTe2 system and rhombohedral pentalayer graphene/hBN moire superlattices with anomalous Hall resistivity quantization number C <= 2/3 including the gapless composite Fermi-liquid state with C = 1/2. Here, we experimentally demonstrate a new system of rhombohedral hexalayer graphene (RHG)/hBN moire superlattices, which exhibit both integer and fractional quantum anomalous Hall effects with rich tunability including electric displacement field, perpendicular magnetic field and in-plane magnetic field. By tuning the electrical and magnetic fields at 0 < v < 1, we have observed a quantum phase transition showing a sign reversal of the Hall resistivity at finite magnetic fields. Surprisingly, the FCI state at v = 2/3 survives in the phase transitions, exhibiting a robust quantized Hall resistivity across both phases. Finally we have further demonstrated the indispensable role moire potential plays in the formation of the flat Chern band from a theoretical perspective. Our work has established RHG/hBN moire superlattices as a promising platform for exploring quasi-particles with fractional charge and non-Abelian anyons at zero magnetic field.

cond-mat.mes-hall↗

arXiv2025

Chern Vector Protected Three-dimensional Quantized Hall Effect

Recently, Chern vector with arbitrary formula $\textbf{C}\!=\!(\mathcal{C}_{yz},\mathcal{C}_{xz},\mathcal{C}_{xy})$ in three-dimensional systems has been experimentally realized [\B{Nature 609, 925 (2022)}]. Motivated by these progresses, we propose the Chern vector $\textbf{C}\!=\!(0,m,n)$-protected quantized Hall effect in three-dimensional systems. By examining samples with Chern vector $\textbf{C}\!=\!(0,m,n)$ and dimensions $L_y$ and $L_z$ along the $y$- and $z$-directions, we demonstrate a topologically protected two-terminal response. This response can be reformulated as the sum of the transmission coefficients along the $x$- and $y$-directions, given by $(mL_y\!+\!nL_z)$. When applied to Hall bar setups, this topological mechanism gives rise to quantized Hall conductances, such as $G_{xy}$ and $G_{xz}$, which are expressed by $\pm(mL_y\!+\!nL_z)$. These Hall conductances exhibit a clear dependency on sample dimensions, illuminating the intrinsic three-dimensional nature. Finally, we propse potential candidates for experimental realization. Our findings not only deepen the understanding of the topological nature of Chern vectors but also enlighten the exploration of their transport properties.

cond-mat.mes-hall↗

arXiv2024

Rules for dissipationless topotronics

Topological systems hosting gapless boundary states have attracted huge attention as promising components for next-generation information processing, attributed to their capacity for dissipationless electronics. Nevertheless, recent theoretical and experimental inquiries have revealed the emergence of energy dissipation in precisely quantized electrical transport. Here, we present a criterion for the realization of truly no-dissipation design, characterized as $N_{in}=N_{tunl}+N_{bs}$, where $N_{in}$, $N_{tunl}$, and $N_{bs}$ represent the number of modes participating in injecting, tunneling, and backscattering processes, respectively. The key lies in matching the number of injecting, tunneling and backscattering modes, ensuring the equilibrium among all engaged modes inside the device. Among all the topological materials, we advocate for the indispensability of Chern insulators exhibiting higher Chern numbers to achieve functional devices and uphold the no-dissipation rule simultaneously. Furthermore, we design the topological current divider and collector, evading dissipation upon fulfilling the established criterion. Our work paves the path for developing the prospective topotronics.

cond-mat.mes-hall↗

arXiv2024

Fractional spin Josephson effect in topological spin superconductors

Topological spin superconductors are $p$-wave spin-triplet exciton insulators whose topological edge modes have been shown to obey non-Abelian braiding statistics. Based on a toy model as the spin counterpart of the Kitaev's chain, we study the spin Josephson effect adopting the $S$-matrix as well as the Green's function method. The on-site energies of these topological edge modes lead to a transition between the fractional and integer spin Josephson effects. Moreover, non-vanishing on-site energies will also induce a charge pump through the spin Josephson junction. These two effects, distinct features of topological spin superconductors and absent in Majorana systems, can be utilized for spin transport detection of topological spin superconductors.

cond-mat.mes-hall↗

arXiv2024

Quantum Griffiths singularity in a three-dimensional superconductor to Anderson critical insulator transition

Disorder is ubiquitous in real materials and can have dramatic effects on quantum phase transitions. Originating from the disorder enhanced quantum fluctuation, quantum Griffiths singularity (QGS) has been revealed as a universal phenomenon in quantum criticality of low-dimensional superconductors. However, due to the weak fluctuation effect, QGS is very challenging to detect experimentally in three-dimensional (3D) superconducting systems. Here we report the discovery of QGS associated with the quantum phase transition from 3D superconductor to Anderson critical insulator in a spinel oxide MgTi2O4 (MTO). Under both perpendicular and parallel magnetic field, the dynamical critical exponent diverges when approaching the quantum critical point, demonstrating the existence of 3D QGS. Among 3D superconductors, MTO shows a relatively strong fluctuation effect featured as a wide superconducting transition region. The enhanced fluctuation, which may arise from the mobility edge of Anderson localization, finally leads to the occurrence of 3D quantum phase transition and QGS. Our findings offer a new perspective to understand quantum phase transitions in strongly disordered 3D systems.

cond-mat.supr-con↗

arXiv2024

Edge supercurrent in Josephson junctions based on topological materials

The interplay between novel topological states and superconductivity has garnered substantial interest due to its potential for topological quantum computing. The Josephson effect serves as a useful probe for edge superconductivity in these hybrid topological materials. In Josephson junctions based on topological materials, supercurrents exhibit unique quantum interference patterns, including the conventional Fraunhofer oscillations, the $Φ_0$-periodic oscillation, and the $2Φ_0$-periodic oscillation in response to the external magnetic field ($Φ_0 = h/2e$ is the flux quantum, $h$ the Planck constant, and $e$ the electron charge). These interference patterns stem from varied Andreev reflection mechanisms and the associated current density profiles. This review seeks to comprehensively examine the theoretical and experimental advancements in understanding the quantum interference patterns of edge supercurrents in Josephson junctions based on quantum spin Hall, quantum Hall, and quantum anomalous Hall systems.

cond-mat.supr-con↗

arXiv2024

Anomalous Fraunhofer-like patterns in quantum anomalous Hall Josephson junction

The intriguing interplay between topology and superconductivity has attracted significant attention, given its potential for realizing topological superconductivity. In the quantum anomalous Hall insulators (QAHIs)-based junction, the supercurrents are carried by the chiral edge states, characterized by a $2Φ_0$ magnetic flux periodicity ($Φ_0 = h/2e$ is the flux quantum, $h$ the Planck constant, and $e$ the electron charge). However, experimental observations indicate the presence of bulk carriers in QAHI samples due to magnetic dopants. In this study, we reveal a systematic transition from edge-state to bulk-state dominant supercurrents as the chemical potential varies from the bulk gap to the conduction band. This results in an evolution from a $2Φ_0$-periodic oscillation pattern to an asymmetric Fraunhofer pattern. Furthermore, a novel Fraunhoher-like pattern emerges due to the coexistence of chiral edge states and bulk states caused by magnetic {\color{black}domains}, even when the chemical potential resides within the gap. These findings not only advance the theoretical understanding but also pave the way for the experimental discovery of the chiral Josephson effect based on QAHI doped with magnetic impurities.

cond-mat.mes-hall↗

arXiv2024

Dispersions and magnetism of strain-induced pseudo Landau levels in Bernal-stacked bilayer graphene

Elastic strain can displace the massless Dirac fermions in monolayer graphene in a space-dependent fashion, similar to the effect of an external magnetic field, thus giving rise to Landau quantization. We here show that the strain-induced Landau quantization can also take place in Bernal-stacked bilayer graphene, where the low-energy excitations are massive rather than Dirac-like. The zigzag ribbon of Bernal-stacked bilayer graphene realizes a two-legged Su-Schrieffer-Heeger model with a domain wall, which coincides with the guiding center of the strain-induced pseudo Landau levels. We reduce the lattice model of the ribbon in the vicinity of the guiding center into an exactly solvable coupled Dirac model and analytically derive the dispersions of the strain-induced pseudo Landau levels. Remarkably, the zeroth and first pseudo Landau levels are dispersionless and sublattice-polarized. We elucidate that the interaction on these two pseudo Landau levels results in a global antiferromagnetic order. Our study extends the strain-induced Landau quantization to the massive excitations and indicates strain as a tuning knob of magnetism.

cond-mat.mes-hall↗

arXiv2024

Quantifying the non-Abelian property of Andreev bound states in inhomogeneous Majorana nanowires

Non-Abelian braiding is a key property of Majorana zero modes (MZMs) that can be utilized for topological quantum computation. However, the presence of trivial Andreev bound states (ABSs) in topological superconductors can hinder the non-Abelian braiding of MZMs. We systematically investigate the braiding properties of ABSs induced by various inhomogeneous potentials in nanowires and quantify the main obstacles to non-Abelian braiding. We find that if a trivial ABSs is present at zero energy with a tiny energy fluctuation, their non-Abelian braiding property can be sustained for a longer braiding time cost, since the undesired dynamic phase is suppressed. Under certain conditions, the non-Abelian braiding of ABSs can even surpass that of MZMs in realistic systems, suggesting that ABSs might also be suitable for topological quantum computation.

cond-mat.mes-hall↗

arXiv2024

Energy Bands of Incommensurate Systems

Energy band theory is a fundamental cornerstone of condensed matter physics. According to conventional wisdom, discrete translational symmetry is mandatory for defining energy bands. Here, we illustrate that, in fact, the concept of energy band can be generalized to incommensurate systems lacking such symmetry, thus transcending the traditional paradigm of energy band. The validity of our theory is verified by extensive numerical calculations in the celebrated Aubry-André-Harper model and a two-dimensional incommensurate model of graphene. Building upon the proposed concept of incommensurate energy bands, we further develop a theory of angle-resolved photoemission spectroscopy (ARPES) for incommensurate systems, providing a clear physical picture for the incommensurate ARPES spectra. Our work establishes a comprehensive energy band theory for incommensurate systems.

cond-mat.mes-hall↗