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Fuming Xu

Publications and source records attributed to Fuming Xu.

At least 19 recordsLinked to original sources
arXiv2026

Andreev reflection mediated by topological corner states in a two-dimensional honeycomb lattice

Topological corner states in two-dimensional second-order topological insulators (SOTIs) are localized in real space. We numerically demonstrate that such localized topological corner states can mediate Andreev reflection when coupled to a superconducting lead. We consider a transport setup based on a two-dimensional honeycomb lattice, consisting of a normal lead, a central SOTI region, and a superconducting lead. The central SOTI region is described by the modified Kane--Mele model with an in-plane Zeeman field and hosts topological corner states in a diamond-shaped flake. Although the central region is insulating, the local density of states shows that incident electrons can turn the localized corner state into an extended scattering state, which forms a resonant tunneling channel to the superconducting lead. This process leads to a perfect Andreev reflection peak near zero energy. Away from this resonance, antiresonance dips appear in the Andreev reflection spectrum, and their positions can be tuned by the Zeeman field strength. We show that the suppression of Andreev reflection is caused by quantum interference and the imbalance between electron and hole dwell times in the central region. These results demonstrate that topological corner states can provide a resonant tunneling path to the superconducting interface and mediate Andreev reflection in second-order topological systems.

cond-mat.mes-hall↗
arXiv2026

Dual Quantum Geometric Tensors and Local Topological Invariant

The conventional quantum geometric tensor (QGT) is Hermitian, with a real symmetric quantum metric and an imaginary antisymmetric Berry curvature. We show that the Zeeman QGT is generically non-Hermitian and admits a natural decomposition into normal and anomalous metric-curvature sectors. The normal sector reduces to the conventional Hermitian structure, whereas the anomalous sector contains an imaginary symmetric metric-like tensor and a real antisymmetric curvature-like tensor with no counterpart in the standard QGT. In a two-dimensional Dirac system, the anomalous Zeeman curvature develops a radial flux singularity that is Hodge-dual to the tangential winding field of the Dirac node. This recasts the same local $\pi_1$ topology into a curvature-flux language, analogous to the flux representation of global $\pi_2$ topology by the conventional Berry curvature. At the level of linear response, the four symmetry-resolved components of the gyrotropic conductivity are in one-to-one correspondence with the four components of the Zeeman QGT, while their distinct low-frequency scalings provide an additional diagnostic for isolating the underlying geometric sector. The reciprocal kinetic magnetoelectric response offers a complementary experimental route to probe the same structure. These results establish a unified framework connecting non-Hermitian Zeeman quantum geometry, local Dirac-node topology, and measurable transport signatures.

quant-ph↗
arXiv2026

Linear thermal noise induced by Berry curvature dipole in a four-terminal system

In this work, we numerically investigate linear thermal noise in a four-terminal system with a finite Berry curvature dipole (BCD) using the nonequilibrium Green's function formalism. By comparing with the semiclassical results for bulk systems, we establish a one-to-one correspondence between terminal-resolved linear noise in multi-terminal systems and direction-resolved noise in bulk transport. Specifically, the auto-correlation function scales as $2 k_B T$ when the driving field is perpendicular to the BCD and vanishes when they are parallel, whereas the cross-correlation scales as $k_B T$. Both the auto- and cross-correlation functions exhibit pronounced peaks near the band edges, consistent with BCD-induced features. In addition, the linear thermal noise increases approximately linearly with $T$ at low temperatures and is suppressed by dephasing effect at high temperatures. Our work bridges semiclassical bulk theory and quantum multi-terminal theory for linear thermal noise, highlighting the symmetry(geometry)-selection rule in quantum transport.

cond-mat.mes-hall↗
arXiv2025

Quantum geometric map of magnetotransport

We propose a quantum geometric map for the magnetononlinear Hall effect (MNHE), the planar Hall effect (PHE), and the ordinary Hall effect (OHE). These magnetotransport phenomena originate from the bilinear charge current of Bloch electrons in electromagnetic fields, incorporating both spin Zeeman coupling and orbital minimal coupling to the applied magnetic field. Benchmarked against Onsager reciprocity, we demonstrate that the spin- and orbital-induced MNHEs are governed by the time-reversal-even Zeeman quantum metric dipole and conventional quantum metric quadrupole, respectively; the spin- and orbital-induced PHEs are dominated by the time-reversal-odd Zeeman Berry curvature dipole and conventional Berry curvature quadrupole, respectively. We further show that the OHE contains an interband contribution that is related to the quantum metric quadrupole, contrary to conventional wisdom. Navigated by this map, we study the previously unexplored spin-induced PHE in the surface Dirac cone of topological insulators, where we uncover a step-like PHE. Our work offers a unified quantum geometric framework for understanding magnetotransport experiments.

cond-mat.mes-hall↗
arXiv2025

Unconventional Hall Effect in Gapless Superconductors: Transverse Supercurrent Converted from Normal Current

A normal metallic system proximitized by a superconductor can exhibit a gapless superconducting state characterized by segmented Fermi surfaces, as confirmed experimentally. In such a state, quasiparticle states remain gapless along one direction, while a superconducting gap opens in the perpendicular direction. This anisotropy enables a novel Hall effect in gapless superconductors, termed the superconducting Hall effect (ScHE), where a longitudinal normal current carried by quasiparticles is converted into a dissipationless transverse supercurrent. Employing both the thermodynamic approach for bulk systems and quantum transport theory for a four-probe setup, we demonstrate the existence of this effect and reveal its intrinsic origin as the quasiparticle Berry curvature. The predicted ScHE can be experimentally verified via the standard angular-dependent Hall measurements performed on gapless superconductors.

cond-mat.mes-hall↗
arXiv2025

Spin separation and filtering assisted by topological corner states in the Kekul\'{e} lattice

Higher-order topological corner states have been realized in two-dimensional Kekul\'{e} lattice, which can be further coupled with spin polarization through the implementation of local magnetization. In this work, we numerically investigate the spin-dependent transport properties assisted by topological corner states in the Kekul\'{e} lattice. By applying local magnetization and electric potential, the topological corner states are spin polarized with opposite spins localized at different corners, thereby demonstrating a spin-corner state locking mechanism. Transport characteristics, including transmission, local density of states, and local current density, are calculated for a two-terminal setup consisting of a diamond-shaped Kekul\'{e} lattice connected to two leads. When opposite local magnetization is applied to the corners, spin-up and spin-down electrons are perfectly separated, forming two spin-polarized conducting channels and leading to spin spatial separation. In the presence of identical local magnetization on both corners and an electric potential at one corner, the spin-polarized corner states can facilitate selective filtering of different spins and generate spin-polarized currents by tuning the energy. Furthermore, spin-resolved transmission diagrams as functions of both the Fermi energy and electric potential are presented, illustrating the global distribution of spin filtering through topological corner states.

cond-mat.mes-hall↗
arXiv2025

General method for calculating transport properties of disordered mesoscopic systems based on the nonequilibrium Green's function formalism

Disorder scattering plays important roles in quantum transport as well as various Hall effects, including the second-order nonlinear Hall effect induced by Berry curvature dipole. Calculation of disorder-averaged transport properties usually requires substantial computational resources, especially for higher-order effects. Existing methods are either limited by approximation conditions or constrained by numerical stability, making it difficult to conveniently obtain average physical quantities over a wide range of disorder strength. In this work, we develop a general method for noninteracting system to obtain analytical expressions of disorder averages in finite orders of disorder strength. This method utilizes the Dyson equation to expand physical quantities expressed in terms of the Green's functions into series of disorder-averaged matrices, and the only approximation involved is the truncation of the Dyson equation. Therefore, this method not only avoids the brute force calculation of disorder samples, but also widely applies to different model systems, types of disorder, and the number of Green's functions in the expressions. We demonstrate the applicability of this general method by calculating averages of the linear conductance of a two-terminal system, the spin Hall conductance and the second-order nonlinear conductance of four-terminal Hall setups. It is found that truncation at the fourth order of disorder strength provides a reasonable accuracy and a convenient Pad\'{e} treatment effectively extends its applicable range. Numerical results also confirms disorder enhancement of the second-order nonlinear Hall current in four-terminal systems. Moreover, more accurate predictions for a broader range of disorder strength can be achieved by including higher-order terms in a similar manner.

cond-mat.mes-hall↗
arXiv2024

Light-induced thermal noise \textit{anomaly} governed by quantum metric

Traditionally, thermal noise in electric currents, arising from thermal agitation, is expected to increase with temperature $T$ and disappear as $T$ approaches zero. Contrary to this expectation, we discover that the resonant DC thermal noise (DTN) in photocurrents not only persists at $T=0$ but also exhibits a divergence proportional to $1/T$. This thermal noise \textit{anomaly} arises from the unique electron-photon interactions near the Fermi surface, manifesting as the interplay between the inherent Fermi-surface property and the resonant optical selection rules of DTN, and thereby represents an unexplored noise regime. Notably, we reveal that this \textit{anomalous} DTN, especially in time-reversal-invariant systems, is intrinsically linked to the quantum metric. We illustrate this \textit{anomalous} DTN in massless Dirac materials, including two-dimensional graphene, the surfaces of three-dimensional topological insulators, and three-dimensional Weyl semimetals, where the quantum metric plays a pivotal role. Finally, we find that the total noise spectrum at low temperatures, which includes both the DC shot noise and the \textit{anomalous} DTN, will universally peak at $\omega_p=2|\mu|$ with $\omega_p$ the frequency of light and $\mu$ the chemical potential of the bulk crystals.

cond-mat.mes-hall↗
arXiv2024

Pseudo-Riemannian metric: a new perspective on the quantum realm

As a fundamental concept in condensed matter physics, quantum geometry within the Riemannian metric elucidates various exotic phenomena, including the Hall effects driven by Berry curvature and quantum metric. In this work, we propose novel quantum geometries within a pseudo-Riemannian framework to explore unique characteristic of quantum matter. By defining distinct distances on pseudo-Riemannian manifolds and incorporating spin degree of freedom, we introduce the Pauli quantum geometric tensor. The imaginary part of this tensor corresponds to the Pauli Berry curvature, leading to the discovery a novel quantum phase: Pauli semimetal in PT-symmetric systems. This phase, characterized by the topological Pauli Chern number, manifests as a two-dimensional Pauli Chern insulator with helical edge states. These topological phases, uniquely revealed by the Pauli-Riemannian metric, go beyond the familiar Riemannian metric, where Berry curvature vanishes due to PT-symmetry. Pauli Chern number can classify helical topological insulator with or without time reversal symmetry. Pseudo-Riemannian metrics offer new insights into quantum materials and extend the scope of quantum geometry.

cond-mat.mes-hall↗
arXiv2024

Quantum intrinsic ${\cal T}$-odd spin Hall effect in altermagnets

Drude weight, historically associated with the longitudinal Drude conductivity, can be generalized to describe the transverse or Hall component of the extrinsic conductivity tensor. In particular, transverse Drude weights, such as band geometric quantities Berry curvature dipole and spin vorticity, manifest themselves through the \textit{extrinsic} second-order nonlinear Hall effect and \textit{extrinsic} linear spin Hall effect (SHE) in diffusive transport, respectively. In this work, we uncover a new class of intrinsic Hall effects in quantum transport regime, termed as quantum intrinsic Hall effect (QIHE), which is the manifestation of system symmetry through intrinsic transport phenomena. For a given Hamiltonian, its transport characteristics can be revealed either intrinsically through QIHE in ballistic regime or extrinsically via the transverse Drude weight in diffusive transport, where both intrinsic and extrinsic effects share the same salient transport features governed by symmetry of the Hamiltonian. The physical origin of QIHE is attributed to quantum boundary scattering of the measurement setup that respects the system symmetry, as exemplified by the contact resistance of a two-terminal ballistic conductor. We demonstrate our finding by studying the quantum ${\cal T}$-odd ($\mathcal{T}$, time-reversal) SHE in altermagnets. Our work paves a way towards the quantum transport manifestation of band geometric characteristics.

cond-mat.mes-hall↗
arXiv2023

Intrinsic gyrotropic magnetic current from Zeeman quantum geometry

Quantum geometric tensor (QGT), which is usually obtained by evaluating the quantum distance between Bloch states parametrized by momentum, plays a key role in exploring the exotic responses of quantum materials. Herein, we revisit the concept of QGT by further taking into account the spin degree of freedom. Besides the conventional QGT relating to momentum translation, we uncover a new QGT (termed Zeeman QGT) relating to momentum translation as well as spin rotation, whose imaginary (real) part gives the Zeeman Berry curvature (quantum metric). Notably, we show that these novel quantum geometric quantities can drive an intrinsic gyrotropic magnetic current (IGMC) in spin-orbit coupled materials when the electron spin is steered by an oscillating magnetic field. With symmetry analysis, we show that a wide range of materials can support the IGMC, as illustrated by model calculations. Finally, we discuss the experimental aspects of detecting the IGMC driven by Zeeman QGT.

cond-mat.mes-hall↗
arXiv2023

Classification of spin Hall effect in two-dimensional systems

Physical properties such as the conductivity are usually classified according to the symmetry of the underlying system using Neumann's principle, which gives an upper bound for the number of independent components of the corresponding property tensor. However, for a given Hamiltonian, this global approach usually can not give a definite answer on whether a physical effect such as spin Hall effect (SHE) exists or not. It is found that the parity and types of spin-orbit interactions (SOIs) are good indicators that can further reduce the number of independent components of the spin Hall conductivity for a specific system. In terms of the parity as well as various Rashba-like and Dresselhaus-like SOIs, we propose a local approach to classify SHE in two-dimensional (2D) two-band models, where sufficient conditions for identifying the existence or absence of SHE in all 2D magnetic point groups are presented.

cond-mat.mes-hall↗
arXiv2023

Bulk-boundary-transport correspondence of the second-order topological insulators

The bulk-boundary correspondence of the second-order topological insulator (SOTI) has been well established, but a universal transport signature for open systems is still absent. For a variety of SOTIs induced by applying in-plane magnetic fields in Z$_2$-invariant first-order TIs, rotating this magnetic field features the spin pump mechanism while maintaining the SOTI phase. We demonstrate that, this spin pump can generate quantized pure spin current when tuning the magnetic field strength, which corresponds to the formation of topological corner states characterizing SOTI in two-dimensional (2D) systems. Quantized spin pump is discovered in various 2D and 3D SOTI models evolved from Z$_2$-invariant TIs, which is robust against disorder and universally independent of system parameters including Fermi energy, system size, magnetic field strength, and pumping frequency. These findings suggest that this universal quantized spin pump can characterize the bulk-boundary-transport correspondence of SOTIs. Quantized spin pump can also be realized by combining pseudo spin such as the orbital degree of freedom with the rotating magnetic field, which could be achieved in higher-order photonic or acoustic topological systems. Such a quantized spin pump is promising as an accurate and stable single-spin source.

cond-mat.mes-hall↗
arXiv2023

Quantum nonlinear ac transport theory at low frequency

Based on the nonequilibrium Green's function (NEGF), we develop a quantum nonlinear theory to study time-dependent ac transport properties in the low frequency and nonlinear bias voltage regimes. By expanding NEGF in terms of time to the linear order in Wigner representation, we can explicitly include the time-dependent self-consistent Coulomb interaction induced by external ac bias. Hence this theory automatically satisfies two basic requirements, i.e., current conservation and gauge invariance. Within this theory, the nonlinear ac current can be evaluated at arbitrarily large bias voltages under the low frequency limit. In addition, we obtain the expression of time-dependent current under the wide band limit and derive the relation between the nonlinear electrochemical capacitance and the bias voltage, which are very useful in predicting the dynamical properties of nanoelectronic devices. This quantum theory can be directly combined with density functional theory to investigate time-dependent ac transport from first-principles calculation.

cond-mat.mes-hall↗
arXiv2023

Unified framework of the microscopic Landau-Lifshitz-Gilbert equation and its application to Skyrmion dynamics

The Landau-Lifshitz-Gilbert (LLG) equation is widely used to describe magnetization dynamics. We develop a unified framework of the microscopic LLG equation based on the nonequilibrium Green's function formalism. We present a unified treatment for expressing the microscopic LLG equation in several limiting cases, including the adiabatic, inertial, and nonadiabatic limits with respect to the precession frequency for a magnetization with fixed magnitude, as well as the spatial adiabatic limit for the magnetization with slow variation in both its magnitude and direction. The coefficients of those terms in the microscopic LLG equation are explicitly expressed in terms of nonequilibrium Green's functions. As a concrete example, this microscopic theory is applied to simulate the dynamics of a magnetic Skyrmion driven by quantum parametric pumping. Our work provides a practical formalism of the microscopic LLG equation for exploring magnetization dynamics.

cond-mat.mes-hall↗
arXiv2023

High-efficiency photoelectric detector based on a p-n homojunction of monolayer black phosphorus

We numerically investigate the high-efficiency photovoltaic effect in lateral p-n homojunction based on monolayer black phosphorus (MBP) by using the non-equilibrium Green's function combined with the density functional theory. Due to the built-in electric field of the p-n junction and the wrinkle structure of MBP, the photocurrent excited by either linearly or elliptically polarized light is significantly enhanced in a wide photon energy range. Moreover, because of the electron-photon interaction, the photocurrent is related to atomic orbitals through the polarizing angle of polarized light. Therefore, we can read the orbital information of the band structure from the polarizing angular distribution of photocurrent. These findings suggest the promising application of MBP-based p-n homojunction in high-efficiency photoelectric devices and orbital-resolved photovoltaic detection.

cond-mat.mes-hall↗
arXiv2023

Gapless superconducting state and mirage gap in altermagnets

The interplay between spin-orbit interaction (SOI) and magnetism produces interesting phenomena in superconductors. When a two-dimensional (2D) system with strong SOI is coupled to an $s$-wave superconductor, an in-plane magnetic field can drive the system into a gapless superconducting state and induce a mirage gap at finite energies for an Ising superconductor. In this work, we demonstrate that when an $s$-wave superconductor is proximitized to an altermagnet, the intrinsic anisotropic spin splitting of the altermagnet can result in a gapless superconducting state and a pair of mirage gaps at finite energy. The gapless superconductivity exhibits spin-polarized segmented Fermi surfaces, with coexisting spin-singlet and spin-triplet pairings that have a $d$-wave character. Importantly, the gapless superconducting and mirage gap features are quantified through quantum transport. Our results suggest that altermagnet is an ideal platform for studying gapless superconducting states and mirage gap physics.

cond-mat.supr-con↗
arXiv2023

Realization of valley-spin polarized current via parametric pump in monolayer $\rm MoS_2$

Monolayer $MoS_2$ is a typical valleytronic material with valley-spin locked valence bands. We numerically investigate the valley-spin polarized current in monolayer $\rm MoS_2$ via adiabatic electron pumping. By introducing an exchange field to break the energy degeneracy of monolayer $MoS_2$, the top of its valence bands is valley-spin polarized and tunable by the exchange field. A device with spin-up polarized left lead, spin-down polarized right lead, and untuned central region is constructed through applying different exchange fields in the corresponding regions. Then, equal amount of pumped currents with opposite valley-spin polarization are simultaneously generated in the left and right leads when periodically varying two pumping potentials. Numerical results show that the phase difference between the pumping potentials can change the direction and hence polarization of the pumped currents. It is found that the pumped current exhibits resonant behavior in the valley-spin locked energy window, which depends strongly on the system size and is enhanced to resonant current peaks at certain system lengths. More importantly, the pumped current periodically oscillates as a function of the system length, which is closely related to the oscillation of transmission. The effects of other system parameters, such as the pumping amplitude and the static potential, are also thoroughly discussed.

cond-mat.mes-hall↗