arXiv subjects

Xukun Feng

Publications and source records attributed to Xukun Feng.

13 recordsLinked to original sources

arXiv2026

Symplectic connection third-order Hall effect in a room-temperature ferromagnet

Third-order nonlinear Hall effects (THE) have recently attracted considerable experimental interest as powerful probes for quantum geometric properties in emergent quantum materials, encompassing quadrupole moments of quantum metric and Berry curvature. Here, we report a fundamentally new THE in room-temperature van der Waals ferromagnet Fe3GaTe2 from second-order Berry connection polarizability, which manifests a higher-order characterization of band geometry called symplectic connection. Our observations show that the third-order transverse response in Fe3GaTe2 is odd to magnetization, vanishes above the Curie temperature and remains independent of driving current directions. Scaling law analysis combined with first-principles calculations establishes this response as the symplectic-connection-induced THE. This discovery opens the door to probing high-order quantum geometric properties beyond Berry curvature and quantum metric through nonlinear transport, unveiling the potential of exploring nonlinear Hall phenomena in broad classes of magnets without breaking inversion symmetry. Moreover, the room-temperature manipulation of THE holds promises for device applications based on harnessing the quantum-geometric connection structure.

cond-mat.mes-hall↗

arXiv2026

Nonlinear Magnetic Orbital Hall Effect Induced by Spin-Orbit Coupling

Electrical readout of 180$^\circ$ switching in strictly compensated collinear antiferromagnets remains a major challenge in antiferromagnetic spintronics. Electrical writing of perpendicularly magnetized ferromagnets by out-of-plane orbital torque remains an important challenge in orbitronics. In this work, we propose a second-order nonlinear magnetic orbital Hall effect in the source antiferromagnet as a simultaneous recipe for both difficulties. This orbitronics effect is induced by spin-orbit coupling and is odd in the N\'eel vector, thus is a unique effect that integrates both functionalities via electric control of the N\'eel vector in the source antiferromagnet. Our first-principles calculations in CuMnAs predict significant non-perturbative orbital effects from spin-orbit coupling, with a orbital Berry-curvature dipole mechanism. These findings unveil new possibilities opened by topological antiferromagnetic orbitronics.

cond-mat.mtrl-sci↗

arXiv2026

Nonperturbative Magnetic Orbital Hall Effect in Altermagnets

Recent studies on altermagnets have focused considerable attention on nonrelativistic effects that persist in the absence of spin-orbit coupling (SOC). As a result, the relative importance of various phenomena in altermagnets has commonly been judged by their dependence on SOC. Here, we challenge this common wisdom by uncovering the magnetic orbital Hall effect, which is nonperturbative in SOC strength. We establish the symmetry properties of this effect, demonstrating that it is strictly forbidden in conventional collinear antiferromagnets yet universally allowed in all ten spin-Laue classes of collinear altermagnets. Counterintuitively, although SOC-induced, it reaches giant magnitudes in altermagnets-comparable to or even exceeding the nonrelativistic spin Hall effect. Moreover, altermagnetic symmetry enables unconventional collinear-polarized orbital currents, allowing field-free manipulation of perpendicular magnetization. Our first-principles calculations predict strong room-temperature responses in the experimentally established altermagnets CrSb and FeSb2. These findings reveal the previously overlooked potential of altermagnetic orbitronics and broaden the horizons for altermagnets in high-performance magnetic memory applications.

cond-mat.mtrl-sci↗

arXiv2025

Out-of-Plane Nonlinear Orbital Hall Torque

Despite recent advances in orbitronics, generating out-of-plane orbital torques essential for field-free deterministic switching of perpendicular magnetization remains a key challenge. Here, we propose a strategy to produce such unconventional torques across broad classes of materials, by leveraging the nonlinear orbital Hall effect. We demonstrate that this nonlinear orbital response is dramatically amplified by topological band degeneracies, where it overwhelmingly dominates the spin response even in systems with strong spin-orbit coupling. These features are confirmed via a quantitative investigation of representative topological metals RhSi, YPtBi, and PbTaSe$_2$, by combining our theory with first-principles calculations. The resulting orbital torques substantially surpass those from linear mechanisms reported thus far. These findings propel the research of orbital transport into the nonlinear regime, broaden the scope of orbital source materials, and establish a new pathway towards high-performance orbitronic devices.

cond-mat.mtrl-sci↗

arXiv2024

Intrinsic Dynamic Generation of Spin Polarization by Time-Varying Electric Field

Electric control of spin in insulators is desired for low-consumption and ultrafast spintronics, but the underlying mechanism remains largely unexplored. Here, we propose an intrinsic effect of dynamic spin generation driven by time-varying electric field. In the intraband response regime, it can be nicely formulated as a Berry curvature effect and leads to two phenomena that are forbidden in the $dc$ limit: linear spin generation in nonmagnetic insulators and intrinsic N{\'e}el spin-orbit torque in $\mathcal{PT}$-symmetric antiferromagnetic insulators. These phenomena are driven by the time derivative of field rather than the field itself, and have a quantum origin in the first-order dynamic anomalous spin polarizability. Combined with first-principles calculations, we predict sizable effects driven by terahertz field in nonmagnetic monolayer Bi and in antiferromagnetic even-layer MnBi$_2$Te$_4$, which can be detected in experiment.

cond-mat.mes-hall↗

arXiv2024

Intrinsic Nonlinear Spin Hall Effect and Manipulation of Perpendicular Magnetization

We propose an intrinsic nonlinear spin Hall effect, which enables the generation of collinearly-polarized spin current in a large class of nonmagnetic materials with the corresponding linear response being symmetry-forbidden. This opens a new avenue for field-free switching of perpendicular magnetization, which is required for the next-generation information storage technology. We develop the microscopic theory of this effect, and clarify its quantum origin in band geometric quantities which can be enhanced by topological nodal features. Combined with first-principles calculations, we predict pronounced effects at room temperature in topological metals $\mathrm{PbTaSe_{2}}$ and PdGa. Our work establishes a fundamental nonlinear response in spin transport, and opens the door to exploring spintronic applications based on nonlinear spin Hall effect.

cond-mat.mes-hall↗

arXiv2024

Quantum Metric Nonlinear Spin-Orbit Torque Enhanced by Topological Bands

Effects manifesting quantum geometry have been a focus of physics research. Here, we reveal that quantum metric plays a crucial role in nonlinear electric spin response, leading to a quantum metric spin-orbit torque. We argue that enhanced quantum metric can occur at band (anti)crossings, so the nonlinear torque could be amplified in topological metals with nodal features close to Fermi level. By applying our theory to magnetic Kane-Mele model and monolayer CrSBr, which feature nodal lines and Weyl points, we demonstrate that the quantum metric torque dominates the response, and its magnitude is significantly enhanced by topological band structures, which even surpasses the previously reported linear torques and is sufficient to drive magnetic switching by itself.

cond-mat.mes-hall↗

arXiv2023

Ultrathick MA$_2$N$_4$(M'N) Intercalated Monolayers with Sublayer-Protected Fermi Surface Conduction States: Interconnect and Metal Contact Applications

Recent discovery of ultrathick $\mathrm{MoSi_2N_4(MoN)_n}$ monolayers open up an exciting platform to engineer 2D material properties via intercalation architecture. Here we computationally investigate a series of ultrathick MA$_2$N$_4$(M'N) monolayers (M, M' = Mo, W; A = Si, Ge) under both homolayer and heterolayer intercalation architectures in which the same and different species of transition metal nitride inner core layers are intercalated by outer passivating nitride sublayers, respectively. The MA$_2$N$_4$(M'N) monolayers are thermally, dynamically and mechanically stable with excellent mechanical strength and metallic properties. Intriguingly, the metallic states around Fermi level are localized within the inner core layers. Carrier conduction mediated by electronic states around the Fermi level is thus spatially insulated from the external environment by the native outer nitride sublayers, suggesting the potential of MA$_2$N$_4$(M'N) in back-end-of-line (BEOL) metal interconnect applications. Nitrogen vacancy defect at the outer sublayers creates `punch through' states around the Fermi level that bridges the carrier conduction in the inner core layers and the outer environment, forming a electrical contact akin to the `vias' structures of metal interconnects. We further show that MoSi$_2$N$_4$(MoN) can serve as a quasi-Ohmic contact to 2D WSe$_2$. These findings reveal the promising potential of ultrathick MA$_2$N$_4$(MN) monolayers as metal electrodes and BEOL interconnect applications.

cond-mat.mtrl-sci↗

arXiv2023

Quadratic nodal point in a two-dimensional noncollinear antiferromagnet

Quadratic nodal point (QNP) in two dimensions has so far been reported only in nonmagnetic materials and in the absence of spin-orbit coupling. Here, by first-principles calculations and symmetry analysis, we predict stable QNP near Fermi level in a two-dimensional kagome metal-organic framework material, Cr$_3$(HAB)$_2$, which features noncollinear antiferromagnetic ordering and sizable spin-orbit coupling. Effective kp and lattice models are constructed to capture such magnetic QNPs. Besides QNP, we find Cr$_3$(HAB)$_2$ also hosts six magnetic linear nodal points protected by mirror as well as $C_{2z}T$ symmetry. Properties associated to these nodal points, such as topological edge states and quantized optical absorbance, are discussed.

cond-mat.mes-hall↗

arXiv2023

Phonon chirality manipulation mechanism in TMD interlayer-sliding ferroelectrics

As an ideal platform, both the theoretical prediction and first experimental verification of chiral phonons are based on transition-metal dichalcogenide materials. The manipulation of phonon chirality in these materials will have a profound impact on the study of chiral phonons. In this work, we utilize the sliding ferroelectric mechanism to study the phonon chirality manipulation mechanism in transition-metal dichalcogenide materials. Based on first-principles calculations, we study the different effects of interlayer sliding on the phonon properties in bilayer and four-layer MoS$_2$ sliding ferroelectrics. We find that sliding can regulate phonon chirality and Berry curvature, which further affects the phonon angular momentum and magnetization under a temperature gradient and the phonon Hall effect under a magnetic field. Our work connects two emerging fields and opens up a new route to manipulate phonon chirality in transition-metal dichalcogenide materials through the sliding ferroelectric mechanism.

cond-mat.mtrl-sci↗

arXiv2023

Half-Valley Ohmic Contact and Contact-Limited Valley-Contrasting Current Injection

Two-dimensional (2D) ferrovalley semiconductor (FVSC) with spontaneous valley polarization offers an exciting material platform for probing Berry phase physics. How FVSC can be incorporated in valleytronic device applications, however, remain an open question. Here we generalize the concept of metal/semiconductor (MS) contact into the realm of valleytronics. We propose a half-valley Ohmic contact based on FVSC/graphene heterostructure where the two valleys of FVSC separately forms Ohmic and Schottky contacts with those of graphene, thus allowing current to be valley-selectively injected through the `Ohmic' valley while being blocked in the `Schottky' valley. We develop a theory of contact-limited valley-contrasting current injection and demonstrate that such transport mechanism can produce gate-tunable valley-polarized injection current. Using RuCl$_2$/graphene heterostructure as an example, we illustrate a device concept of valleytronic barristor where high valley polarization efficiency and sizable current on/off ratio, can be achieved under experimentally feasible electrostatic gating conditions. These findings uncover contact-limited valley-contrasting current injection as an efficient mechanism for valley polarization manipulation, and reveals the potential of valleytronic MS contact as a functional building block of valleytronic device technology.

cond-mat.mes-hall↗

arXiv2023

Upper bound of a band complex

Band structure for a crystal generally consists of connected components in energy-momentum space, known as band complexes. Here, we explore a fundamental aspect regarding the maximal number of bands that can be accommodated in a single band complex. We show that in principle a band complex can have no finite upper bound for certain space groups. It means infinitely many bands can entangle together, forming a connected pattern stable against symmetry-preserving perturbations. This is demonstrated by our developed inductive construction procedure, through which a given band complex can always be grown into a larger one by gluing a basic building block to it. As a by-product, we demonstrate the existence of arbitrarily large accordion type band structures containing $N_C=4n$ bands, with $n\in\mathbb{N}$.

cond-mat.mtrl-sci↗

arXiv2022

Emergent Anti-ferromagnetism in a Y -Shaped Kekul\'e Graphene

Antiferromagnetic (AF) transitions of birefringent Dirac fermions created by a Y-shaped Kekul\'e distortion in graphene are investigated by the mean-field theory and the determinant quantum Monte Carlo simulations. We show that the quantum critical point can be continuously tuned by the bond-modulation strength, and the universality of the quantum criticality remains in the Gross-Neveu-Heisenberg class. The critical interaction scales with the geometric average of the two velocities of the birefringent Dirac cones and decreases monotonically between the uniform and the completely depleted limits. Since the AF critical interaction can be tuned to very small values, antiferromagnetism may emerge automatically, realizing the long-sought magnetism in graphene. These results enrich our understanding of the semimetal-AF transitions in Dirac-fermion systems and open a new route to achieving magnetism in graphene.

cond-mat.str-el↗