arXiv subjects

Shen Lai

Publications and source records attributed to Shen Lai.

14 recordsLinked to original sources

arXiv2026

Quantum Geometric Origin of Nonlinear Current Induced Orbital Magnetization

Electric generation of magnetization is a focus of condensed matter research, and has recently been advanced into the nonlinear regime. However, due to the nonlocal nature of orbital magnetism, how to properly formulate nonlinear current-induced orbital magnetization remains a fundamental challenge. Here, we develop the proper theory for this effect. This is based on the microscopic derivation of field-corrected orbital magnetic moment of a Bloch electron, a critical missing piece in the present theory. We show that the quantum geometric origin of this phenomenon lies in both the anomalous orbital polarizability and the Berry-connection polarizability, which often provide competing contributions. Combining our theory with first-principles calculations, we predict significant, experimentally accessible nonlinear orbital magnetization generated in strained bilayer graphene, monolayer 1T' $\mathrm{MoS_2}$ and $\mathrm{MoTe_2}$. Remarkably, nonlinear orbital magnetization can dominate over its spin counterpart in materials with topological band features, irrespective of the spin-orbit coupling strength.

cond-mat.mes-hall↗

arXiv2026

Eccentricity valley Hall effect

Valleytronics harnesses the valley degree of freedom -- energy-degenerate extrema in the electronic band structure -- for information storage and processing. Valley Hall effect (VHE) is a cornerstone of valleytronics, enabling electric generation of pure valley currents. While extensively studied in systems with valleys located at time-reversal-breaking points, here, we shift the paradigm to valleytronic platforms with time-reversal-invariant valleys (TRIVs), revealing a novel phenomenon: eccentricity VHE. Unlike conventional VHE, the valley Hall angle for eccentricity VHE is an intrinsic geometric property, governed solely by the eccentricity of the valley Fermi surface, rendering it highly robust against variations in temperature or carrier density. Eccentricity VHE emerges universally across all 25 layer groups supporting TRIVs. We demonstrate these distinctive features in monolayer GeS$_{2}$ via first-principles calculations, predicting a significant valley Hall angle of 0.74. This effect can be detected through nonlocal transport measurements exhibiting characteristic scaling behavior, or, in certain cases, through valley-layer coupling. Our findings reveal a critical overlooked facet of valley Hall physics, transcend the established VHE paradigm, and significantly broadens the scope of valleytronics.

cond-mat.mes-hall↗

arXiv2025

Lorentz Skew Scattering Nonreciprocal Magneto-Transport

In materials with broken inversion symmetry, nonreciprocal magneto-transport (NRMT) manifests as a bilinear dependence of charge conductivity on applied electric (E) and magnetic (B) fields. This phenomenon is deeply rooted in symmetry and electronic quantum geometry, holding promise for novel rectification and detector technologies. Existing experimental studies generally attribute NRMT to Zeeman-driven mechanisms and exhibit quadratic scaling with conductivity. Here, we report a previously unknown NRMT microscopic mechanism - Lorentz skew scattering (LSK) - revealed through the discovery of an unprecedented quartic scaling law of NRMT as well as quantitative agreement between theory and experiment in BiTeBr. LSK emerges from the interplay of Lorentz force and skew scattering, bridging classical field effect to quantum scattering effect on the Fermi surface. We demonstrate that the LSK dominates NRMT in BiTeBr, and elucidate that this dominance over other possible contributions stems from high mobility and strong Rashba splitting. The finding of LSK mechanism is of unique importance because it unveils the leading NRMT effect in high-mobility systems and suggests a universal principle towards strong NRMT by enhancing electronic relaxation time in topological materials, rendering a new designing idea for low-dissipation rectifiers and high-performance quantum electronics.

cond-mat.mes-hall↗

arXiv2025

Intrinsic nonlinear valley Nernst effect

We investigate the intrinsic nonlinear valley Nernst effect, which induces a transverse valley current via a second-order thermoelectric response to a longitudinal temperature gradient. The effect arises from the Berry connection polarizability dipole of valley electrons and is permissible in both inversion-symmetric and inversion-asymmetric materials. We demonstrate that the response tensor is connected to the intrinsic nonlinear valley Hall conductivity through a generalized Mott relation, with the two being directly proportional at low temperatures, scaled by the Lorenz number. We elucidate the symmetry constraints governing this effect and develop a theory for its nonlocal measurement, revealing a nonlocal second-harmonic signal with a distinct $\rho^2$ scaling. This signal comprises two scaling terms, with their ratio corresponding to the square of the thermopower normalized by the Lorenz number. Key characteristics are demonstrated using a tilted Dirac model and first-principles calculations on bilayer WTe$_2$. Possible extrinsic contributions and alternative experimental detection methods, e.g., by valley pumping and by nonreciprocal directional dichroism, are discussed. These findings underscore the significance of band quantum geometry on electron dynamics and establish a theoretical foundation for nonlinear valley caloritronics.

cond-mat.mes-hall↗

arXiv2025

Nonlinear N\'eel Spin-Orbit Torque in Centrosymmetric Antiferromagnets

Electric control of N\'eel vector is a central task of antiferromagnetic (AFM) spintronics. The major scheme so far relies on the linear N\'eel torque, which however is restricted to AFMs with broken inversion symmetry. Here, we propose a nonlinear N\'eel 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

Theory of Nonlocal Transport from Nonlinear Valley Responses

We develop a theory for the nonlocal measurement of nonlinear valley Hall effect. Different from the linear case where the direct and the inverse processes are reciprocal, we unveil that the nonlinear inverse valley Hall effect needed to generate nonlocal voltage signal must have a distinct symmetry character and involve distinct mechanisms compared to the nonlinear valley Hall response it probes. Particularly, it must be valley-even, in contrast to both linear and nonlinear valley Hall effects which are valley-odd. Layer groups that permit such nonlocal valley responses are obtained via symmetry analysis, and formulas for the nonlocal signals are derived. In the presence of both linear and nonlinear valley responses, we show that the different responses can be distinguished by their distinct scaling behaviors in the different harmonic components, under a low-frequency ac driving. Combined with first-principles calculations, we predict sizable nonlocal transport signals from nonlinear valley responses in bilayer $T_{d}$-WTe$_{2}$. Our work lays a foundation for nonlocal transport studies on the emerging nonlinear valleytronics.

cond-mat.mes-hall↗

arXiv2024

Tunable linear and nonlinear anomalous Hall transport in two-dimensional CrPS$_{4}$

Few-layer CrPS$_{4}$ is a two-dimensional (2D) magnetic material with excellent stability in ambient environment, which attracted significant interest in recent research. Here, via first-principles calculations, we show that 2D CrPS$_{4}$ hosts a variety of anomalous Hall transport phenomena, owing to its layer-dependent magnetism and symmetry character. Monolayer CrPS$_{4}$ can display a sizable linear anomalous Hall effect, while its nonlinear anomalous Hall response is forbidden. In contrast, bilayer CrPS$_{4}$ can produce pronounced intrinsic nonlinear anomalous Hall response from Berry-connection polarizability, in the absence of linear anomalous Hall effect. We clarify that the large peaks in these responses originate from gapped Dirac points in the band structure. Furthermore, we show that linear anomalous Hall effect can be induced and controlled in bilayer CrPS$_{4}$ by gate electric field or in-plane magnetic field, which break the spacetime inversion symmetry. Our findings unveil the interesting layer-dependent Hall transport physics in 2D CrPS$_{4}$ magnets, suggesting its potential in electronic and spintronic device applications.

cond-mat.mes-hall↗

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

Superionic surface Li-ion transport in carbonaceous materials

Unlike Li-ion transport in the bulk of carbonaceous materials, little is known about Li-ion diffusion on their surface. In this study, we have discovered an ultra-fast Li-ion transport phenomenon on the surface of carbonaceous materials, particularly when they have limited Li insertion capacity along with a high surface area. This is exemplified by a carbon black, Ketjen Black (KB). An ionic conductivity of 18.1 mS cm-1 at room temperature is observed, far exceeding most solid-state ion conductors. Theoretical calculations reveal a low diffusion barrier for the surface Li species. The species is also identified as Li*, which features a partial positive charge. As a result, lithiated KB functions effectively as an interlayer between Li and solid-state electrolytes (SSE) to mitigate dendrite growth and cell shorting. This function is found to be electrolyte agnostic, effective for both sulfide and halide SSEs. Further, lithiated KB can act as a high-performance mixed ion/electron conductor that is thermodynamically stable at potentials near Li metal. A graphite anode mixed with KB instead of a solid electrolyte demonstrates full utilization with a capacity retention of ~85% over 300 cycles. The discovery of this surface-mediated ultra-fast Li-ion transport mechanism provides new directions for the design of solid-state ion conductors and solid-state batteries.

cond-mat.mtrl-sci↗

arXiv2022

Nonlinear Hall effect with non-centrosymmetric topological phase in ZrTe$_5$

The non-centrosymmetric topological material has attracted intense attention due to its superior characters as compared to the centrosymmetric one. On one side, the topological phase coming from global geometric properties of the quantum wave function remains unchanged, on the other side, abundant exotic phenomena are predicted to be merely emerged in non-centrosymmetric ones, due to the redistribution of local quantum geometry. Whereas, probing the local quantum geometry in non-centrosymmetric topological material remains challenging. Here, we report a non-centrosymmetric topological phase in ZrTe$_5$, probed by the nonlinear Hall (NLH) effect. The angle-resolved and temperature-dependent NLH measurement reveals the inversion and ab-plane mirror symmetries breaking under 30 K, consistent with our theoretical calculation. Our findings identify a new non-centrosymmetric phase of ZrTe$_5$ and provide a platform to probe and control local quantum geometry via crystal symmetries.

cond-mat.mtrl-sci↗

arXiv2021

Berry connection polarizability tensor and third-order Hall effect

One big achievement in modern condensed matter physics is the recognition of the importance of various band geometric quantities in physical effects. As prominent examples, Berry curvature and the Berry curvature dipole are connected to the linear and the second-order Hall effects, respectively. Here, we show that the Berry connection polarizability (BCP) tensor, as another intrinsic band geometric quantity, plays a key role in the third-order Hall effect. Based on the extended semiclassical formalism, we develop a theory for the third-order charge transport and derive explicit formulas for the third-order conductivity. Our theory is applied to the two-dimensional (2D) Dirac model to investigate the essential features of the BCP and the third-order Hall response. We further demonstrate the combination of our theory with the first-principles calculations to study a concrete material system, the monolayer FeSe. Our work establishes a foundation for the study of third-order transport effects, and reveals the third-order Hall effect as a tool for characterizing a large class of materials and for probing the BCP in band structure.

cond-mat.mes-hall↗

arXiv2021

Room-temperature bipolar valleytronic transistor in MoS2/WSe2 heterostructures

Valley degree of freedom in the 2D semiconductor is a promising platform for the next generation optoelectronics. Electrons in different valleys can have opposite Berry curvature, leading to the valley Hall effect (VHE). However, VHE without the plasmonic structure's assistance has only been reported in cryogenic temperature, limiting its practical application. Here, we report the observation of VHE at room temperature in the MoS2/WSe2 heterostructures. We also uncover that both the magnitude and the polarity of the VHE in the 2D heterostructure is gate tunable. We attribute this to the opposite VHE contribution from the electron and hole in different layers. These results indicate the bipolar transport nature of our valleytronic transistor. Utilizing this gate tunability, we demonstrate a bipolar valleytronic transistor. Our results can be used to improve the ON/OFF ratio of the valleytronic transistor and to realize more versatile valleytronics logic circuits.

cond-mat.mes-hall↗

arXiv2019

Magnetic Proximity Effect in 2D Ferromagnetic CrBr3/Graphene van der Waals Heterostructures

Two-dimensional (2D) van der Waals heterostructures serve as a promising platform to exploit various physical phenomena in a diverse range of novel spintronic device applications. The efficient spin injection is the prerequisite for these devices. The recent discovery of magnetic 2D materials leads to the possibility of fully 2D van der Waals spintronics devices by implementing spin injection through magnetic proximity effect (MPE). Here, we report the investigation of magnetic proximity effect in 2D CrBr3/graphene van der Waals heterostructures, which is probed by Zeeman spin Hall effect through non-local measurements. Zeeman splitting field estimation demonstrates a significant magnetic proximity exchange field even in a low magnetic field. Furthermore, the observed anomalous longitudinal resistance changes at the Dirac point R_(XX,D)with increasing magnetic field at {\nu} = 0 may attribute to the MPE induced new ground state phases. This MPE revealed in our CrBr3/graphene van der Waals heterostructures therefore provides a solid physics basis and key functionality for next generation 2D spin logic and memory devices.

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