arXiv subjects

Harsh Varshney

Publications and source records attributed to Harsh Varshney.

11 recordsLinked to original sources

arXiv2026

Quantum oscillation spectroscopy of Fermi-surface topologies in tetralayer graphene

Quantum oscillations offer a direct probe of Fermi-surface topology and electronic degeneracy, yet disentangling both simultaneously across the Lifshitz transitions of multiband systems has remained an open experimental challenge. Here, we use Shubnikov-de Haas spectroscopy on a high-mobility, dual-gated Bernal-stacked tetralayer graphene (B-4LG) device to quantitatively reconstruct the complete sequence of six distinct Fermi-surface topologies-gully, annular, singly connected, and multiband pockets. The extracted oscillation frequencies determine the extremal momentum-space areas and their spin, valley, and gully-resolved degeneracies, in quantitative agreement with our tight-binding calculations. We further show that a perpendicular magnetic-field, combined with displacement-field lifts the valley degeneracy through an orbital-Zeeman coupling, producing a single-particle valley splitting of several $meV$, far larger than in bilayer or trilayer graphene. Our work demonstrates a framework for tracking Fermi-surface topologies and their flavor degeneracies in multiband quantum materials.

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arXiv2026

Longitudinal Nonreciprocal Charge Transport with Time Reversal Symmetry

Longitudinal nonreciprocal charge transport is usually associated with broken time-reversal symmetry, either from magnetic order or an external magnetic field. Here, we show that it can also arise in nonmagnetic conductors preserving time-reversal symmetry through disorder-induced asymmetric scattering. Within a semiclassical Boltzmann theory, skew-scattering and side-jump processes generate a finite longitudinal current quadratic in the electric field. Our symmetry analysis identifies 42 point groups that allow this longitudinal nonreciprocal response. As a concrete example, gated Bernal-stacked bilayer graphene shows a gate-tunable nonreciprocal response with clear enhancement near its Lifshitz transition. These results identify disorder-driven asymmetric scattering as a route to bulk longitudinal nonreciprocal charge transport in crystalline conductors.

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arXiv2026

Asymmetric Scattering Drives Large Nonlinear Nernst and Seebeck Effects

The nonlinear Nernst and Seebeck effects (NNE and NSE) offer promising routes for thermoelectric energy conversion in non-magnetic systems. While intrinsic mechanisms such as the nonlinear Drude and Berry-curvature-dipole terms are well established, extrinsic contributions to thermoelectric responses arising from disorder-induced asymmetric scattering remain comparatively less explored, despite growing experimental evidence of their dominance. Here, we develop a unified semiclassical theory of NNE and NSE that incorporates skew scattering and side-jump processes, identifying four distinct extrinsic contributions to NNE and two for NSE. A systematic symmetry analysis shows that these responses are allowed in time-reversal-symmetric non-magnets, PT-symmetric antiferromagnets, and non-centrosymmetric magnetic systems such as altermagnets. As a case study, we demonstrate that ABA-stacked trilayer graphene hosts large nonlinear Nernst and Seebeck responses dominated by extrinsic scattering, in excellent agreement with recent experiments. Our results establish the microscopic origin of these effects and provide guiding principles for designing high-efficiency nonlinear thermoelectric devices.

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arXiv2025

Spin band geometry drives intrinsic thermal spin magnetization and current

Generating spin magnetization and spin currents without magnetic or electric fields is a key frontier in spin caloritronics. Spin responses driven by thermal gradients offer a promising route, though the band geometric origin of intrinsic mechanisms, especially in non-magnetic materials, remains poorly understood. Here we develop a unified quantum theory of thermal spin magnetization and spin currents in itinerant electrons, rooted in spin band geometry with both Fermi-surface and Fermi-sea contributions. We identify two key geometric quantities: the spin-velocity metric tensor, which governs thermal spin magnetization, and the spin geometric tensor, combining spin Berry curvature and spin quantum metric, which generates thermal spin currents. These intrinsic contributions persist and can even dominate in non-magnetic insulators. Numerical calculations for chiral metal RhGe and antiferromagnet CuMnAs demonstrate sizable thermal spin responses near band crossings. Our results establish the band geometric origin of thermal spin transport and provide guiding principles for discovering and engineering next-generation spin caloritronic materials.

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arXiv2025

Detecting Lifshitz Transitions Using Nonlinear Conductivity in Bilayer Graphene

The second-order nonlinear electrical response (NLER) is an intrinsic property of inversion symmetry-broken systems which can provide deep insights into the electronic band structures of atomically thin quantum materials. However, the impact of Fermi surface reconstructions, also known as Lifshitz transitions, on the NLER has remained elusive. We investigated NLER in bilayer graphene (BLG), where the low-energy bands undergo Lifshitz transitions. Here, NLER undergoes a sign change near the Lifshitz transitions even at elevated temperatures $T\gtrsim10~$K. At the band edge, NLER in BLG is modulated by both extrinsic scattering and interfacial-strain-induced intrinsic Berry curvature dipole, both of which can be finely tuned externally by varying doping and interlayer potential. Away from the band edge, BLG exhibits second-order conductivity exceeding $30~\mu$mV$^{-1}\Omega^{-1}$ at 3K higher than any previous report. Our work establishes NLER as a reliable tool to probe Lifshitz transitions in quantum materials.

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arXiv2025

Planar Nernst effect from hidden band geometry in layered two-dimensional materials

The Nernst effect is a versatile phenomenon relevant for energy harvesting, magnetic sensing, probing band topology and charge-neutral excitations. The planar Nernst effect (PNE) generates an in-plane voltage transverse to an applied temperature gradient under an in-plane magnetic field. Conventional Berry curvature-induced PNE is absent in two-dimensional (2D) systems, as the out-of-plane Berry curvature does not couple to the in-plane electron velocity. We challenge this notion by demonstrating a distinct planar Nernst effect in quasi-2D materials (2DPNE). We show that the 2DPNE originates from previously overlooked planar components of Berry curvature and orbital magnetic moment, arising from inter-layer tunneling in multilayered 2D systems. We comprehensively analyze the band-geometric origin and crystalline symmetry constraints on 2DPNE responses. We illustrate its experimental feasibility in strained bilayer graphene. Our findings significantly expand the theoretical understanding of planar Nernst effects, providing a clear pathway for next-generation magnetic sensing and energy-harvesting applications.

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arXiv2024

Intrinsic nonlinear Nernst and Seebeck effect

The Nernst and Seebeck effects are crucial for thermoelectric energy harvesting. However, the linear anomalous Nernst effect requires magnetic materials with intrinsically broken time-reversal symmetry. In non-magnetic systems, the dominant transverse thermoelectric response is the nonlinear Nernst current. Here, we investigate nonlinear Nernst and Seebeck effects to reveal intrinsic scattering-free Seebeck and Nernst currents arising from band geometric effects in bipartite antiferromagnets (parity-time-reversal symmetric systems). We show that these contributions, independent of scattering time, originate from the Berry connection polarizability tensor which depends on the quantum metric. Using CuMnAs as a model system, we demonstrate the dominance of intrinsic nonlinear Seebeck and Nernst currents over other scattering-dependent contributions. Our findings deepen the fundamental understanding of nonlinear thermoelectric phenomena and provide the foundation for using them to develop more efficient, next-generation energy harvesting devices.

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arXiv2024

Nonlinear electrical transport unveils Fermi surface malleability in a moir\'e heterostructure

Van Hove singularities enhance many-body interactions and induce collective states of matter ranging from superconductivity to magnetism. In magic-angle twisted bilayer graphene, van Hove singularities appear at low energies and are malleable with density, leading to a sequence of Lifshitz transitions and resets observable in Hall measurements. However, without a magnetic field, linear transport measurements have limited sensitivity to the band's topology. Here, we utilize nonlinear longitudinal and transverse transport measurements to probe these unique features in twisted bilayer graphene at zero magnetic field. We demonstrate that the nonlinear responses, induced by the Berry curvature dipole and extrinsic scattering processes, intricately map the Fermi surface reconstructions at various fillings. Importantly, our experiments highlight the intrinsic connection of these features with the moir\'e bands. Beyond corroborating the insights from linear Hall measurements, our findings establish nonlinear transport as a pivotal tool for probing band topology and correlated phenomena.

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arXiv2024

Planar Hall Effect in Quasi-Two-Dimensional Materials

The planar Hall effect in 3D systems is an effective probe for their Berry curvature, topology, and electronic properties. However, the Berry curvature-induced conventional planar Hall effect is forbidden in 2D systems as the out-of-plane Berry curvature cannot couple to the band velocity of the electrons moving in the 2D plane. Here, we demonstrate a unique 2D planar Hall effect (2DPHE) originating from the hidden planar components of the Berry curvature and orbital magnetic moment in quasi-2D materials. We identify all planar band geometric contributions to 2DPHE and classify their crystalline symmetry restrictions. Using gated bilayer graphene as an example, we show that in addition to capturing the hidden band geometric effects, 2DPHE is also sensitive to the Lifshitz transitions. Our work motivates further exploration of hidden planar band geometry-induced 2DPHE and related transport phenomena for innovative applications.

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arXiv2023

Intrinsic nonlinear thermal Hall transport of magnons: A Quantum kinetic theory approach

We present a systematic study of the nonlinear thermal Hall responses in bosonic systems using the quantum kinetic theory framework. We demonstrate the existence of an intrinsic nonlinear boson thermal current, arising from the quantum metric which is a wavefunction dependent band geometric quantity. In contrast to the nonlinear Drude and nonlinear anomalous Hall contributions, the intrinsic nonlinear thermal conductivity is independent of the scattering timescale. We demonstrate the dominance of this intrinsic thermal Hall response in topological magnons in a two-dimensional ferromagnetic honeycomb lattice without Dzyaloshinskii-Moriya interaction. Our findings highlight the significance of band geometry induced nonlinear thermal transport and motivate experimental probe of the intrinsic nonlinear thermal Hall response with implications for quantum magnonics.

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arXiv2022

Quantum kinetic theory of nonlinear thermal current

We investigate the second-order nonlinear electronic thermal transport induced by temperature gradient. We develop the quantum kinetic theory framework to describe thermal transport in presence of a temperature gradient. Using this, we predict an intrinsic scattering time independent nonlinear thermal current in addition to the known extrinsic nonlinear Drude and Berry curvature dipole contributions. We show that the intrinsic thermal current is determined by the band geometric quantities and is non-zero only in systems where both the space inversion and time-reversal symmetries are broken. We employ the developed theory to study the thermal response in tilted massive Dirac systems. We show that besides the different scattering time dependence, the various current contributions have distinct temperature dependence in the low temperature limit. Our systematic and comprehensive theory for nonlinear thermal transport paves the way for future theoretical and experimental studies on intrinsic thermal responses.

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