arXiv subjects

A. D. Levin

Publications and source records attributed to A. D. Levin.

At least 19 recordsLinked to original sources

arXiv2026

Hydrodynamic magnetotransport in a GaAs Corbino geometry

We report the observation of positive magnetoresistance in high-mobility GaAs Corbino devices. Over a broad intermediate-temperature range, the resistance exhibits a quadratic dependence on perpendicular magnetic field. We analyze the data within hydrodynamic theories of magnetotransport in the Corbino geometry, which describe the crossover between the diffusive and viscous regimes, including finite-slip boundary conditions appropriate for current-penetrable contacts. The extracted relaxation rates are consistent with an approximately $T^2$ temperature dependence of the electron-electron scattering contribution. The extracted viscous relaxation parameters are consistent with those obtained from Hall-bar measurements. Comparison with theory indicates that the observed magnetoresistance is predominantly governed by the bulk hydrodynamic response, while finite slip modifies the Stokes--Ohm crossover quantitatively and the field-dependent boundary voltage provides a separate correction. These results show that Corbino magnetotransport can serve as a complementary bulk-sensitive probe of viscous electron flow.

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arXiv2026

Electron Hydrodynamics and Bernoulli Effect in Venturi-Shaped 2D Systems

The study of electron hydrodynamics provides a powerful framework for understanding transport in ultraclean conductors, yet experimental evidence has thus far been largely restricted to the linear-response regime. Here, we report the direct observation of a strongly nonlinear transport regime in a high-mobility two-dimensional electron system. By engineering devices with a Venturi-shaped wedge geometry specifically designed to enhance convective nonlinearities, we uncover a pronounced nonlinear voltage response and large diodicity in the current-voltage characteristics. Our experimental findings show quantitative agreement with a theoretical model that attributes the observed nonlinearity to the convective acceleration of the electron fluid, analogous to the Bernoulli effect. These results provide compelling evidence for the applicability of the hydrodynamic framework to two-dimensional electron transport and open new avenues for exploring nonlinear and preturbulent phenomena in solid-state systems.

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arXiv2026

Spectroscopy of Heat Transport and Violation of the Wiedemann--Franz Law in a GaAs Hydrodynamic Mesoscopic Channel

The Wiedemann--Franz law, which determines the universality of the ratio of thermal conductivity to electrical conductivity, is studied in the hydrodynamic electron transport regime, where electron--electron scattering predominates over scattering by disorder. In this case, the different relaxation of electric and thermal currents can lead to a violation of the Wiedemann--Franz law, which is expected to be even more pronounced in mesoscopic electron systems. This paper reports the propagation of hot electrons in a GaAs hydrodynamic narrow channel, studied using micrometer-resolution photoluminescence thermometry. A temperature dependence of the Lorenz number was obtained, indicating a violation of the Wiedemann--Franz law. The important role of narrow constrictions in this violation was also demonstrated, and theoretical arguments are presented.

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arXiv2026

Viscous Electron Flow and Nonlinear Magnetotransport in 2D Channels

We examine nonlinear transport in a viscous two-dimensional electron fluid within narrow GaAs channels. The differential magnetoresistance shows nonmonotonic behavior, a signature of electron pairing in the hydrodynamic regime. Theoretical models that account for both the influence of these interactions on shear stress relaxation and viscosity changes from electron heating show good agreement with the data. The nonlinear regime thus reveals how such correlated states govern the hydrodynamic behavior of the electron fluid. Our findings establish the nonlinear transport regime as a powerful probe for dissecting the complex interplay of correlated electron states and momentum relaxation in the hydrodynamic flow of an electron fluid.

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arXiv2026

Interaction Induced Magnetotransport in a 2D Dirac-Heavy Hole Hybrid Band System

While electron-electron (e-e) interactions are known to influence resistivity in non-Galilean invariant two-dimensional (2D) systems, their effect on magnetotransport is not fully understood. Conventional models for simple bands often predict a vanishing magnetoresistivity from e-e interactions alone. In this work, we investigate magnetotransport in a gapless 6.3 nm HgTe quantum well, a hybrid 2D band system that hosts coexisting holes with both linear (Dirac-like) and parabolic energy bands. Focusing on the high temperature regime where particle-particle collisions dominate scattering, we observe significant corrections to both the magnetoresistivity and the Hall effect. The high temperature transport coefficients are in good agreement with the theoretical model describing transport in massive-massless fermion mixtures governed by a frictional mechanism and intervalley scattering. Our findings provide strong experimental validation for this theoretical framework, demonstrating that collisions between particles with different dispersions are a key mechanism governing magnetotransport in hybrid band semimetals.

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arXiv2025

Magnetotransport in a 2D Hybrid Band System: Dirac and Heavy Hole Interplay

We investigate magnetoresistivity and the Hall effect in a 6.3 nm gapless HgTe quantum well - a two-dimensional hybrid band system featuring coexisting linear (Dirac-like) and parabolic hole energy bands at low energies. Using a classical two-subband model that incorporates intervalley scattering, we reveal a striking tenfold enhancement of the Hall resistance, primarily driven by the dominant transport contribution of Dirac holes. A comprehensive magnetotransport analysis enables us to extract key parameters, such as the mobilities of both carrier types, thereby providing insight into their complex interplay. These results establish the HgTe quantum well as a distinctive platform for exploring novel transport phenomena in hybrid band systems and deepen our understanding of mixed-carrier magnetotransport.

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arXiv2025

Resistivity of non-Galilean invariant two dimensional Dirac system

We revisited the influence of electron-electron scattering on the resistivity of a two-dimensional system with linear spectrum. In conventional systems with parabolic spectrum, where Umklapp scattering is either prohibited or ineffective due to small Fermi surface, particle-particle scattering does not contribute to conductivity because it does not change the total momentum. However, within the framework of Boltzmann kinetic model, we demonstrate that electron-electron scattering in Dirac systems can significantly contribute to conductivity, producing distinct temperature-dependent corrections: a T\textsuperscript{4} behavior at low temperatures and T\textsuperscript{2} dependence at moderate temperatures. While the predicted T\textsuperscript{4} scaling is not observed experimentally -- likely suppressed by dominant weak localization effects -- the T\textsuperscript{2} scaling is clearly confirmed in our measurements. Specifically, temperature-dependent resistivity data from gapless single-valley HgTe quantum well exhibit T\textsuperscript{2} corrections, which align well with theoretical predictions. Thus, we challenge the paradigm that T\textsuperscript{2} term in resistivity is absent in single-band 2D metals.

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arXiv2025

Obstacle-Induced Gurzhi Effect and Hydrodynamic Electron Flow in Two-Dimensional Systems

The viscous flow of electrons in a narrow channel requires both strong electron-electron interactions and no-slip boundary conditions. However, introducing obstacles within the liquid can significantly increase flow resistance and, as a result, amplify the effects of viscosity. Even in samples with smooth walls, the presence of an obstacle can strongly alter electron behavior, leading to pronounced hydrodynamic effects. We investigated transport in mesoscopic samples containing a disordered array of obstacles. In contrast to samples without obstacles, which do not show a decrease in resistivity with rising temperature, samples with obstacles exhibit a significant resistivity reduction as temperature increases (the Gurzhi effect). By measuring the negative magnetoresistance, we extracted shear viscosity and other parameters through comparison with theoretical predictions. Consequently, narrow-channel samples with a disordered obstacle array provide a valuable platform for studying hydrodynamic electron flow independently of boundary conditions.

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arXiv2024

Bulk and shear viscosities in multicomponent 2D electron system

We investigated magnetotransport in mesoscopic samples containing electrons from three different subbands in GaAs triple wells. At high temperatures, we observed positive magnetoresistance, which we attribute to the imbalance between different types of particles that are sensitive to bulk viscosities. At low temperatures, we found negative magnetoresistance, attributed to shear viscosity. By analyzing the magnetoresistance data, we were able to determine both viscosities. Remarkably, the electronic bulk viscosity was significantly larger than the shear viscosity. Studying multicomponent electron systems in the hydrodynamic regime presents an intriguing opportunity to further explore the physics in systems with high bulk viscosity.

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arXiv2024

Interaction-controlled transport in a two-dimensional massless-massive Dirac system: Transition from degenerate to nondegenerate regimes

The resistivity of two-dimensional (2D) metals generally exhibits insensitivity to electron-electron scattering. However, it's worth noting that Galilean invariance may not hold true in systems characterized by a spectrum containing multiple electronic branches or in scenarios involving electron-hole plasma. In the context of our study, we focus on 2D electrons confined within a triple quantum well (TQW) based on HgTe. This system displays a coexistence of energy bands featuring both linear and parabolic-like spectra at low energy and, therefore, lacks the Galilean invariance. This research employs a combined theoretical and experimental approach to investigate the transport properties of this two-component system across various regimes. By manipulating carrier density and temperature, we tune our system from a fully degenerate regime, where resistance follows a temperature-dependent behavior proportional to $T^2$, to a regime where both types of electrons adhere to Boltzmann statistics. In the non-degenerate regime, electron interactions lead to resistance that is weakly dependent on temperature. Notably, our experimental observations closely align with the theoretical predictions derived in this study. This work establishes the HgTe-based TQW as a promising platform for exploring different interaction dominant scenarios for the massless-massive Dirac system.9 pages, 8 figures

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arXiv2024

Interaction dominated transport in 2D conductors: from degenerate to partially-degenerate regime

In this study, we investigate the conductivity of a two-dimensional (2D) system in HgTe quantum well comprising two types of carriers with linear and quadratic spectra, respectively. The interactions between the two-dimensional Dirac holes and the heavy holes lead to the breakdown of Galilean invariance, resulting in interaction-limited resistivity. Our exploration of the transport properties spans from low temperatures, where both subsystems are fully degenerate, to higher temperatures, where the Dirac holes remain degenerate while the heavy holes follow Boltzmann statistics, creating a partially degenerate regime. Through a developed theory, we successfully predict the behavior of resistivity as $ρ\sim T^2$ and $ρ\sim T^{3}$ for the fully degenerate and partially degenerate regimes, respectively, which is in reasonable agreement with experimental observations. Notably, at elevated temperatures, the interaction-limited resistivity surpasses the resistivity caused by impurity scattering by a factor of 5-6. These findings imply that the investigated system serves as a versatile experimental platform for exploring various interaction-limited transport regimes in two component plasma.

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arXiv2023

Geometric engineering of viscous magnetotransport in a two-dimensional electron system

In this study, we present our experimental investigation on the magnetotransport properties of a two-dimensional electron system in GaAs quantum wells utilizing a variety of device geometries, including obstacles with thin barriers and periodic width variations. Our primary focus is to explore the impact of these geometries on the electron viscous flow parameters, enabling precise manipulation of hydrodynamic effects under controlled conditions. Through an analysis of the large negative magnetoresistivity and zero field resistivity, we deduce the scattering times for electron-electron and electron-phonon interactions, as well as the effective channel width. Our findings confirm that the system under investigation serves as a tunable experimental platform for investigating hydrodynamic transport regimes at temperatures above 10 K.

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arXiv2022

Quantum transport of Dirac fermions in HgTe gapless quantum wells

We study transport properties of HgTe quantum wells with critical well thickness, where the band gap is closed, and the low energy spectrum is described by a single Dirac cone. In this work, we examined both macroscopic and micron-sized (mesocopic) samples. In micron-sized samples, we observe a magnetic field induced, quantized resistance ($\sim h/2e^{2}$) at Landau filling factor $ν=0$, corresponding to the formation of helical edge states centered at the charge neutrality point (CNP). In macroscopic samples, the resistance near zero Landau level (LL) reveals strong oscillations, which we attribute to scattering between the edge $ν=0$ state and bulk $ν\neq 0$ hole LL. We provide a model taking an empirical approach to construct a LL diagram based on a reservoir scenario, formed by the heavy holes.

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arXiv2021

Thermoelectric transport in a three-dimensional HgTe topological insulator

The thermoelectric response of 80-nm-thick strained HgTe films of a three-dimensional topological insulator (3D TI) has been studied experimentally. An ambipolar thermopower is observed where the Fermi energy moves from conducting to the valence bulk band. The comparison between theory and experiment shows that the thermopower is mostly due to the phonon drag contribution. In the region where the 2D Dirac electrons coexist with bulk hole states, the Seebeck coefficient is modified due to 2D electron - 3D hole scattering.

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arXiv2021

Viscous magnetotransport and Gurzhi effect in bilayer electron system

We observe a large negative magnetoresistance and a decrease of resistivity with increasing temperature, known as the Gurzhi effect, in a bilayer electron (BL) system formed by a wide GaAs quantum well. A hydrodynamic model for the single fluid transport parameters in narrow channels is employed and successfully describes our experimental findings. We find that the electron-electron scattering in the bilayer is more intensive in comparison with a single-band well (SW). The hydrodynamic assumption implies a strong dependence on boundary conditions, which can be characterized by slip length, describing the behavior of a liquid near the edge. Our results reveal that slip length in a BL is shorter than in a SW, and that the BL system goes deeper into the hydrodynamic regime. This is in agreement with the model proposed where the slip length is of the order of the electron-electron mean free path.

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arXiv2020

Manifestations of classical size effect and electronic viscosity in the magnetoresistance of narrow two-dimensional conductors: Theory and experiment

We develop a classical kinetic theory of magnetotransport of 2D electrons in narrow channels with partly diffusive boundary scattering and apply it to description of magnetoresistance measured in the temperature interval 4.2-30 K in long mesoscopic bars fabricated from high-purity GaAs quantum well structures. Both experiment and theory demonstrate a number of characteristic features in the longitudinal and Hall resistances caused by the size effect in two dimensions owing to the high ballisticity of the transport. In addition to the features described previously, we also reveal a change in the slope of the first derivative of magnetoresistance when the cyclotron orbit diameter equals to half of the channel width. These features are suppressed with increasing temperature as a result of the electronic viscosity due to electron-electron interaction. By comparing theory and experiment, we determine the characteristic time of relaxation of angular distribution of electrons caused by electron-electron scattering.

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arXiv2020

Stokes flow around an obstacle in viscous two-dimensional electron liquid

The electronic analog of the Poiseuille flow is the transport in a narrow channel with disordered edges that scatter electrons in a diffuse way. In the hydrodynamic regime, the resistivity decreases with temperature, referred to as the Gurzhi effect, distinct from conventional Ohmic behaviour. We studied experimentally an electronic analog of the Stokes flow around a disc immersed in a two-dimensional viscous liquid. The circle obstacle results in an additive contribution to resistivity. If specular boundary conditions apply, it is no longer possible to detect Poiseuille type flow and the Gurzhi effect. However, in flow through a channel with a circular obstacle, the resistivity decreases with temperature. By tuning the temperature, we observed the transport signatures of the ballistic and hydrodynamic regimes on the length scale of disc size. Our experimental results confirm theoretical predictions.

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arXiv2019

Electronic thermal conductivity in 2D topological insulator in a HgTe quantum well

We have measured the differential resistance in a two-dimensional topological insulator (2DTI) in a HgTe quantum well, as a function of the applied dc current. The transport near the charge neutrality point is characterized by a pair of counter propagating gapless edge modes. In the presence of an electric field, the energy is transported by counter propagating channels in the opposite direction. We test a hot carrier effect model and demonstrate that the energy transfer complies with the Wiedemann Franz law near the charge neutrality point in the edge transport regime.

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