arXiv subjects

Alex Levchenko

Publications and source records attributed to Alex Levchenko.

At least 37 records · Page 2Linked to original sources

arXiv2025

Dragging of electric current by hydrodynamic flow at charge neutrality

We develop a theory of drag in graphene double layers near charge neutrality. We work in the regime of electron hydrodynamics and account for interlayer correlations of charge puddle disorder. The drag resistivity is expressed in terms of the viscosity, intrinsic conductivity of the electron liquid, and the correlation function of the puddle disorder. The contributions of the interlayer transfer of momentum and energy to drag have opposite signs. This leads to a nonmonotonic dependence of the drag resistivity on the carrier density. For layer-symmetric doping, the drag resistivity changes sign as a function of the carrier density. At interlayer separations shorter than the disorder correlation length, the transconductivity saturates to the disorder-induced enhancement of the intralayer conductivity. We provide quantitative estimates of the effect for Dirac electron liquids in monolayer graphene and bilayer graphene double-layer devices.

cond-mat.mes-hall↗

arXiv2025

Theories of Superconducting Diode Effects

Superconducting diode effects (SDE), both in bulk superconductors and in Josephson junctions, have garnered a lot of attention due to potential applications in classical and quantum computing, as well as superconducting sensors. Here we review various mechanisms that have been theoretically proposed for their realization. We first provide a brief historical overview and discuss the basic but subtle phenomenological Ginzburg-Landau theory of SDE, emphasizing the need to the simultaneous breaking of time-reversal and inversion symmetries. We then proceed to more microscopic treatments, focusing especially on implementations in noncentrosymmetric materials described by the Rashba-Zeeman model. Finally, we review proposals based on other condensed matter systems such as altermagnets, valley polarized and topological materials, and systems out of equilibrium.

cond-mat.supr-con↗

arXiv2025

Helical phases and Bogoliubov Fermi surfaces probed by superconducting diode effects

Noncentrosymmetric superconductors (NCSs) with Rashba spin-orbit coupling (SOC) and in-plane magnetic fields have emerged as natural platforms for realizing both the bulk superconducting diode effect (SDE) and the Josephson diode effect (JDE) - phenomena characterized by unequal critical currents in opposite directions due to the simultaneous breaking of time-reversal and inversion symmetries. Using the quasiclassical Eilenberger formalism, we systematically investigate both the bulk SDE and the JDE in a clean NCS with Rashba SOC and in-plane magnetic fields. For the bulk system, we find that the diode efficiency can nominally approach its maximal value at the critical endpoint of the first-order Lifshitz transition between weak and strong helical phases featuring finite-momentum Cooper pairs, the latter marked by the emergence of Bogolyubov Fermi surfaces (BFSs). In a Josephson junction, we show that finite-momentum pairing in the superconducting leads is the dominant mechanism behind the JDE in short junctions, whereas in long junctions it is primarily governed by the Zeeman field in the normal region. In the long-junction regime, the diode efficiency additionally oscillates between positive and negative values as a function of magnetic field at low fields, providing a route toward a highly tunable Josephson diode. At higher fields, the onset of BFSs in the strong helical phase leads to a sharp suppression of both the JDE and the Josephson current when the current direction is aligned with momenta along the BFS, resulting in strong anisotropy. We propose that this anisotropy in the Josephson current offers an alternative method for detecting BFSs, applicable to systems with or without a JDE.

cond-mat.supr-con↗

arXiv2025

Josephson Diode Effect from Nonequilibrium Current in a Superconducting Interferometer

We investigate the Josephson diode effect in a superconducting interferometer under nonequilibrium conditions. In contrast to its thermodynamic counterpart, which requires the simultaneous breaking of time-reversal and inversion symmetry, we demonstrate that a diode-like asymmetry of the critical current can emerge solely due to a dissipative current in the normal region of an otherwise symmetric Josephson junction. This effect is driven entirely by the nonequilibrium conditions, without the need for additional inversion symmetry breaking. Using the standard quasiclassical Keldysh Green's function formalism, we explicitly calculate the diode coefficient from the supercurrent-phase relation of the interferometer. Remarkably, within certain ranges of control parameters, such as applied voltage, temperature, and the geometric aspect ratio of the device, the diode coefficient can exceed its nominal perfect value.

cond-mat.supr-con↗

arXiv2025

Nonreciprocal Coulomb drag in electron bilayers

We propose a mechanism and develop a theory for nonreciprocal Coulomb drag resistance. This effect arises in electron double-layer systems in the presence of an in-plane magnetic field in noncentrosymmetric conductors or in bilayers with spontaneously broken time-reversal symmetry and without Galilean invariance. We demonstrate the significance of this effect by examining the hydrodynamic regime of the electron liquid. The nonreciprocal component of the transresistance is shown to sensitively depend on the intrinsic conductivity, viscosity of the fluid, and the emergent nonreciprocity parameter.

cond-mat.mes-hall↗

arXiv2025

Quasi-1D Coulomb drag in the nonlinear regime

One-dimensional Coulomb drag has been an essential tool to probe the physics of interacting Tomonaga-Luttinger liquids. To date, most experimental work has focused on the linear regime while the predictions for Luttinger liquids beyond the linear response theory remain largely untested. In this letter, we report measurements of momentum transfer induced Coulomb drag between vertically-coupled quasi-one-dimensional quantum wires in the nonlinear regime. Measurements were performed at ultra-low temperatures between wires only 15 nm apart. Our results reveal a nonlinear dependence of the drag voltage as a function of the drive current superimposed with an oscillatory contribution, in agreement with theoretical predictions for Coulomb drag between Tomonaga-Luttinger liquids. Additionally, the observed current-voltage ($I$-$V$) characteristics exhibit a nonmonotonic temperature dependence, further corroborating the presence of non-Fermi-liquid behavior in our system. These findings are observed both in the single and in the multiple subband regimes and in the presence of disorder, extending the onset of this behavior beyond the clean single channel Tomonaga-Luttinger regime where the predictions were originally formulated.

cond-mat.mes-hall↗

arXiv2025

Hydrodynamic Coulomb drag in odd electron liquids

We consider the problem of Coulomb drag resistance in bilayers of electron liquids with spontaneously broken time-reversal symmetry. In the hydrodynamic regime, the viscosity tensor of such fluids has a nonvanishing odd component. In this scenario, fluctuating viscous stresses drive the propagation of plasmons, whose dispersion relations are modified by nondissipative odd viscous waves. Coulomb coupling of electron density fluctuations induces a drag force exerted by one layer on the other in the presence of a steady flow. This drag force can be expressed through the dynamic structure factor of the electron liquid, which is peaked at frequencies corresponding to plasmon resonances in the bilayer. As a result, the drag resistivity depends on the dissipationless odd viscosity of the fluid. We quantify this effect and present a general theory of hydrodynamic fluctuations applicable to odd electron liquids, both with and without Galilean invariance.

cond-mat.mes-hall↗

arXiv2025

Stokes flow in an electronic fluid with odd viscosity

We investigate the transition between elastic and viscous regimes for time-reversal broken Weyl semimetals. In these materials, Hall transport occurs through two parallel channels: the Fermi sea and the Fermi surface. The Fermi sea part remains unaffected by electron-electron scattering, whereas the Fermi surface is influenced by it. We model the disorder by dilute impenetrable spherical impurities. We analyze the flow of an electronic fluid with a finite odd viscosity in the presence of such disorder and compute the conductivity tensor. We find that in the generic case of finite intrinsic conductivity, the Hall angle in the viscous regime is parametrically suppressed compared to the elastic regime. In the special case where the intrinsic conductivity vanishes, the ratio between the transverse and the longitudinal resistivities matches the ratio between the odd and even components of the viscosity tensor.

cond-mat.mes-hall↗

arXiv2025

Quasi-1D Coulomb drag between spin-polarized quantum wires

One-dimensional (1D) quantum wires provide a versatile platform for studying strong electron-electron interactions and collective excitations under confinement. Coulomb drag between 1D systems offers a powerful probe of Tomonaga-Luttinger liquid (TLL) physics, with theoretical predictions suggesting distinct power-law in temperature dependencies between the spin-full and the spin-polarized regimes. However, experimental verification has thus far remained limited. Here, we report measurements of reciprocal and nonreciprocal Coulomb drag between vertically coupled quasi-1D quantum wires in the spin-polarized regime. Clear signatures of spin splitting are observed in both the wires conductance and the drag signal. We observed a connection between electron-hole asymmetry and negative drag, and demonstrated different power-law behaviors in spin-full and spin-polarized regimes, yielding consistent TLL interaction parameters. These results validate the theoretical predictions for backscattering induced drag in the reciprocal regime and extend them to the nonreciprocal and the multiple subband regimes. Furthermore, the nonmonotonic density dependence of the reciprocal interaction parameter correlates with the subband occupation of the drag wire, revealing the complexity of the scattering mechanisms in multichannel systems.

cond-mat.mes-hall↗

arXiv2025

Lorenz ratios of diagonal and Hall conductivities in a Dirac electron liquid

The magnetotransport properties of a two-dimensional electron liquid in graphene are analyzed in the hydrodynamic limit under isothermal conditions. It is shown that the Wiedemann-Franz law does not hold in this regime and that the Lorenz ratios constructed from the diagonal and Hall conductivities generally differ from each other. The breakdown of the Wiedemann-Franz law is most pronounced near charge neutrality, where the Lorenz ratio can exceed its nominal universal Sommerfeld value by an order of magnitude. The corresponding Lorenz ratios exhibit a nonmonotonic dependence on the magnetic field, which is caused by emergent magnetic friction arising in liquids lacking Galilean invariance and possessing finite intrinsic conductivity.

cond-mat.mes-hall↗

arXiv2025

Superconducting diode effect in Ising superconductors

We study the superconducting diode effect (SDE) in an Ising superconductor with broken basal mirror symmetry in a parallel magnetic field. We show that in the presence of a small Rashba spin splitting, $Δ_R$, the dominant Ising spin-orbit coupling ($Δ_I >> Δ_R$) dramatically enhances the SDE efficiency compared to a Rashba superconductor with $Δ_I = 0$ and the same $Δ_R$. The suppression of the SDE for $Δ_I = 0$ at $Δ_R$ much larger than the critical temperature ($T_c$) is accidental. At $Δ_R << T_c$, the SDE efficiency is small because, to linear order, $Δ_R$ can be removed by a gauge transformation. These two factors -- the accidental suppression of the SDE at large $Δ_R$ and the systematic suppression at small $Δ_R$ -- are eliminated by Ising spin-orbit coupling. As a result, SDE efficiency is substantially enhanced for $Δ_I >> Δ_R \neq 0$.

cond-mat.supr-con↗

arXiv2025

Resonant second harmonic generation in a two-dimensional electron system

We consider the nonlinear response of a disordered two-dimensional electronic system, lacking inversion symmetry, to an external alternating electric field. The application of an in-plane static magnetic field induces local contributions to the current density that are quadratic in the electric field and linear in the magnetic field. This current oscillates at twice the frequency of the external irradiation and there are two linearly independent vector combinations that contribute to the current density. This particular mechanism coexists with the topological Berry-dipole contribution to the second harmonic of the current density, which can be generated by quantum confinement. Additional nonlocal terms in the current density are possible in the regime away from the normal incidence. The total current exhibits a nonreciprocal character upon reversal of the magnetic field direction. We evaluate the magnitude of this effect by computing its dependence on the strength of spin-orbit coupling and the disorder scattering rate. Importantly, we show that these local second-harmonic contributions can be resonantly excited when the frequency of the external radiation approaches the energy separation between the spin-orbit split bands.

cond-mat.mes-hall↗

arXiv2025

Superconducting diode efficiency from singlet-triplet mixing in disordered systems

The superconducting diode effect (SDE) -- the nonreciprocity of the critical current in a bulk superconductor -- has garnered significant attention due to its potential applications in superconducting electronics. However, the role of disorder scattering in SDE has rarely been considered, despite its potential qualitative impact, as we demonstrate in this work. We investigate SDE in a disordered Rashba superconductor under an in-plane magnetic field, employing a self-consistent Born approximation to derive the corresponding Ginzburg-Landau theory. Our analysis reveals two surprising effects. First, in the weak Rashba spin-orbit coupling (SOC) regime, disorder can reverse the direction of the diode effect, indicated by a sign change in the superconducting diode efficiency coefficient. Second, in the strong Rashba SOC regime, disorder becomes the driving mechanism of SDE, which vanishes in its absence. In this case, we show that disorder-induced mixing of singlet and triplet superconducting orders underlies the effect.

cond-mat.supr-con↗

arXiv2025

Magnetism from multiparticle ring exchange in moiré Wigner crystals

We investigate the multiparticle ring exchange couplings of the two-dimensional triangular Wigner crystal in external commensurate triangular and honeycomb potentials, using a semiclassical approach valid in the regime where Coulomb interactions dominate over electronic kinetic energy. In this limit, increasing the strength of the potential drives a transition from a ferromagnet to a $120^\circ$ Néel antiferromagnet for both external potential types. In the triangular case, we find that the transition occurs already for a weak potential, whereas in the honeycomb case, it occurs when the potential is nearly two orders of magnitude larger. Our results are relevant to the magnetism of generalized Wigner crystal phases observed at certain rational fillings of the moiré superlattice in transition-metal dichalcogenide heterobilayers.

cond-mat.str-el↗

arXiv2024

Spin-caloric resistance of Dirac plasma in a graphene Corbino device

The thermal resistance of a spin-polarized hydrodynamic Dirac plasma in graphene is considered. A mechanism for the coupling of heat and spin flows is discussed, demonstrating that spin diffusion and spin thermocurrent modify viscous dissipation, leading to a significant enhancement of thermal resistance. Practical calculations are then presented for graphene devices in the Corbino geometry.

cond-mat.mes-hall↗

arXiv2024

Superconducting Diode Effect in Multiphase Superconductors

We identify a new mechanism for the intrinsic superconducting diode effect (SDE) in multiphase superconductors. Using a Ginzburg-Landau and a microscopic two-band model, we find phase transitions into a mixed phase with finite-momentum Cooper pairs and SDE with high (including maximal) diode efficiencies, despite the individual phases exhibiting no SDE and equal inversion parity. We thus show that parity mixing $-$ invoked in previous proposals $-$ is not a crucial ingredient for SDE. The new mechanism may be relevant in a multitude of known multiphase superconductors like UTe$_2$.

cond-mat.supr-con↗

arXiv2024

Casimir effect in Josephson junctions

In a Josephson junction, the supercurrent is determined by both the discrete sub-gap part of the spectrum due to Andreev bound states and the continuous part of the spectrum from energy states outside the superconducting gap. We consider the cohesive force exerted on a junction, which is thermodynamically conjugated to the superflow, and comment on its connection to the Casimir effect in quantum electrodynamics. In contrast to the supercurrent, it is shown that in ballistic short junctions, the force is predominantly contributed by the continuum. Its magnitude is universally defined by the energy gap and coherence length of the superconductor per spin-dependent transverse mode. This force scales non-analytically with the junction length and is periodic with the superconducting phase. For long ballistic junctions, the force results from the interplay of oscillatory contributions originating from both bound states and the continuum. The resulting asymptotic limit for the force is established, including the correction terms. Thermal and impurity effects on the force are briefly discussed.

cond-mat.supr-con↗

arXiv2024

Fine structure of current noise spectra in nanoelectromechanical resonators

We study the frequency-dependent noise of a suspended carbon nanotube quantum dot nanoelectromechanical resonator induced by electron-vibration coupling. Using a rigorous Keldysh diagrammatic technique, we establish a formal framework connecting the vibrational properties to electrical measurements. We find that the noise power spectrum exhibits a narrow resonant peak at the frequency of the vibrational modes. However, this fine structure tends to disappear due to a coherent cancellation effect when the tunneling barriers are tuned to a symmetric point. Notably, measuring the electrical current noise spectra provides a sensitive alternative method for detecting the damping and dephasing of quantum vibrational modes.

cond-mat.mes-hall↗