arXiv subjects

Leonid Levitov

Publications and source records attributed to Leonid Levitov.

At least 19 recordsLinked to original sources

arXiv2026

Anomalous Hall Effect Driven by Chiral Superconductivity

Direct dc-current signatures of unconventional superconductivity remain scarce. Existing probes of unconventional pairing are typically indirect, relying on phase-diagram anomalies, responses to external fields, or optical measurements. Here we propose a zero-field Hall drag effect as a direct transport signature of chiral superconductivity. The effect arises from Coulomb drag between quasiparticles in a chiral superconductor and those in an adjacent time-reversal-symmetric normal layer. We develop a minimal hydrodynamic theory that includes both quasiparticle normal current and condensate supercurrent in the superconducting layer. In an open-circuit superconducting layer, the condensate generates a counterflowing supercurrent that cancels the net layer current, while a finite quasiparticle current remains and mediates the transverse drag response. This results in anomalous Hall voltage signal appearing abruptly when $T$ is lowered below $T_c$, of the sign reflecting the sign of the superconducting order parameter phase winding.

cond-mat.supr-con↗

arXiv2026

Particle-Hole Ghost Interference in Superconductors

Mirror-assisted optical interference can improve the fidelity of Young's double-slit interference. Here we discuss an electron analogue: particle-hole interference in superconductors produced by a single impurity near a line defect, terrace edge, or phase boundary. Quasiparticle waves scattered directly from the impurity interfere with waves reflected by the boundary, generating a ``ghost'' interference pattern that combines conventional $2k_F$ Friedel oscillations with additional hyperbolic fringes. Compared to the recently studied two-impurity Young's interference, this effect appears already at first order in the impurity potential and is therefore parametrically stronger. The resulting spatial modulation extends beyond $\lambda_F/2$ and is directly sensitive to the quasiparticle structure of the paired state, including possible Fermi-surface anisotropy of the superconducting order parameter. These findings point to boundary-assisted impurity interference as a robust local probe of superconducting electronic order, with clear signatures accessible to STM/STS measurements.

cond-mat.supr-con↗

arXiv2026

Tunable viscosity across the BCS-BEC crossover

Tunable interactions make ultracold quantum gases a unique platform for exploring hydrodynamic properties in the strongly correlated regime. Of particular interest are turbulent flows possible in the regime of high Reynolds numbers. Since the system size and flow velocity are limited in experimentally realistic systems, we propose an alternative approach to enhance the Reynolds numbers in an ultracold Fermi gas by minimizing the shear viscosity in the vicinity of the Feshbach resonance. By employing the Keldysh formulation of the linear response theory, we theoretically demonstrate that the shear viscosity can vary by several orders of magnitude in the vicinity of the BCS-BEC crossover. It is also shown that while Drude-like contributions generally dominate at large Feshbach detunings, higher-order vertex corrections, including the Maki-Thompson contribution, become significant and suppress singular behavior in the near-resonant regime. Our results provide a roadmap for achieving tunable Reynolds numbers in ultracold quantum fluids, which can serve as table-top turbulence simulators.

cond-mat.quant-gas↗

arXiv2026

Berry-Flux-Controlled Cascade of Chiral Superconducting States

Motivated by recent interest in chiral superconductivity in narrow bands, we develop a general framework to clarify how band topology and quantum geometry affect superconducting pairing and connect to the two-body problem. Berry curvature does not merely favor a chiral pairing channel; it produces a sequence of chiral pairing instabilities indexed by angular momentum, controlled by the Berry flux through the Fermi sea, with a Little-Parks-like periodicity in momentum space. We show that Berry curvature converts a nonchiral attractive interaction into a geometrically frustrated Cooper problem in momentum space. The relevant control parameter is the Berry-curvature flux enclosed by the Fermi sea, $\Phi = b k_F^2$, which acts as an effective Aharonov-Bohm flux for the order parameter defined on the Fermi surface. As $\Phi$ is tuned, the leading pairing instability switches between odd angular-momentum channels $m=1,3,5...$, producing a cascade of first-order transitions and Little-Parks-like oscillations of $T_c$.

cond-mat.mes-hall↗

arXiv2025

Spontaneous Running Waves and Self-Oscillatory Transport in Dirac Fluids

We predict hydrodynamic Turing instability of current-carrying Dirac electron fluids that drives spontaneous self-oscillatory transport. The instability arises near charge neutrality, where carrier kinetics make current dissipation strongly density dependent. Above a critical drift velocity, a uniform electronic flow becomes unstable and undergoes a dynamical transition to a state with coupled spatial modulation and temporal oscillations--an electronic analogue of Kapitsa roll waves in viscous films. The transition exhibits two clear signatures: a nonanalytic, second-order-like onset in the time-averaged current and narrow-band electromagnetic emission at a tunable washboard frequency $f=u/λ$. Although reminiscent of sliding charge-density waves, the mechanism is intrinsic and disorder independent. Owing to the small effective mass of Dirac carriers, hydrodynamic time scales translate into emission frequencies in the tens to hundreds of gigahertz range, establishing Dirac materials as a platform for high-frequency self-oscillatory electron hydrodynamics.

cond-mat.mes-hall↗

arXiv2025

Strain Response as a Probe of Spinons in Quantum Spin Liquids

Quantum spin liquids (QSLs) host emergent, fractionalized fermionic excitations that are charge-neutral. Identifying clear experimental signatures of these excitations remains a central challenge in the field of strongly correlated systems, as they do not couple to conventional electromagnetic probes. Here, we propose lattice strain as a powerful and tunable probe: Mechanical deformation of the lattice generates large pseudomagnetic fields, inducing pseudo-Landau levels that serve as distinctive spectroscopic signatures of these excitations. Using the Kitaev model on the honeycomb lattice, we show that distinct QSL phases exhibit strikingly different strain responses. The semimetallic Kitaev spin liquid and the gapped chiral spin liquid display pronounced Landau quantization and a diamagnetic-like response to strain, whereas the Majorana metal phase shows a paramagnetic-like response without forming Landau levels. These contrasting behaviors provide a direct route to experimentally identifying and distinguishing QSL phases hosting fractionalized excitations. We further outline how local resonant ultrasound spectroscopy can detect the strain-induced resonances associated with these responses, offering a practical pathway towards identifying fractionalized excitations in candidate materials.

cond-mat.str-el↗

arXiv2025

Proposal for spin superfluid quantum interference device

In easy-plane magnets, the spin superfluid phase was predicted to facilitate coherent spin transport. So far, experimental evidence remains elusive. In this Letter, we propose an indirect way to sense this effect via the spin superfluid quantum interference device (spin SQUID), inspired by its superconducting counterpart (rf SQUID). The spin SQUID is constructed as a quasi-one-dimensional (1D) magnetic ring with a single Josephson weak link, functioning as an isolated device with a microwave response. The spin current is controlled by an in-plane electric field through Dzyaloshinskii-Moriya interaction. This interaction can be interpreted as a gauge field that couples to the spin supercurrent through the Aharonov-Casher effect. By investigating the static and dynamic properties of the device, we show that the spin current and the harmonic frequencies of the spin superfluid are periodic with respect to the accumulated Aharonov-Casher phase and are, therefore, sensitive to the radial electric flux through the ring in units of an electric flux quantum, suggesting a potential electric-field sensing functionality. For readout, we propose to apply spectroscopic analysis to detect the frequency shift of the harmonic modes induced by this magnonic Stark effect.

cond-mat.mes-hall↗

arXiv2025

Chiral Wigner crystal phases induced by Berry curvature

We consider the impact of Berry phase on the Wigner crystal (WC) state of a two-dimensional electron system. We consider first a model of Bernal bilayer graphene with a perpendicular displacement field, and we show that Berry curvature leads to a new kind of WC state in which the electrons acquire a spontaneous orbital angular momentum when the displacement field exceeds a critical value. We determine the phase boundary of the WC state in terms of electron density and displacement field at low temperature. We then derive the general effective Hamiltonian that governs the ordering of the physical electron spin. We show that this Hamiltonian includes a chiral term that can drive the system into chiral spin-density wave or spin liquid phases. The phenomena we discuss are relevant for the valley-polarized Wigner crystal phases observed in multilayer graphene.

cond-mat.str-el↗

arXiv2025

Tomographic imaging of superconducting order using particle-hole interference

Superconducting phases with exotic symmetries that differ from the underlying crystalline lattice are at the focus of superconductivity research. Yet, despite intense interest, detecting the order parameter symmetry and topology remains a major challenge. Real-space imaging near atomic impurities with scanning tunneling microscopy (STM) has been highly successful in revealing nodes of the superconducting gap, in particular in cuprate superconductors, however the order parameter phase winding has so far remained inaccessible by STM techniques. We demonstrate that STM can access this phase information by exploiting Young-type quasiparticle interference patterns generated by pairs of impurities acting as beam splitters. Superconducting order parameter tomography (SOPT), a technique proposed here, utilizes the response of real-space interference patterns of Bogoliubov quasiparticles to the controlled rotation of impurity configurations, allowing us to reconstruct the momentum space structure of the gap function $\Delta(\vec{k})$. As a concrete example, we consider Strontium Ruthenate, whose superconducting order remains a subject of ongoing debate, and demonstrate how SOPT can distinguish between competing order parameter candidates. The Young's interference fringes, nodal directions, and rotating beams, detected by SOPT, encode information about both the nodes and phase winding of the superconducting order parameter. This method provides a broadly applicable route to identifying unconventional and topological superconductivity and establishes particle-hole interference as a new imaging modality for superconducting order.

cond-mat.supr-con↗

arXiv2025

Linear-in-temperature conductance in two-dimensional electron fluids

Linear temperature dependence of transport coefficients in metals is often ascribed to non-Fermi-liquid physics. Here we demonstrate the $T$-linear behavior of nonlocal conductivity in a clean 2D electron fluid, where carrier collisions assist conduction and lead to hydrodynamic transport with conductance rather than resistance growing with temperature. The key aspect is the occurrence of multiple hydrodynamic modes representing odd-parity modulations of the Fermi surface evolving in space and time. A cascade of such modes results in a linear $T$ dependence that extends to lowest temperatures, as well as a Kolmogorov-like fractional power $-5/3$ scaling of conductivity vs. wavenumber. These dependences provide a smoking gun for nonclassical hydrodynamics driven by such modes, expected to be generic for 2D electron fluids with simple near-circular Fermi surfaces.

cond-mat.mes-hall↗

arXiv2024

Chirality-induced pseudo-magnetic fields, flat bands and enhancement of superconductivity

Systems in which exchange interactions couple carrier spins to a spin texture with a net chirality exhibit a spin-dependent Aharonov-Bohm effect, where the geometric gauge field and pseudo-magnetic field have opposite signs for carriers with opposite spins. As a result, Cooper pairs see a net zero vector potential and superconducting pairing is not hindered by pair-breaking effects. This allows superconductivity to occur even when the geometric field induces quantized Landau levels. We identify the dominant pairing order as an s-wave pair density wave of an FFLO type. Flat Landau levels can significantly enhance superconducting $T_c$, favoring superconductivity over competing orders. This exotic paired state features tell-tale signatures such as flat bands of Bogoliubov-deGennes quasiparticles, manifest through Landau level-like resonances in the quasiparticle density of states.

cond-mat.supr-con↗

arXiv2024

Spin chirality and fermion stirring in topological bands

We demonstrate that in metals, both normal and superconducting, orbital currents present in the ground state when time reversal symmetry (TRS) is broken, generate spin chirality. Nonzero chirality can emerge in the absence of any spin-dependent interactions, even when the ground state remains spin-unpolarized. The chirality effect is derived diagrammatically and illustrated for Haldane model and the topological superconductivity problem. Chirality in the carrier band results in a chiral three-spin RKKY interaction between localized spins coupled to carriers by s-d Hamiltonian, an effect that can be detected by local probes such as spin-sensitive STM. In systems where detecting TRS breaking by conventional means is challenging, such as topological superconductors, local detection of spin chirality can serve as a reliable diagnostic of superconducting topological phases.

cond-mat.mes-hall↗

arXiv2024

Signatures of electronic ordering in transport in graphene flat bands

Recently, a wide family of electronic orders was unveiled in graphene flat bands, such as spin- and valley-polarized phases as well as nematic momentum-polarized phases, stabilized by exchange interactions via a generalized Stoner mechanism. Momentum polarization involves orbital degrees of freedom and is therefore expected to impact resistivity in a way that is uniquely sensitive to the ordering type. Under pocket polarization, carrier distribution shifts in $k$ space and samples the band mass in regions defined by the displaced momentum distribution. This makes transport coefficients sensitive to pocket polarization, resulting in the Ohmic resistivity decreasing with temperature. In addition, it leads to current switching and hysteresis under strong $E$ field. Being robust in the presence of electron-phonon scattering, this behavior can serve as a telltale sign of pocket polarization order. The fast timescale and low dissipation of the switching cycle may be advantageous for highly applicable memory-dependent resistors, i.e., memristors.

cond-mat.str-el↗

arXiv2024

Nonlocal conductivity, continued fractions and current vortices in electron fluids

Vortices in electron fluids are a key indicator of electron hydrodynamics. However, a comprehensive framework linking macroscopic vorticity measurements with microscopic interactions and scattering mechanisms has been lacking. We employ wavenumber-dependent conductivity $σ(k)$ incorporating realistic microscopic scattering processes, aiming to clarify the relationship between nonlocal response and vortices across ballistic and hydrodynamic phases. Vorticity is found to take similar values in both phases but feature very different sensitivity to momentum-relaxing scattering, with ballistic vortical flows being orders-of-magnitude more resilient than the hydrodynamic ones. This behavior can serve as a simple diagnostic of the microscopic origin of vorticity in electron fluids.

cond-mat.mes-hall↗

arXiv2024

Chiral Stoner magnetism in Dirac bands

Stoner magnetism in bands endowed with Berry curvature is shown to be profoundly influenced by the coupling between spin chirality density $\vec s\cdot(\partial_x\vec s\times\partial_y\vec s)$ and Berry's orbital magnetization. The key effect is that carriers moving in the presence of a spin texture see it as a source of a geometric magnetic field coupled to the carrier's orbital motion through a spin-dependent Aharonov-Bohm effect. This emergent spin-orbit interaction effect was recently predicted to enable chiral magnons propagating along system boundaries. Here we show that it also favors chiral spin textures such as skyrmions -- the topologically protected objects with particle-like properties, stabilized in the ground state. The threshold for Stoner instability is found to soften, rendering chiral spin-ordered phases accessible under realistic conditions. We present a detailed analysis of the chiral effect for Bernal bilayer graphene and discuss the unique properties of skyrmion textures in graphene multilayers.

cond-mat.mes-hall↗

arXiv2024

Charge and spin density wave orders in field-biased Bernal bilayer graphene

This paper aims to clarify the nature of a surprising ordered phase recently reported in biased Bernal bilayer graphene that occurs at the phase boundary between the isospin-polarized and unpolarized phases. Strong nonlinearity of transport at abnormally small currents, with $dI/dV$ vs. $I$ sharply rising and then falling back, is typical for a charge/spin-density-wave state (CDW or SDW) sliding transport. Here, however, it is observed at an isospin-order phase boundary, prompting a question about the CDW/SDW mechanism and its relation to the quantum critical point. We argue that the observed phase diagram cannot be understood within a standard weak-coupling picture. Rather, it points to a mechanism that relies on an effective interaction enhancement at a quantum critical point. We develop a detailed strong-coupling framework accounting for the soft collective modes that explain these observations.

cond-mat.str-el↗

arXiv2023

Hofstadter quasicrystals, hidden symmetries and irrational quantum oscillations

Landau levels perturbed by a periodic potential is a prime setting to design quantum systems with exotic fractal spectra. Motivated by recent advances in twistronics, we introduce `Hofstadter quasicrystal' problem describing Landau levels perturbed by a set of incommensurate cosine waves. We illustrate the underlying physics for moiré quasicrystals with octagonal and dodecagonal symmetries, finding spectra that are vastly more complex than the Hofstadter spectrum. Surprisingly, due to the high spatial symmetry, the quasicrystal problem exhibits hidden `inner' symmetry arising at special `magic' values of the magnetic field. The $1/B$-periodic pattern of magic field values explains striking wide-range oscillations in the observed spectra that have irrational periodicity incommensurate with the Aharonov-Bohm and Brown-Zak periodicities. The prominent character of these oscillations makes them readily accessible in state-of-the-art moiré graphene systems.

cond-mat.mes-hall↗

arXiv2023

Two-dimensional electron gases as non-Newtonian fluids

Two-dimensional electron systems offer an appealing platform to explore long-lived excitations arising due to collinear carrier scattering enabled by phase-space constraints at the Fermi surface. Recently it was found that these effects can boost excitation lifetimes over the fundamental bound set by Landau's Fermi-liquid theory by a factor as large as $(T_F/T)^α$ with $α\approx 2$. Long-lived degrees of freedom possess the capability to amplify the response to weak perturbations, producing lasting collective memory effects. This leads to non-Newtonian hydrodynamics in 2D electron fluids driven by multiple viscous modes with scale-dependent viscosity. We describe these modes as Fermi surface modulations of odd parity evolving in space and time, and discuss their implications for experimental studies of electron hydrodynamics.

cond-mat.str-el↗