arXiv subjects

Oleksiy Kashuba

Publications and source records attributed to Oleksiy Kashuba.

At least 19 recordsLinked to original sources

arXiv2026

Voltage characteristics of hydrodynamic Dirac electron nozzles with supersonic flow

In clean Dirac electron systems such as graphene, electron-electron interactions can dominate over other relaxation mechanisms such as phonon or impurity scattering. In this limit, collective electron dynamics can be described by hydrodynamic equations. The prerequisites for electron hydrodynamics have already been fulfilled in experiments, and signatures of hydrodynamic flow have been identified in transport measurements. Here, we derive the pressure-driven hydrodynamic flow profile across a de Laval nozzle profile for Dirac electrons in the subsonic and supersonic regimes. Based on this, we resolve the local voltage characteristics, which provide clear signatures of supersonic hydrodynamic flow. In particular, we identify two distinct features in the experimentally measurable potential profile: a pronounced asymmetry of the local voltage profile on opposite sides of the nozzle, and a sharp differential resistance signature induced by an electron shock wave on the exit side of the nozzle.

cond-mat.mes-hall↗

arXiv2025

On-demand analog space-time in superconducting networks: grey holes, dynamical instability and exceptional points

There has been considerable effort to mimic analog black holes and wormholes in solid state systems. Lattice realizations in particular present specific challenges. One of those is that event horizons in general have both white and black hole (grey hole) character, a feature guaranteed by the Nielsen-Ninomiya theorem. We here explore and extend the capability of superconducting circuit hardware to implement on-demand spacetime geometries on lattices, combining nonreciprocity of gyrators with the nonlinearity of Josephson junctions. We demonstrate the possibility of the metric sharply changing within a single lattice point, thus entering a regime where the modulation of system parameters is "trans-Planckian", and the Hawking temperature ill-defined. Instead of regular Hawking radiation, we find an instability in the form of an exponential burst of charge and phase quantum fluctuations over short time scales - a robust signature even in the presence of an environment. Moreover, we present a loop-hole for the typical black/white hole ambiguity in lattice simulations: exceptional points in the dispersion relation allow for the creation of pure black (or white) hole horizons, at the expense of a radical change in the dynamics of the wormhole interior.

cond-mat.mes-hall↗

arXiv2023

Counting interacting electrons in one dimension

The calculation of the full counting statistics of the charge within a finite interval of an interacting one-dimensional system of electrons is a fundamental, yet as of now unresolved problem. Even in the non-interacting case, charge counting turns out to be more difficult than anticipated because it necessitates the calculation of a nontrivial determinant and requires regularization. Moreover, interactions in a one-dimensional system are best described using bosonization. However, this technique rests on a long-wavelength approximation and is a priori inapplicable for charge counting due to the sharp boundaries of the counting interval. To mitigate these problems, we investigate the counting statistics using several complementary approaches. To treat interactions, we develop a diagrammatic approach in the fermionic basis, which makes it possible to obtain the cumulant generating function up to arbitrary order in the interaction strength. Importantly, our formalism preserves charge quantization in every perturbative order. We derive an exact expression for the noise and analyze its interaction-dependent logarithmic cutoff. We compare our fermionic formalism with the results obtained by other methods, such as the Wigner crystal approach and numerical calculations using the density-matrix renormalization group. Surprisingly, we show good qualitative agreement with the Wigner crystal for weak interactions, where the latter is in principle not expected to apply.

cond-mat.str-el↗

arXiv2021

Electron-hole scattering limited transport of Dirac fermions in a topological insulator

We experimentally investigate the effect of electron temperature on transport in the two-dimensional Dirac surface states of the three-dimensional topological insulator HgTe. We find that around the minimal conductivity point, where both electrons and holes are present, heating the carriers with a DC current results in a non-monotonic differential resistance of narrow channels. We show that the observed initial increase in resistance can be attributed to electron-hole scattering, while the decrease follows naturally from the change in Fermi energy of the charge carriers. Both effects are governed dominantly by a van Hove singularity in the bulk valence band. The results demonstrate the importance of interband electron-hole scattering in the transport properties of topological insulators.

cond-mat.mes-hall↗

arXiv2020

Dirac quantum kinetic equation: minimal conductivity revisited

The kinetic equation used for the description of Dirac systems does not fully take into account two features that play an important role in the vicinity of the Dirac point: (i) the spin degree of freedom, in particular if the spin-flip energy $2 vp$ is not large anymore; and (ii) the failure of the semiclassical approximation due to the large Fermi wavelength. In our work, we propose a novel quantum kinetic equation, which does not have these two drawbacks. Exploiting it in the presence of short range disorder, we demonstrate how it predicts the correct minimal conductivity in 2D Dirac system, a result that has so far been obtained only by other methods like the Kubo formula. The nature of the presented kinetic equation opens up the possibility for the kinetic description of deeply quantum and even strongly correlated Dirac systems.

cond-mat.mes-hall↗

arXiv2019

Semiclassical conservation of spin and large transverse spin current in Dirac systems

In Dirac materials, the low energy excitations obey the relativistic Dirac equation. This dependence implies that the electrons are exposed to strong spin-orbit coupling. Hence, real spin conservation is believed to be violated in Dirac materials. We show that this point of view needs to be refined in the semiclassical picture which applies to the case of doped Dirac materials (away from the nodal point in the spectrum). We derive a novel type of Boltzmann equation for these systems if they are brought slightly out-of-equilibrium. Remarkably, spin-momentum locking is softened and a generalized spin conservation law can be formulated. The most striking observable consequence of our theory is a large transverse spin current in a nearly ballistic transport regime.

cond-mat.mes-hall↗

arXiv2018

Raman Scattering as a Selective Probe of Chiral Electronic Excitations in Bilayer Graphene

We report a symmetry resolved electronic Raman scattering (ERS) study of a bilayer graphene device under gate voltage. We show that the ERS continuum is dominated by interband chiral excitations of $A_{2}$ symmetry and displays a characteristic Pauli-blocking behavior similar to the monolayer case. Crucially, we show that non-chiral excitations make a vanishing contribution to the Raman cross-section due to destructive interference effects in the Raman amplitude matrix elements. This is in a marked contrast to optical absorption measurements and opens interesting venues for the use of Raman scattering as a selective probe of chiral degrees of freedom in topological matter and other 2D crystals.

cond-mat.mes-hall↗

arXiv2018

Auto- and cross-correlations in the spinful topological Kondo model

The correlations in charge transport in multiple Majorana fermions systems contain much richer physics and carry more information than the conventional transport coefficients. We calculate the auto- and cross-correlations in the spinful topological Kondo model and point out the frequencies dependencies of the correlations that may serve as a proof of the existence of the Majorana fermions in our system, particularly the change of the cross-correlation from positive to negative at higher frequency.

cond-mat.str-el↗

arXiv2018

Relativistic Gurzhi effect in channels of Dirac materials

Charge transport in channel-shaped 2D Dirac systems is studied employing the Boltzmann equation. The dependence of the resistivity on temperature and chemical potential is investigated. An accurate understanding of the influence of electron-electron interaction and material disorder allows us to identify a parameter regime, where the system reveals hydrodynamic transport behavior. We point out the conditions for three Dirac fermion specific features: heat flow hydrodynamics, pseudo\-diffusive transport, and the electron-hole scattering dominated regime. It is demonstrated that for clean samples the relativistic Gurzhi effect, a definite indicator of hydrodynamic transport, can be observed.

cond-mat.mes-hall↗

arXiv2018

Collective Yu-Shiba-Rusinov states in magnetic clusters at superconducting surfaces

We study the properties of collective Yu-Shiba-Rusinov (YSR) states generated by multiple magnetic adatoms (clusters) placed on the surface of a superconductor. For magnetic clusters with equal distances between their constituents, we demonstrate the formation of effectively spin-unpolarized YSR states with subgap energies independent of the spin configuration of the magnetic impurities. We solve the problem analytically for arbitrary spin structure and analyze both spin-polarized (dispersive energy levels) and spin-unpolarized (pinned energy levels) solutions. While the energies of the spin-polarized solutions can be characterized solely by the net magnetic moment of the cluster, the wave functions of the spin-unpolarized solutions effectively decouple from it. This decoupling makes them stable against thermal fluctuation and detectable in scanning tunneling microscopy experiments.

cond-mat.supr-con↗

arXiv2017

Dynamical transport measurement of the Luttinger parameter in helical edges states of 2D topological insulators

One-dimensional (1D) electron systems in the presence of Coulomb interaction are described by Luttinger liquid theory. The strength of Coulomb interaction in the Luttinger liquid, as parameterized by the Luttinger parameter K, is in general difficult to measure. This is because K is usually hidden in powerlaw dependencies of observables as a function of temperature or applied bias. We propose a dynamical way to measure K on the basis of an electronic time-of-flight experiment. We argue that the helical Luttinger liquid at the edge of a 2D topological insulator constitutes a preeminently suited realization of a 1D system to test our proposal. This is based on the robustness of helical liquids against elastic backscattering in the presence of time reversal symmetry.

cond-mat.str-el↗

arXiv2016

The Majorana STM as a perfect detector of odd-frequency superconductivity

We propose a novel scanning tunneling microscope (STM) device in which the tunneling tip is formed by a Majorana bound state (MBS). This peculiar bound state exists at the boundary of a one-dimensional topological superconductor. Since the MBS has to be effectively spinless and local, we argue that it is the smallest unit that shows itself odd-frequency superconducting pairing. Odd-frequency superconductivity is characterized by an anomalous Green function which is an odd function of the time arguments of the two electrons forming the Cooper pair. Interestingly, our Majorana STM can be used as the perfect detector of odd-frequency superconductivity. The reason is that a supercurrent between the Majorana STM and any other superconductor can only flow if the latter system exhibits itself odd-frequency pairing. To illustrate our general idea, we consider the tunneling problem of the Majorana STM coupled to a quantum dot in vicinity to a conventional superconductor. In such a (superconducting) quantum dot, the effective pairing can be tuned from even- to odd-frequency behavior if an external magnetic field is applied to it.

cond-mat.supr-con↗

arXiv2016

Influence of spin dynamics of defects on weak localization in paramagnetic 2D metals

Spin-flip scattering of charge carriers in metals with magnetic defects leads to the low-temperature saturation of the decoherence time, $τ_φ$, of electrons at the value comparable to their spin relaxation time, $τ_s$. In two-dimensional (2D) conductors such a saturation can be lifted by an in-plane magnetic field, $B_\parallel$, which polarizes spins of scatterers without affecting orbital motion of free carriers. Here, we show that in 2D conductors with substantially different values of the g-factors of electrons ($g_e$) and magnetic defects ($g_i$), the decoherence time $τ_φ(B_\parallel)$ (reflected by the curvature of magnetoconductance) displays an anomaly: it first gets shorter, decaying on the scale $B_\parallel\sim \hbar/|g_i-g_e|μ_B τ_s$, before becoming longer at higher values of $B_\parallel$.

cond-mat.mes-hall↗

arXiv2015

Influence of impurity spin dynamics on quantum transport in epitaxial graphene

Experimental evidence from both spin-valve and quantum transport measurements points towards unexpectedly fast spin relaxation in graphene. We report magnetotransport studies of epitaxial graphene on SiC in a vector magnetic field showing that spin relaxation, detected using weak-localisation analysis, is suppressed by an in-plane magnetic field, $B_{\parallel}$, and thereby proving that it is caused at least in part by spinful scatterers. A non-monotonic dependence of effective decoherence rate on $B_{\parallel}$ reveals the intricate role of scatterers' spin dynamics in forming the interference correction to conductivity, an effect that has gone unnoticed in earlier weak localisation studies

cond-mat.mes-hall↗

arXiv2015

Interaction-Induced Renormalization of Tunneling into Multiple Majorana End States of a Topological Superconducting Wire

We investigate a system of multiple Majorana states at the end of a topological superconducting wire coupled to a normal lead. For a minimum of three Majorana fermions at the interface, we find nontrivial renormalization physics. Interface tunneling processes can be classified in terms of spin-1/2 and spin-3/2 irreducible representations of the SU(2) group. We show that the renormalization of the tunneling amplitudes belonging to different representations is completely different in that one type is suppressed, whereas the other is enhanced, depending on the sign of the interaction coupling. This results in distinct temperature dependencies of the tunneling current through the interface and different spin polarizations of this current.

cond-mat.mes-hall↗

arXiv2013

The quench dynamics of a dissipative quantum system: a renormalization group study

We study dissipation in a small quantum system coupled to an environment held in thermodynamic equilibrium. The relaxation dynamics of a system subject to an abrupt quench in the parameters of the underlying Hamiltonian is investigated using two complementary renormalization group approaches. The methods are applied to the Ohmic spin-boson model close to the coherent-to-incoherent transition. In particular, the role of non-Markovian memory for the relaxation before and after the quench of the spin-boson coupling and the Zeeman splitting of the up and down spin is investigated.

cond-mat.mes-hall↗

arXiv2013

Transient dynamics of open quantum systems

We present a renormalization group (RG) method which allows for an analytical study of the transient dynamics of open quantum systems on all time scales. Whereas oscillation frequencies and decay rates of exponential time evolution follow from the fixed point positions, the long-time behavior of pre-exponential functions is related to the scaling behavior around the fixed points. We show that certain terms of the RG flow are only cut off by inverse time, which leads to a difference between infrared and ultraviolet scaling. An evaluation for the ohmic spin boson model at weak damping reveals significant deviations from previous predictions in the long-time regime. We propose that weak coupling problems for stationary quantities can in principle turn into strong coupling ones for the determination of the long-time behavior.

cond-mat.str-el↗

arXiv2013

Interplay between uniaxial strain and magnetophonon resonance in graphene

We study the fine structure of the phonon G peak in the inelastic light scattering spectra of strained graphene in the presence of a quantizing magnetic field. We show that under the conditions of the magnetophonon resonance (MPR), the spectral line shape in Raman spectra of the Γ-point longitudinal optic-transverse optical phonon undergoes multiple splitting and acquires a strong dependence on the polarization of the incoming and detected light. Interference of strain and MPR leads to the modification of the anticrossing branches, introducing the variation of the light polarization preferences depending on the proximity to the resonance, and their better resolution between neighboring anticrossings.

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