arXiv subjects

P. M. Ostrovsky

Publications and source records attributed to P. M. Ostrovsky.

At least 19 recordsLinked to original sources

arXiv2025

Density of scattering resonances in a disordered system

Reflection of particles from a disordered or chaotic medium is characterized by a scattering matrix that can be represented as a superposition of resonances. Each resonance corresponds to an eigenstate inside the medium and has a width related to the decay time of this eigenstate. We develop a general approach to study the distribution function of these resonance widths based on the nonlinear sigma model. We derive an integral representation of the distribution function that works equally well for systems of any symmetry and for any type of coupling to the measuring device. From this integral representation we find explicit analytic expressions for the distribution function in the case of disordered metallic grains. We also compare the analytic results to large-scale numerical simulations and observe their perfect agreement.

cond-mat.dis-nn↗

arXiv2025

Effect of superconductivity on non-uniform magnetization in dirty SF junctions

We study proximity effect in a tunnel junction between a bulk superconductor and a thin disordered ferromagnetic layer on its surface. Cooper pairs penetrating from the superconductor into the ferromagnet tend to destabilize its uniform magnetic order. The competition of this effect and the intrinsic magnetic stiffness of the ferromagnet leads to a second order phase transition between uniform and non-uniform magnetic states. Using the quasiclassical Usadel equation, we derive the Landau functional for this transition and construct the complete phase diagram of the effect. We identify a special point of "resonance" at which the characteristic energy scale of the proximity effect equals the exchange field of the ferromagnet. At this point, the uniform magnetic state is unstable even in the limit of large stiffness. We further explore the parameter regime far beyond the transition and determine the properties of the resulting strongly non-uniform magnetic state.

cond-mat.dis-nn↗

arXiv2024

Andreev conductance in disordered SF junctions with spin-orbit scattering

We calculate the conductance of a junction between a disordered superconductor and a very strong half-metallic ferromagnet admitting electrons with only one spin projection. A usual mechanism of Andreev reflection is strongly suppressed in this case since Cooper pairs are composed of electrons with opposite spins. However, this obstacle can be overcome if we take into account spin-orbit scattering inside the superconductor. Spin-orbit scattering induces a fluctuational (zero on average) spin-triplet component of the superconducting condensate, which is enough to establish Andreev transport into a strong ferromagnet. This remarkably simple mechanism is quite versatile and can explain long-range triplet proximity effect in a number of experimental setups. One particular application of the suggested effect is to measure the spin-orbit scattering time $τ_{\text{SO}}$ in disordered superconducting materials. The value of Andreev conductance strongly depends on the parameter $Δτ_\text{SO}$ and can be noticeable even in very disordered but relatively light metals like granular aluminum.

cond-mat.supr-con↗

arXiv2024

Electron transport in a weakly disordered Weyl semimetal

Weyl semimetal is a solid material with isolated touching points between conduction and valence bands in its Brillouin zone -- Weyl points. Low energy excitations near these points exhibit a linear dispersion and act as relativistic massless particles. Weyl points are stable topological objects robust with respect to most perturbations. We study effects of weak disorder on the spectral and transport properties of Weyl semimetals in the limit of low energies. We use a model of Gaussian white-noise potential and apply dimensional regularization scheme near three dimensions to treat divergent terms in the perturbation theory. In the framework of self-consistent Born approximation, we find closed expressions for the average density of states and conductivity. Both quantities are analytic functions in the limit of zero energy. We also include interference terms beyond the self-consistent Born approximation up to the third order in the disorder strength. These interference corrections are stronger than the mean-field result and non-analytic as functions of energy. Our main result is the dependence of conductivity (in units $e^2/h$) on the electron concentration $σ= σ_0 - 0.891\, n^{1/3} + 0.115\, (n^{2/3}/σ_0) \ln|n|$.

cond-mat.dis-nn↗

arXiv2020

Anomalous Hall effect with massive Dirac fermions

Anomalous Hall effect arises in systems with both spin-orbit coupling and magnetization. Generally, there are three mechanisms contributing to anomalous Hall conductivity: intrinsic, side jump, and skew scattering. The standard diagrammatic approach to the anomalous Hall effect is limited to computation of ladder diagrams. We demonstrate that this approach is insufficient. An important additional contribution comes from diagrams with a single pair of intersecting disorder lines. This contribution constitutes an inherent part of skew scattering on pairs of closely located defects and essentially modifies previously obtained results for anomalous Hall conductivity. We argue that this statement is general and applies to all models of anomalous Hall effect. We illustrate it by an explicit calculation for two-dimensional massive Dirac fermions with weak disorder. In this case, inclusion of the diagrams with crossed impurity lines reverses the sign of the skew scattering term and strongly suppresses the total Hall conductivity at high electron concentrations.

cond-mat.dis-nn↗

arXiv2020

Anisotropy of spin-transfer torques and Gilbert damping induced by Rashba coupling

Spin-transfer torques (STT), Gilbert damping (GD), and effective spin renormalization (ESR) are investigated microscopically in a 2D Rashba ferromagnet with spin-independent Gaussian white-noise disorder. Rashba spin-orbit coupling induced anisotropy of these phenomena is thoroughly analysed. For the case of two partly filled spin subbands, a remarkable relation between the anisotropic STT, GD, and ESR is established. In the absence of magnetic field and other torques on magnetization, this relation corresponds to a current-induced motion of a magnetic texture with the classical drift velocity of conduction electrons. Finally, we compute spin susceptibility of the system and generalize the notion of spin-polarized current.

cond-mat.mes-hall↗

arXiv2020

Anomalous Hall effect in 2D Rashba ferromagnet

Skew scattering on rare impurity configurations is shown to dominate the anomalous Hall effect in a 2D Rashba ferromagnet. The mechanism originates in scattering on rare impurity pairs separated by distances of the order of the Fermi wave length. Corresponding theoretical description goes beyond the conventional non-crossing approximation. The mechanism provides the only contribution to the anomalous Hall conductivity in the most relevant metallic regime and strongly modifies previously obtained results for lower energies in the leading order with respect to impurity strength.

cond-mat.mes-hall↗

arXiv2019

Spin-torque resonance due to diffusive dynamics at a surface of topological insulator

We investigate spin-orbit torques on magnetization in an insulating ferromagnetic (FM) layer that is brought into a close proximity to a topological insulator (TI). In addition to the well-known field-like spin-orbit torque, we identify an anisotropic anti-damping-like spin-orbit torque that originates in a diffusive motion of conduction electrons. This diffusive torque is vanishing in the limit of zero momentum (i. e. for spatially homogeneous electric field or current), but may, nevertheless, have a strong effect on spin-torque resonance at finite frequency provided external field is neither parallel nor perpendicular to the TI surface. The required electric field configuration can be created by a grated top gate.

cond-mat.mes-hall↗

arXiv2018

Mesoscopic supercurrent fluctuations in diffusive magnetic Josephson junctions

We study the supercurrent in quasi-one-dimensional Josephson junctions with a weak link involving magnetism, either via magnetic impurities or via ferromagnetism. In the case of weak links longer than {\color{black}the magnetic pair-breaking} length, the Josephson effect is dominated by mesoscopic fluctuations. We establish the supercurrent-phase dependence $I(φ)$ along with statistics of its sample-dependent properties in junctions with transparent contacts between leads and link. High transparency gives rise to the inverse proximity effect, while the direct proximity effect is suppressed by magnetism in the link. We find that all harmonics are present in $I(φ)$. Each harmonic has its own sample-dependent amplitude and phase shift with no correlation between different harmonics. Depending on the type of magnetic weak link, the system can realize a $φ_0$ or $φ$ junction in the fluctuational regime. Full supercurrent statistics is obtained at arbitrary relation between temperature, superconducting gap, and the Thouless energy of the weak link.

cond-mat.mes-hall↗

arXiv2018

Cooper pair splitting in diffusive magnetic SQUIDs

We study Josephson junctions with weak links consisting of two parallel disordered arms with magnetic properties -- ferromagnetic, half-metallic or normal with magnetic impurities. In the case of long links, the Josephson effect is dominated by mesoscopic fluctuations. In this regime, the system realises a $φ_0$ junction with sample-dependent $φ_0$ and critical current. Cooper pair splitting between the two arms plays a major role and leads to $2Φ_0$ periodicity of the current as a function of flux between the arms. We calculate the current and its flux and polarization dependence for the three types of magnetic links.

cond-mat.mes-hall↗

arXiv2017

Sensitivity of anomalous Hall effect to disorder correlations

Both longitudinal and anomalous Hall conductivity are computed in the model of two-dimensional Dirac fermions with a mass in the presence of arbitrary correlated weak disorder. The anomalous Hall conductivity is shown to be highly sensitive to the correlation properties of the random potential, such as the correlation length, while it remains independent of the integral disorder strength. This property extends beyond the Dirac model making the anomalous Hall effect an interesting tool to probe disorder correlations.

cond-mat.dis-nn↗

arXiv2017

Localization effects on magnetotransport of a disordered Weyl semimetal

We study magnetotransport in a disordered Weyl semimetal taking into account localization effects exactly. In the vicinity of a Weyl node, a single chiral Landau level coexists with a number of conventional non-chiral levels. Disorder scattering mixes these topologically different modes leading to peculiar localization effects. We derive the average conductance as well as the full distribution function of transmission probabilities along the field direction. Remarkably, we find that localization of the non-chiral modes is greatly enhanced in a strong magnetic field with the typical localization length scaling as $1/B$. Technically, we use the non-linear sigma-model formalism with a topological term describing the chiral states. The problem is solved exactly by mapping to an equivalent transfer matrix Hamiltonian.

cond-mat.mes-hall↗

arXiv2017

Dynamics of Anderson localization in disordered wires

We consider the dynamics of an electron in an infinite disordered metallic wire. We derive exact expressions for the probability of diffusive return to the starting point in a given time. The result is valid for wires with or without time-reversal symmetry and allows for the possibility of topologically protected conducting channels. In the absence of protected channels, Anderson localization leads to a nonzero limiting value of the return probability at long times, which is approached as a negative power of time with an exponent depending on the symmetry class. When topologically protected channels are present (in a wire of either unitary or symplectic symmetry), the probability of return decays to zero at long time as a power law whose exponent depends on the number of protected channels. Technically, we describe the electron dynamics by the one-dimensional supersymmetric non-linear sigma model. We derive an exact identity that relates any local dynamical correlation function in a disordered wire of unitary, orthogonal, or symplectic symmetry to a certain expectation value in the random matrix ensemble of class AIII, CI, or DIII, respectively. The established exact mapping from one- to zero-dimensional sigma model is very general and can be used to compute any local observable in a disordered wire.

cond-mat.mes-hall↗

arXiv2017

Ultrafast lithium diffusion in bilayer graphene

Solid mixed conductors with significant ionic as well as electronic conduction play a pivotal role for mass transfer and storage as required in battery electrodes. Single-phase materials with simultaneously high electronic and ionic conductivity at room temperature are hard to come by and therefore multi-phase systems with separate ion and electron channels have been put forward instead. Here, we explore bilayer graphene as a true single phase mixed conductor and demonstrate ultrafast lithium diffusion exceeding diffusion in bulk graphite by an order of magnitude and even surpassing diffusion of sodium chloride in liquid water. To this end, an innovative electrochemical cell architecture has been developed where the redox-reaction forcing lithium intercalation is localized at a protrusion of the device only. Its remainder consists of pristine bilayer graphene unperturbed by an electrolyte. The geometry lends itself to the use of magnetotransport machinery known from mesoscopic low-dimensional physics. Time dependent Hall measurements across spatially displaced Hall probes deliver a direct view on the in-plane diffusion kinetics. The device layout with a perimeterial electrochemical cell is transferable to other 2D materials as well as thin films and may promote a paradigm shift on the use of electrolytes in on-chip experiments.

cond-mat.mes-hall↗

arXiv2016

Boundary scattering effects on magnetotransport of narrow metallic wires and films

Electron transport in thin metallic wires and films is strongly influenced by the quality of their surface. Weak localization and magnetoconductivity are also sensitive to the electron scattering at the edges of the sample. We study weak localization effects in a two-dimensional electron gas patterned in the form of a narrow quasi-one-dimensional channel in transverse magnetic field. The most general boundary conditions interpolating between the limits of mirror and diffuse edge scattering are assumed. We calculate magnetoconductivity for an arbitrary width of the sample including the cases of diffusive and ballistic lateral transport as well as the crossover between them. We find that in a broad range of parameters, the electron mobility is limited by the boundary roughness while the magnetotransport is only weakly influenced by the quality of the edges. In addition, we calculate magnetoconductivity for a metallic cylinder in the transverse field and a quasi-two-dimensional metallic film in the parallel field.

cond-mat.mes-hall↗

arXiv2016

Lévy flights due to anisotropic disorder in graphene

We study transport properties of graphene with anisotropically distributed on-site impurities (adatoms) that are randomly placed on every third line drawn along carbon bonds. We show that stripe states characterized by strongly suppressed back-scattering are formed in this model in the direction of the lines. The system reveals Lévy-flight transport in stripe direction such that the corresponding conductivity increases as the square root of the system length. Thus, adding this type of disorder to clean graphene near the Dirac point strongly enhances the conductivity, which is in stark contrast with a fully random distribution of on-site impurities which leads to Anderson localization. The effect is demonstrated both by numerical simulations using the Kwant code and by an analytical theory based on the self-consistent $T$-matrix approximation.

cond-mat.dis-nn↗

arXiv2016

Critical exponents at the unconventional disorder-driven transition in a Weyl semimetal

Disordered non-interacting systems in sufficiently high dimensions have been predicted to display a non-Anderson disorder-driven transition that manifests itself in the critical behaviour of the density of states and other physical observables. Recently the critical properties of this transition have been extensively studied for the specific case of Weyl semimetals by means of numerical and renormalisation-group approaches. Despite this, the values of the critical exponents at such a transition in a Weyl semimetal are currently under debate. We present an independent calculation of the critical exponents using a two-loop renormalisation-group approach for Weyl fermions in $2-\varepsilon$ dimensions and resolve controversies currently existing in the literature.

cond-mat.mes-hall↗

arXiv2016

Semiclassical electron transport at the edge of a 2D topological insulator: Interplay of protected and unprotected modes

We study electron transport at the edge of a generic disordered two-dimensional topological insulator, where some channels are topologically protected from backscattering. Assuming the total number of channels is large, we consider the edge as a quasi-one-dimensional quantum wire and describe it in terms of a non-linear sigma model with a topological term. Neglecting localization effects, we calculate the average distribution function of transmission probabilities as a function of the sample length. We mainly focus on the two experimentally relevant cases: a junction between two quantum Hall (QH) states with different filling factors (unitary class) and a relatively thick quantum well exhibiting quantum spin Hall (QSH) effect (symplectic class). In a QH sample, the presence of topologically protected modes leads to a strong suppression of diffusion in the other channels already at the scales much shorter than the localization length. On the semiclassical level, this is accompanied by the formation of a gap in the spectrum of transmission probabilities close to unit transmission, thereby suppressing shot noise and conductance fluctuations. In the case of a QSH system, there is at most one topologically protected edge channel leading to weaker transport effects. In order to describe `topological' suppression of nearly perfect transparencies, we develop an exact mapping of the semiclassical limit of the one-dimensional sigma model onto a zero-dimensional sigma model of a different symmetry class, allowing us to identify the distribution of transmission probabilities with the average spectral density of a certain random-matrix ensemble. We extend our results to other symmetry classes with topologically protected edges in two dimensions.

cond-mat.mes-hall↗