arXiv subjects

A. V. Shumilin

Publications and source records attributed to A. V. Shumilin.

At least 19 recordsLinked to original sources

arXiv2024

Impact of the impurity symmetry on orbital momentum relaxation and orbital Hall effect studied by the quantum Boltzmann equation

We develop a quantum Boltzman equation approach that incorporates microscopic impurity models into the theory of orbital transport, revealing, how impurity properties, including their symmetry, influence the relaxation of orbital momentum and the orbital Hall effect. Specifically, we demonstrate that when the impurity potential has axial symmetry, the relaxation is governed by the Dyakonov-Perel mechanism. In contrast, when this symmetry is broken, scattering can result in a rapid Elliot-Yafet relaxation of orbital momentum. The details of impurity potential also affect the intrinsic orbital Hall effect even when the impurity concentration is very small. Impurities that alter the orbital texture can also give rise to a skew-scattering contribution to the orbital Hall effect, although this does not necessarily dominate in materials that are nearly pristine due to the Dyakonov-Perel relaxation.

cond-mat.mes-hall↗

arXiv2023

Interface disorder as a cause for kinetic Rashba-Edelstein effect and interface Spin-Hall effect at the metal-insulator boundary

The spin phenomena observed at a clean metall-insulator interface are typically reduced to Rashba-Edelstein effect, that leads to spin accumulation over a few monolayers. We demonstrate that the presence of interface disorder significantly expands the range of potential phenomena. Specifically, the skew scattering at the metal - insulator boundary gives rise to the "kinetic Rashba-Edelstein effect", where spin accumulation occurs on a much larger length scale comparable to mean free path. Moreover, at higher orders of spin-orbit interaction, skew scattering is accompanied with spin relaxation resulting in the interface spin-Hall effect - a conversion of electrical current to spin current at the metal surface. Unlike the conventional spin-Hall effect, this phenomenon persists even within the Born approximation. These two predicted phenomena can dominate the spin density and spin current in devices of intermediate thickness.

cond-mat.mes-hall↗

arXiv2023

Indication of novel magnetoresistance mechanism in (Bi,Sb)$_2$(Te,Se)$_3$ 3D topological insulator thin films

Electron states with the spin-momentum-locked Dirac dispersion at the surface of a three-dimensional (3D) topological insulator are known to lead to weak antilocalization (WAL), i.e. low temperature and low-magnetic field quantum interference-induced positive magnetoresistance (MR). In this work we report on the MR measurements in (Bi,Sb)$_2$(Te,Se)$_3$ 3D topological insulator thin films epitaxially grown on Si(111), demonstrating an anomalous WAL amplitude. This anomalously high amplitude of WAL can not be explained by parabolic or linear MR and indicates the existence of an additional, MR mechanism. Another supporting observation is not linear in the classically weak magnetic field Hall effect in the same films. The increase of the low-field Hall coefficient, with respect to the higher-field value, reaches 10$\%$. We consistently explain both transport features within a two-liquid model, where the mobility of one of the components drops strongly in a weak magnetic field. We argue that this dependence may arise from the Zeeman field induced gap opening mechanism.

cond-mat.mes-hall↗

arXiv2023

Spin light emitting diode based on exciton fine structure tuning in quantum dots

We propose a concept of quantum dot based light emitting diode that produces circularly polarized light due to the tuning of the exciton fine structure by magnetic field and electron nuclear hyperfine interaction. The device operates under injection of electrons and holes from nonmagnetic contacts in a small field of the order of milliteslas. Its size can be parametrically smaller than the light wavelength, and circular polarization degree of electroluminescence can reach 100%. The proposed concept is compatible with the micropillar cavities, which allows for the deterministic electrical generation of single circularly polarized photons.

cond-mat.mes-hall↗

arXiv2023

Optical orientation of excitons in a longitudinal magnetic field in indirect band gap (In,Al)As/AlAs quantum dots with type-I band alignment

The exciton recombination and spin dynamics in (In,Al)As/AlAs quantum dots (QDs) with indirect band gap and type-I band alignment are studied. The negligible (less than $0.2~μ$eV) value of the anisotropic exchange interaction in these QDs prevents a mixing of the excitonic basis states with pure spin and allows for the formation of spin polarized bright excitons for quasi-resonant circularly polarized excitation. In a longitudinal magnetic field, the recombination and spin dynamics of the excitons are controlled by the hyperfine interaction between the electron and nuclear spins. A QD blockade by dark excitons is observed in magnetic field eliminating the impact of the nuclear spin fluctuations. A kinetic equation model, which accounts for the population dynamics of the bright and dark exciton states as well as for the spin dynamics, has been developed, which allows for a quantitative description of the experimental data.

cond-mat.mes-hall↗

arXiv2021

Dynamic spin polarization in organic semiconductors with intermolecular exchange interaction

It is shown that in organic semiconductors where organic magnetoresistance (OMAR) is observed, the exchange interaction between electrons and holes localized at different molecules leads to dynamic spin polarization in the direction of the applied magnetic field. The polarization appears even at room temperature due to the non-equilibrium conditions. The strong spin polarization requires exchange energy to be comparable with Zeeman energy in the external field and be larger or comparable with the energy of hyperfine interaction of electron and nuclear spins. The exchange interaction also modifies the lineshape of OMAR.

cond-mat.dis-nn↗

arXiv2021

Nuclear spin dynamics and noise in anisotropic large box model

We consider the central spin model in the box approximation taking into account external magnetic field and anisotropy of the hyperfine interaction. From the exact Hamiltonian diagonalization we obtain analytical expressions for the nuclear spin dynamics in the limit of many nuclear spins. We predict the nuclear spin precession in zero magnetic field for the case of anisotropic interaction between electron and nuclear spins. We calculate and describe the nuclear spin noise spectra in the thermodynamic equilibrium. The obtained results can be used for the analysis of the nuclear spin induced current fluctuations in organic semiconductors.

cond-mat.mes-hall↗

arXiv2021

Dynamic polarization of electron spins in indirect band gap (In,Al)As/AlAs quantum dots in weak magnetic field: experiment and theory

A novel spin orientation mechanism - dynamic electron spin polarization has been recently suggested in Phys. Rev. Lett. $\mathbf{125}$, 156801 (2020). It takes place for unpolarized optical excitation in weak magnetic fields of the order of a few millitesla. In this paper we demonstrate experimentally and theoretically that the dynamic electron spin polarization degree changes sign as a function of time, strength of the applied magnetic field and its direction. The studies are performed on indirect band-gap (In,Al)As/AlAs quantum dots and their results are explained in the framework of a theoretical model developed for our experimental setting.

cond-mat.mes-hall↗

arXiv2021

Nuclear spin dynamics, noise, squeezing and entanglement in box model

We obtain a compact analytical solution for the nonlinear equation for the nuclear spin dynamics in the central spin box model in the limit of many nuclear spins. The total nuclear spin component along the external magnetic field is conserved and the two perpendicular components precess or oscillate depending on the electron spin polarization, with the frequency, determined by the nuclear spin polarization. As applications of our solution, we calculate the nuclear spin noise spectrum and describe the effects of nuclear spin squeezing and many body entanglement in the absence of a system excitation.

cond-mat.mes-hall↗

arXiv2020

Tunneling magnetoresistance in ensembles of ferromagnetic granules with exchange interaction and random easy axes of magnetic anisotropy

We study the tunneling magnetoresistance in the ensembles of ferromagnetic granules with random easy axes of magnetic anisotropy taking into account the exchange interaction between granules. It is shown that due to the exchange interaction magnetoresistance is effectively decoupled from magnetization, i.e. the strongest negative magnetoresistance can be observed at the field where magnetization is almost saturated. Under some conditions, the sign of magnetoresistance can be reversed and tunneling magnetoresistance can become positive at certain magnetic fields. Our theory agrees with measurements of magnetoresistance in ensembles of Fe granules in SiCxNy matrix.

cond-mat.mes-hall↗

arXiv2020

Electric current noise in mesoscopic organic semiconductors

We demonstrate that nuclear spin fluctuations lead to the electric current noise in the mesoscopic samples of organic semiconductors showing the pronounced magnetoresistance in weak fields. For the bipolaron and electron-hole mechanisms of organic magnetoresistance, the current noise spectrum consists of the high frequency peak related to the nuclear spin precession in the Knight field of the charge carriers and the low frequency peak related to the nuclear spin relaxation. The shape of the spectrum depends on the external magnetic and radiofrequency fields, which allows one to prove the role of nuclei in magnetoresistance experimentally.

cond-mat.mes-hall↗

arXiv2020

Microscopic theory of OMAR based on kinetic equations for quantum spin correlations

The correlation kinetic equation approach is developed that allows describing spin correlations in a material with hopping transport. The quantum nature of spin is taken into account. The approach is applied to the problem of the bipolaron mechanism of organic magnetoresistance (OMAR) in the limit of large Hubbard energy and small applied electric field. The spin relaxation that is important to magnetoresistance is considered to be due to hyperfine interaction with atomic nuclei. It is shown that the lineshape of magnetoresistance depends on short-range transport properties. Different model systems with identical hyperfine interaction but different statistics of electron hops lead to different lineshapes of magnetoresistance including the two empirical laws $H^2/(H^2 + H_0^2)$ and $H^2/(|H| + H_0)^2$ that are commonly used to fit experimental results.

cond-mat.dis-nn↗

arXiv2019

The system of correlation kinetic equations and the generalized equivalent circuit for hopping transport

We derive the system of equations that allows to include non-equilibrium correlations of filling numbers into the theory of the hopping transport. The system includes the correlations of arbitrary order in a universal way and can be cut at any place relevant to a specific problem to achieve the balance between rigor and computation possibilities. In the linear-response approximation, it can be represented as an equivalent electric circuit that generalizes the Miller-Abrahams resistor network. With our approach, we show that non-equilibrium correlations are essential to calculate conductivity and distribution of currents in certain disordered systems. Different types of disorder affect the correlations in different applied fields. The effect of energy disorder is most important at weak electric fields while the position disorder by itself leads to non-zero correlations only in strong fields.

cond-mat.dis-nn↗

arXiv2018

Hall effect in 2D systems with hopping transport and strong disorder

We reconsider the theory of Hall effect in the systems with hopping conduction. The purpose of the present study is to compare the percolation approach based on the optimal triad model with numerical simulations and recent experimental results. We show that, in the nearest neighbor hopping regime, the results of the percolation theory agree to the simulation. However, in the variable range hopping (VRH) regime, the optimal triad model fails to describe the numerical results. It is related to the extremely small probability to find the optimal triad of sites in the percolation cluster in the VRH regime. The contribution of these triads to the Hall effect appears to be small. We describe the Hall mobility in the VRH regime with the empirical law obtained from the numerical results. The law is in agreement with our recent experimental data in 2D quantum dot arrays with the hopping transport.

cond-mat.dis-nn↗

arXiv2018

Spin-related phenomena in two-dimensional hopping regime in magnetic field

The spin relaxation time of localized charge carriers is few orders of magnitude larger than that of free electrons and holes. Therefore mutual conversion of spin polarization, charge current and spin current turns out to be underlined in the hopping conductivity regime. We reveal different regimes of the coupled spin and charge dynamics depending on the relation between spin relaxation time and the characteristic hopping time. We derive kinetic equations to describe electrical spin orientation, dc spin-Hall effect, and spin galvanic effect in the transverse magnetic field. The generalized macroscopic conductivities describing these effects are calculated using percolation theory supported by numerical simulation. The conductivities change the sign at least once as functions of magnetic field for all values of the spin relaxation time.

cond-mat.mes-hall↗

arXiv2018

Magnetoresistance in organic semiconductors: including pair correlations in the kinetic equations for hopping transport

We derive the kinetic equations for polaron hopping in organics that explicitly take into account the double occupation possibility and pair intersite correlations. The equations include simplified phenomenological spin dynamics and provide a self-consistent framework for the description of the bipolaron mechanism of the organic magnetoresistance. At low applied voltages the equations can be reduced to effective resistor network that generalizes the Miller-Abrahams network and includes the effect of spin relaxation on the system resistivity. Our theory discloses the close relationship between the organic magnetoresistance and the intersite correlations. Moreover, in the absence of correlations, as in ordered system with zero Hubbard energy, the magnetoresistance vanishes.

cond-mat.dis-nn↗

arXiv2016

Upper critical field in the model with finite range interaction between electrons

We develop a theory of the upper critical field in a BCS superconductor with a nonlocal interaction between electrons. We have shown that the nonlocal interaction is characterized by the parameter $k_Fρ_0$ where $k_F$ is the Fermi momentum and $ρ_0$ is the radius of electron-electron interaction. The presence of the external magnetic field leads to the generation of additional components of the order parameter with different angular momentum. This effect leads to the enhancement of the upper critical field above the orbital limiting field. In addition the upward curvature in the temperature dependence of $H_{c2}(T)$ in the clean limit is predicted. The impurity scattering suppresses the effect in the dirty limit.

cond-mat.supr-con↗

arXiv2016

Spin excitations in systems with hopping electron transport and strong position disorder in a large magnetic field

We discuss the spin excitations in systems with hopping electron conduction and strong position disorder. We focus on the problem in a strong magnetic field when the spin Hamiltonian can be reduced to the effective single-particle Hamiltonian and treated with conventional numerical technics. It is shown that in a 3D system with Heisenberg exchange interaction the spin excitations have a delocalized part of the spectrum even in the limit of strong disorder, thus leading to the possibility of the coherent spin transport. The spin transport provided by the delocalized excitations can be described by a diffusion coefficient. Non-homogenous magnetic fields lead to the Anderson localization of spin excitations while anisotropy of the exchange interaction results in the Lifshitz localization of excitations. We discuss the possible effect of the additional exchange-driven spin diffusion on the organic spin-valve devices.

cond-mat.dis-nn↗