arXiv subjects

V. M. Kovalev

Publications and source records attributed to V. M. Kovalev.

At least 37 records · Page 2Linked to original sources

arXiv2023

Optically induced resonant tunneling of electrons in nanostructures

We developed the theory of elastic electron tunneling through a potential barrier driven by a strong high-frequency electromagnetic field. It is demonstrated that the driven barrier can be considered as a stationary two-barrier potential which contains the quasi-stationary electron states confined between these two barriers. When the energy of an incident electron coincides with the energy of the quasi-stationary state, the driven barrier becomes fully transparent for the electron (the resonant tunneling). The developed theory is applied to describe electron transport through a quantum point contact irradiated by an electromagnetic wave.

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arXiv2023

Elementary excitations in the hybrid Bose-Fermi system induced by circularly polarized light in a two-dimensional gas of charge carriers with different masses

We developed a theory describing elementary excitations in the Bose-Fermi system induced by circularly polarized light in a two-dimensional (2D) gas of charge carriers with different masses. In such a hybrid system, the Fermi subsystem is a degenerate Fermi gas, whereas the Bose subsystem is a condensate of the light-induced composite bosons consisting of two fermions (electrons or holes) with different effective masses. The interaction of the single-particle excitations and the collective excitations (plasmons) in the Fermi subsystem with the Bogoliubov collective modes (bogolons) in the Bose subsystem is analyzed. The renormalization and damping (lifetime) of the excitations are calculated, and the possibility of their experimental observation is discussed. The developed theory can be applied to describe 2D condensed-matter structures containing charge carriers with different effective masses, including transition metal dichalcogenide monolayers and semiconductor quantum wells.

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arXiv2023

Optically induced delocalization of electrons bound by attractive potentials

Within the Floquet theory of periodically driven quantum systems, we demonstrate that a circularly polarized off-resonant electromagnetic field can destroy the electron states bound by three-dimensional attractive potentials. As a consequence, the optically induced delocalization of bound electrons appears. The effect arises from the changing of topological structure of a potential landscape under a circularly polarized off-resonant electromagnetic field which turns simply connected potentials into doubly connected ones. Possible manifestations of the effect are discussed for conduction electrons in condensed-matter structures.

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arXiv2022

Electron diffusion induced valley Hall effect and nonlinear galvanodiffusive transport in hexagonal 2D Dirac monolayer materials

Diffusion currents are theoretically examined in two-dimensional Dirac materials, such as those of the transition metal dichalcogenides (TMD) family. The transversal effects are analogues of the valley Hall (VHE) and photogalvanic (PGE) transport phenomena in case when the electron driving force is not an electric field but a gradient of electron density distribution in the sample. The latter can be created by a finite-sized laser spot or by the injection of electrons from other materials. We develop the theory of diffusive VHE effect assuming the anisotropic electron-short-range-impurity skew scattering. The electron PGE-like transport caused by higher electron-density derivatives is analyzed assuming the trigonal warping anisotropy of electron valleys in a TMD monolayer. The nonlinear responses on electron-density gradient are studied as well. The isotropic processes of electron scattering off the short-range and Coulomb centers are taken into account in the PGE-like transport theory.

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arXiv2022

Valley spin-acoustic resonance in MoS${\bf _2}$ monolayers

The band structure of a monolayer MoS$_2$ comprises of spin-split subbands, owing to the mutual presence of broken inversion symmetry and strong spin-orbit coupling. In the conduction band, spin-valley coupled subbands cross each other at finite momenta, and they are valley-degenerate. When exposed to surface acoustic waves, the emerging strain-induced effective magnetic field can give rise to spin-flip transitions between the spin-split subbands in the vicinity of subbands crossing point, resulting in the emergence of a spin-acoustic resonance and the acoustoelectric current. An external magnetic field breaks the valley degeneracy resulting in the valley-selective splitting of spin-acoustic resonances both in surface acoustic wave absorption and acoustoelectric current.

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arXiv2022

Photogalvanic transport in fluctuating Ising superconductors

In a two-dimensional noncentrosymmetric Ising superconductor in the fluctuating regime under the action of a uniform external electromagnetic field there emerge two contributions to the photogalvanic effect due to the trigonal warping of the valleys. The first contribution stems from the current of the electron gas in its normal state, while the second contribution is of Aslamazov-Larkin nature: it originates from the presence of fluctuating Cooper pairs when the ambient temperature approaches (from above) the temperature of superconducting transition in the sample. The way to lift the valley degeneracy is the application of a weak out-of-plane external magnetic field producing a Zeeman effect. The Boltzmann equations approach for the electron gas in the normal state and the time-dependent Ginzburg-Landau equations for the fluctuating Cooper pairs allow for the study of the photogalvanic current in two-dimensional transition metal dichalcogenide Ising superconductors.

cond-mat.supr-con↗

arXiv2022

All-optical control of excitons in semiconductor quantum wells

Applying the Floquet theory, we developed the method to control excitonic properties of semiconductor quantum wells by a high-frequency electromagnetic field. It is demonstrated, particularly, that the field induces the blue shift of exciton emission from the quantum wells and narrows width of the corresponding spectral line. As a consequence, the field strongly modifies optical properties of the quantum wells and, therefore, can be used to tune characteristics of the optoelectronic devices based on them.

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arXiv2021

Optically induced hybrid Bose-Fermi system in quantum wells with different charge carriers

It is demonstrated theoretically that the circularly polarized irradiation of two-dimensional conducting systems can produce the composite bosons consisting of two electrons with different effective masses (different charge carriers), which are stable due to the Fermi sea of conduction electrons. As a result, the optically induced mixture of paired electrons and normal conduction electrons (the hybrid Bose-Fermi system) appears. Elementary excitations in such a hybrid system are analyzed and possible manifestations of the light-induced electron pairing are discussed for semiconductor quantum wells.

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arXiv2021

Optically induced persistent current in carbon nanotubes

We demonstrate theoretically that an off-resonant circularly polarized electromagnetic field can induce a persistent current in carbon nanotubes, which corresponds to electron rotation about the nanotube axis. As a consequence, the nanotubes acquire magnetic moment along the axis, which depends on their crystal structure and can be detected in state-of-the-art measurements. This effect and related phenomena are analyzed within the developed Floquet theory describing the electronic properties of the nanotubes irradiated by the field.

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arXiv2021

Bose-Einstein condensate-mediated superconductivity in graphene

We propose a mechanism for robust BCS-like superconductivity in graphene placed in the vicinity of a Bose-Einstein condensate. Electrons in the graphene interact with the excitations above the condensate, called Bogoliubov quasiparticles (or bogolons). It turns out that bogolon-pair-mediated interaction allows us to surpass the long-standing problem of the vanishing density of states of particles with a linear spectrum. This results in a dramatic enhancement of the superconducting properties of graphene while keeping its relativistic dispersion. We study the behavior of the superconducting gap and calculate critical temperatures in cases with single-bogolon and bogolon-pair-mediated pairing processes, accounting for the complex band structure of graphene. We also compare the critical temperature of the superconducting transition with the BKT temperature.

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arXiv2021

Theory of BCS-like bogolon-mediated superconductivity in transition metal dichalcogenides

We report on a novel mechanism of BCS-like superconductivity, mediated by a pair of Bogoliubov quasiparticles (bogolons). It takes place in hybrid systems consisting of a two-dimensional electron gas in a transition metal dichalcogenide monolayer in the vicinity of a Bose-Einstein condensate. Taking a system of two-dimensional indirect excitons as a testing ground we show, that the bogolon-pair-mediated electron pairing mechanism is stronger than phonon-mediated and single bogolon-mediated ones. We develop a microscopic theory of bogolon-pair-mediated superconductivity, based on the Schrieffer-Wolff transformation and the Gor'kov's equations, study the temperature dependence of the superconducting gap and estimate the critical temperature of superconducting transition for various concentrations of the electron gas and the condensate densities.

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arXiv2021

Strong-coupling theory of condensate-mediated superconductivity in 2D materials

We develop a strong-coupling theory of Bose-Einstein condensate-mediated superconductivity in a hybrid system, which consists of a two-dimensional electron gas with either (i) parabolic spectrum or (ii) relativistic Dirac spectrum in the vicinity of a two-dimensional solid-state condensate of indirect excitons. The Eliashberg equations are derived and the expressions for the electron pairing self-energy due to the exchange interaction between electrons mediated by a single Bogoliubov excitation (a bogolon) and the bogolon pairs are found. Furthermore, we find the superconducting order parameter and estimate the critical temperature of the superconducting transition. The critical temperature reveals its linear dependence on the dimensionless coupling constant. It is shown, that the bogolon-pair-mediated interaction is the dominant mechanism of electron pairing in hybrid systems in both the weak and strong coupling regimes. We calculate the effective bogolon-electron interaction constant for both parabolic and linear electron dispersions and examine the dependence of the critical temperature of electron gas superconducting transition on exciton condensate density.

cond-mat.supr-con↗

arXiv2021

Nonlinear circular valley photogalvanic effect

We develop a theory of circular photogalvanic effect in non-gyrotropic two-dimensional transition metal dichalcogenide monolayers under interband optical transitions. Oblique incidence of circularly-polarized electromagnetic field or normal incidence of elliptically polarized electromagnetic field is assumed. In contrast to the linear-in-intensity conventional photogalvanic effect, the effect considered here arises in the second intensity order. The effect is conditioned by i) the predominant population of the valleys by the circular in-plane electromagnetic field component and ii) the direct drift of the photo-excited carriers by the linear-polarized in-plane electromagnetic field component in the presence of trigonal valley asymmetry.

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arXiv2021

Impurity-band optical transitions in two-dimensional Dirac materials under strain-induced synthetic magnetic field

We develop a theory of optical transitions in Coulomb impurity-doped two-dimensional transition metal dichalcogenide monolayers and study the transitions from the spin-resolved valence band to the (Coulomb) donor and acceptor impurities under the influence of a synthetic valley-selective magnetic field produced by a mechanical strain. It is shown that the optical properties of the system are determined by the strength of the synthetic magnetic field, which uncovers an experimental tool, which can be used to manipulate the properties of two-dimensional materials in valley magneto-optoelectronics.

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arXiv2021

Optical valleytronics of impurity states in two-dimensional Dirac materials

We analyze the valley selection rules for optical transitions from impurity states to the conduction band in two-dimensional Dirac materials, taking a monolayer of MoS2 as an example. We employ the analytical model of a shallow impurity potential which localizes electrons described by a spinor wave function, and, first, find the system eigenstates taking into account the presence of two valleys in the Brillouin zone. Then, we find the spectrum of the absorbance and calculate the photon-drag electric current due to the impurity-band transitions, drawing the general conclusions regarding the valley optical selection rules for the impurity-band optical transitions in gapped Dirac materials.

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arXiv2021

Nonequilibrium Theory of Photoinduced Valley Hall Effect

A recent scientific debate has arisen: Which processes underlie the actual ground of the valley Hall effect (VHE) in two-dimensional materials? The original VHE emerges in samples with ballistic transport of electrons due to the anomalous velocity terms resulting from the Berry phase effect. In disordered samples though, alternative mechanisms associated with electron scattering off impurities have been suggested: (i) asymmetric electron scattering, called skew scattering, and (ii) a shift of the electron wave packet in real space, called a side jump. It has been claimed that the side jump not only contributes to the VHE but fully offsets the anomalous terms regardless of the drag force for fundamental reasons and, thus, the side-jump together with skew scattering become the dominant mechanisms. However, this claim is based on equilibrium theories without any external valley-selective optical pumping, which makes the results fundamentally interesting but incomplete and impracticable. We develop in this paper a microscopic theory of the photoinduced VHE using the Keldysh nonequilibrium diagrammatic technique and find that the asymmetric skew scattering mechanism is dominant in the vicinity of the interband absorption edge. This allows us to explain the operation of optical transistors based on the VHE.

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arXiv2020

Acoustoelectric effect in two-dimensional Dirac materials exposed to Rayleigh surface acoustic waves

We study the acoustoelectric effect in two-dimensional materials like transition metal dichalcogenide monolayers located on a non-piezoelectric substrate and exposed to the Rayleigh surface acoustic waves. We investigate the behavior of the Hall component of the electric current density which appears due to the trigonal warping of the valleys in k-space. We calculate the spectrum of the current density and study its dependence on the electron effective lifetime and density in the sample. We distinguish between the drift and diffusive components of the current and show, which components turn out predominant. Furthermore, we compare the effect of the Rayleigh and Bluestein-Gulyaev acoustic waves, which appear if the sample is located on a piezoelectric substrate.

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arXiv2020

Light-induced bound electron states in two-dimensional systems: Contribution to electron transport

In two-dimensional (2D) electron systems, an off-resonant high-frequency circularly polarized electromagnetic field can induce the quasi-stationary bound electron states of repulsive scatterers. As a consequence, the resonant scattering of conduction electrons through the quasi-stationary states and the capture of conduction electrons by the states appear. The present theory describes the transport properties of 2D electron gas irradiated by a circularly polarized light, which are modified by these processes. Particularly, it is demonstrated that irradiation of 2D electron systems by the off-resonant field results in the quantum correction to conductivity of resonant kind.

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