arXiv subjects

V. M. Kovalev

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

At least 55 records · Page 3Linked to original sources

arXiv2020

Coherent photogalvanic effect in fluctuating superconductors

We develop a theory of the coherent photogalvanic effect (CPGE) in low-dimensional superconductors in the fluctuating regime. It manifests itself in the appearance of a stationary electric current of Cooper pairs under the action of two coherent electromagnetic fields of light with frequencies lying in the sub-terahertz range. We derive the general formula for the electric current density, study the particular cases of linear and circular polarizations of the external light fields, and show that the current might have a non-monotonous spectrum at certain polarization angles and turns out very sensitive to the proximity of the ambient temperature to the critical temperature of superconducting transition: Approaching the critical temperature, the peak in the spectrum becomes narrower, its frequency experiences a redshift, and the intensity of the peak drastically grows.

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arXiv2020

Floquet engineering of the Luttinger Hamiltonian

Within the Floquet theory of periodically driven quantum systems, we developed the theory of light-induced modification of electronic states in semiconductor materials described by the Luttinger Hamiltonian (the electronic term $Γ_8$). Particularly, exact solutions of the Floquet problem are found for the band edge in the cases of linearly and circularly polarized irradiation. It is shown that the irradiation changes electron effective masses near the band edge, induces anisotropy of the electron dispersion and splits the bands. It is demonstrated that the light-induced band splitting strongly depends on the light polarization. Namely, the circularly polarized light acts similarly to a stationary magnetic field and lifts the spin degeneracy of electron branches, whereas a linearly polarized light does not affect the spin degeneracy and only splits the bands in the center of the Brillouin zone. The present theory can be applied to describe electronic properties of various semiconductor structures irradiated by an electromagnetic field in the broad frequency range.

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arXiv2020

Proposal for Plasmon Spectroscopy of Fluctuations in Low-Dimensional Superconductors

We propose to employ an optical spectroscopy technique to monitor the superconductivity and properties of superconductors in the fluctuating regime. This technique is operational close to the plasmon resonance frequency of the material, and it intimately connects with the superconducting fluctuations slightly above the critical temperature $T_c$. We find the Aslamazov-Larkin corrections to AC linear and DC nonlinear electric currents in a generic two-dimensional system exposed to an external longitudinal electromagnetic field. First, we study the plasmon resonance of normal electrons near $T_c$, taking into account their interaction with superconducting fluctuations, and show that fluctuating Cooper pairs reveal a redshift of the plasmon dispersion and an additional mechanism of plasmon scattering, which surpasses both the electron-impurity and the Landau dampings. Second, we demonstrate the emergence of a drag effect of superconducting fluctuations by the external field resulting in considerable, experimentally measurable corrections to the electric current in the vicinity of the plasmon resonance.

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arXiv2020

Interplay between collective modes in hybrid electron gas-superconductor structures

We study hybridization of collective plasmon and Carlson-Goldman-Artemenko-Volkov modes in a hybrid system, consisting of a two-dimensional layers of electron gas in the normal state and superconductor, coupled by long-range Coulomb forces. The interaction between these collective modes is not possible in a regular single-layer two-dimensional system since they exist in non-overlapping domains of dimensionless parameter $ωτ$, where $ω$ is the external electromagnetic field frequency and $τ$ is electron scattering time. Thus, in a single-layer structure, these modes are mutually exclusive. However, the coupling may become possible in a hybrid system consisting of two separated in space materials with different properties, in particular, the electron scattering time. We investigate the electromagnetic power absorption by the hybrid system and reveal the conditions necessary for the hybridization of collective modes.

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arXiv2020

Acoustomagnetoelectric effect in two-dimensional materials: Geometric resonances and Weiss oscillations

We study electron transport in two-dimensional materials with parabolic and linear (graphene) dispersions of the carriers in the presence of surface acoustic waves and an external magnetic field using semiclassical Boltzmann equations approach. We observe an oscillatory behavior of both the longitudinal and Hall electric currents as functions of the surface acoustic wave frequency at a fixed magnetic field and as functions of the inverse magnetic field at a fixed frequency of the acoustic wave. We explain the former by the phenomenon of geometric resonances, while we relate the latter to the Weiss-like oscillations in the presence of the dynamic superlattice created by the acoustic wave. Thus we demonstrate the dual nature of the acoustomagnetoelectric effect in two-dimensional electron gas.

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arXiv2019

Exciton-Polariton Topological Insulator with an Array of Magnetic Dots

Recently there have been several proposals on exciton-polariton topological insulators, most requiring strong external magnetic fields induced by bulky superconducting coils. We propose an alternate design for a polariton topological insulator, where excitons are in a proximity of an additional layer of a ferromagnetic material with a predefined magnetic moment located between the pillars of the cavity. Our design supports a variety of topological phases and transitions between Chern numbers $\pm 2$ and $\pm 1$ by varying either the magnetic moment of the ferromagnetic material or the spin-orbit coupling between different spin projections, thus enabling compact polariton devices harnessing switchable topological edge modes.

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arXiv2019

Valley Acoustoelectric Effect

We report on novel valley acoustoelectric effect, which can arise in a 2D material, like a transition metal dichalcogenide monolayer, residing on a piezoelectric substrate. The essence of this effect lies in the emergence of a drag electric current (and a spin current) due to a propagating surface acoustic wave. This current consists of three contributions, one independent of the valley index and proportional to the acoustic wave vector, the other arising due to the trigonal warping of the electron dispersion, and the third one is due to the Berry phase, which Bloch electrons acquire traveling along the crystal. As a result, there appear components of the current orthogonal to the acoustic wave vector. Further, we build an angular pattern, encompassing nontrivial topological properties of the acoustoelectric current, and suggest a way to run and measure the conventional diffusive, warping, and acoustoelectric valley Hall currents independently. We develop a theory, which opens a way to manipulate valley transport by acoustic methods, expanding the applicability of valleytronic effects on acoustoelectronic devices.

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arXiv2019

Quantum Anomalous Valley Hall Effect for Bosons

We predict the emergence and quantization of the valley Hall current of indirect excitons residing in noncentrosymmetric two-dimensional materials in the absence of an external magnetic field. Thus we report on the quantum anomalous valley Hall effect (QAVHE) for bosons. The crucial ingredients of its existence are the finite anomalous group velocity of exciton motion and the presence of the Bose-Einstein condensate, from which the particles are pushed out by external illumination to contribute to the stationary nonequilibrium current, which exhibits a quantized behavior as a function of the frequency of light.

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arXiv2019

Coulomb drag of excitons in Bose-Fermi mixtures

We develop a microscopic theory of the Coulomb drag effect in a hybrid system consisting of spatially separated two-dimensional quantum gases of degenerate electrons and dipolar excitons. We consider both the normal-phase and condensate regimes of the exciton subsystem and investigate the cross-mobility of the system being the kinetic coefficient, which couples the static electric field applied to the electron layer with the particle density current (flux) in the exciton subsystem. We study the temperature dependence of the cross-mobility and its dependence on the interlayer separation. We show that exciton-exciton interaction plays a dramatic role. If the exciton gas is in the normal phase, then the screening of interlayer interaction by the exciton subsystem results in an exponential damping of the cross-mobility with the decrease of temperature, while at low temperatures, the interactions result in a robust bosonic transport due to the emergence of the Bogoliubov quasiparticles.

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arXiv2018

Bogolon-mediated electron scattering in graphene in hybrid Bose-Fermi systems

We report on an unconventional mechanism of electron scattering in graphene in hybrid Bose- Fermi systems. We study energy-dependent electron relaxation time, accounting for the processes of emission and absorption of a Bogoliubov excitation (a bogolon). Then using the Bloch-Gruneisen approach, we find the finite-temperature resistivity of graphene and show that its principal behavior is $\sim T^4$ in the limit of low temperatures and linear at high temperatures. We show that bogolon-mediated scattering can surpass the acoustic phonon-assisted relaxation. It can be controlled by the distance between the layers and the condensate density, giving us additional degrees of freedom and a useful tool to render electron mobility by the sample design and external pump.

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arXiv2018

Photogalvanic currents in dynamically gapped Dirac materials

We develop a microscopic theory of an unconventional photogalvanic effect in two-dimensional materials with the Dirac energy spectrum of the carriers of charge under strong driving. As a test bed, we consider a layer of a transition metal dichalcogenide, exposed to two different electromagnetic fields. The first pumping field is circularly-polarized, and its frequency exceeds the material bandgap. It creates an extremely nonequilibrium distribution of electrons and holes in one valley (K) and opens dynamical gaps, whereas the other valley (K') remains empty due to the valley-dependent interband selection rules. The second probe field has the frequency much smaller than the material bandgap. It generates intraband perturbations of the nonequilibrium carriers density, resulting in the photogalvanic current due to the trigonal asymmetry of the dispersions. This current shows threshold-like behavior due to the dynamical gap opening and renormalizations of the density of states and velocity of quasiparticles.

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arXiv2018

Photogalvanic Effect in 2D Dichalcogenides Under Double Illumination

We study the photogalvanic effect caused by a simultaneous action of circular-polarized interband and linearly-polarized intraband illuminations. It is found that, in such conditions, the steady photocurrent appears. The effect originates from the valley-selective pumping by the circular light, the trigonal asymmetry of the valleys together with the even asymmetry of the linearly-polarized light, that produces a polar in-plane asymmetry of the electron and hole distribution functions, leading to the photocurrent. The approach is based on the solution of the classical kinetic equation for carriers with accounting for the quantum interband excitation.

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arXiv2018

Photoinduced electric currents in Bose-Einstein condensates

We calculate a light-induced electric current which can occur from a Bose-Einstein condensate under the action of an external electromagnetic field with the frequency exceeding the ionization potential of the bosons, taking a system of indirect excitons as a testbed. We show that the ionization can be accompanied by the excitation of collective Bogoliubov modes. As a result, the current consists of two principal components: one regular, which has a counterpart in bosonic systems in the normal phase, and the other one specific for condensates since the photoabsorption is mediated by the emission of Bogoliubov quasiparticles. Surprisingly, the latter component soon becomes predominant with the increase of light frequency above the ionization potential.

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arXiv2018

Valley Hall Transport of Photon-Dressed Quasiparticles in 2D Dirac Semiconductors

We present a theory of the photovoltaic valley-dependent Hall effect in a two-dimensional Dirac semiconductor subject to an intense near-resonant electromagnetic field. Our theory captures and elucidates the influence of both the field-induced resonant interband transitions and the nonequilibrium carrier kinetics on the resulting valley Hall transport in terms of photon-dressed quasiparticles. The non-perturbative renormalization effect of the pump field manifests itself in the dynamics of the photon-dressed quasiparticles, with a quasienergy spectrum characterized by {dynamical gaps $δ_η$ ($η$ is the valley index)} that strongly depend on field amplitude and polarization. Nonequilibrium carrier distribution functions are determined by the pump field frequency $ω$ as well as the ratio of intraband relaxation time $τ$ and interband recombination time $τ_{\mathrm{rec}}$. We obtain analytic results in three regimes, when (I) all relaxation processes are negligible, (II) $τ\ll τ_{\mathrm{rec}}$, and (III) $τ\gg τ_{\mathrm{rec}}$, and display corresponding asymptotic dependences on $δ_η$ and $ω$. We then apply our theory to two-dimensional transition-metal dichalcogenides, and find a strong enhancement of valley-dependent Hall conductivity as the pump field frequency approaches the transition energies between the pair of spin-resolved conduction and valence bands at the two valleys.

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arXiv2018

Proposal for frequency-selective photodetector based on the resonant photon drag effect in a condensate of indirect excitons

We present a microscopic theory of a photon drag effect that appears in a Bose-Einstein condensate of neutral particles, considering indirect excitons in a double quantum well nanostructure under the action of a circularly polarized electromagnetic field. It is shown that the dynamical polarization of excitons results in a resonant behavior of the exciton photon drag flux when the frequency of light is close to the gap between two energy levels of internal exciton motion. Specifically, we consider the ground and first excited energy states characterized by the angular momentum difference $\pm 1$, and thus, the helicity of light matters. We show that the resulting drag current is caused by both Bose-condensed particles and the particles in the normal state. As a result, the total current represents a superposition of thresholdlike and resonant contributions, - property, which can be used in frequency-selective photodetection.

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arXiv2018

Shedding light on topological superconductors

We propose an optical approach to monitor superconductors in conjunction with a normal metal layer. Effectively such a hybrid system represents a resonator, where electrons are strongly coupled with light. We show that the interaction of light with the superconductor turns out strongly boosted in the presence of the neighboring metal and as a result, the electromagnetic power absorption of the system is dramatically enhanced. It manifests itself in a giant Fanolike resonance which can uniquely characterize the elementary excitations of the system. Our approach is especially promising for topological superconductors, where the Majorana fermions can be revealed and controlled by light.

cond-mat.supr-con↗

arXiv2018

Magnetoplasmon Fano resonance in Bose-Fermi mixtures

We investigate theoretically the magnetoplasmon (cyclotron) resonance in a hybrid system consisting of spatially separated two-dimensional layers of electron and dipolar exciton gases coupled via the Coulomb forces. We study the dynamics of this system under the action of weak alternating external electromagnetic field in the presence of uniform magnetic field, perpendicular to the layers. We reveal that the electromagnetic power absorption exhibits a double-resonance spectrum. We show that the first resonance is associated with the conventional well-studied magnetoplasmon excitations of the electron gas and it has a standard Lorentzian shape, whereas the second resonance is a peculiarity attributed to the Bose-condensed exciton gas. Further, we explicitly demonstrate that the spectrum of the system exhibits an asymmetric Fano-type profile, where the excitonic peak is extremely narrow in comparison with the magnetoplasmon one. We show that the shape of the resonance and the position of the peaks depends on the magnitude of the applied magnetic field, exciton condensate density and exciton-impurity scattering time. In particular, the Fano profile turns into a Lorentzian shape with decreasing exciton-impurity scattering time and the position of the plasmon-associated resonance is mainly sensitive and determined by the magnetic field strength, whereas the exciton-condensate peak position is determined by exciton condensate density. It opens the experimental possibility to determine the latter quantity in cyclotron resonance experiment.

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arXiv2018

Bogolon-mediated electron capture by impurities in hybrid Bose-Fermi systems

We investigate the processes of electron capture by a Coulomb impurity center residing in a hybrid system consisting of spatially separated two-dimensional layers of electron and Bose-condensed dipolar exciton gases coupled via the Coulomb forces. We calculate the probability of the electron capture accompanied by the emission of a single Bogoliubov excitation (bogolon), similar to regular phonon-mediated scattering in solids. Further, we study the electron capture mediated by the emission of a pair of bogolons in a single capture event and show that these processes not only should be treated in the same order of the perturbation theory, but also they give more important contribution than single bogolon-mediated capture, in contrast with regular phonon scattering.

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