arXiv subjects

Alexey A. Sokolik

Publications and source records attributed to Alexey A. Sokolik.

13 recordsLinked to original sources

arXiv2026

Tunneling characteristics of twisted double bilayer graphene heterostructures

Electron tunneling between sheets of bilayer Bernal graphene twisted at different small angles was studied experimentally and theoretically. The current-voltage characteristics exhibit resonant peaks, steps, and regions of negative differential resistance, the origin of which is explained by the intersections of energy- and momentum-shifted electron dispersions of adjacent layers. A theoretical analysis of tunneling transport demonstrated that the key to understanding this phenomenon lies in the competition between two contributions: between like (conductivity-conductivity or valence-valence) and unlike (conductivity-valence) bands of parallel bilayer graphene sheets. A systematic evolution of the tunneling current patterns with increase of the twist angle is investigated. Polarization of electron wave function across graphene sublayers caused by displacement field within bilayer graphene is shown to strongly affect the tunneling probability, thus enhancing negative differential resistance due to Van Hove singularities at the band edges.

cond-mat.mes-hall↗

arXiv2025

Probing the features of electron dispersion by tunneling between slightly twisted bilayer graphene sheets

Tunneling conductance between two bilayer graphene (BLG) sheets separated by 2 nm-thick insulating barrier was measured in two devices with the twist angles between BLGs less than 1°. At small bias voltages, the tunneling occurs with conservation of energy and momentum at the points of intersection between two relatively shifted Fermi circles. Here, we experimentally found and theoretically described signatures of electron-hole asymmetric band structure of BLG: since holes are heavier, the tunneling conductance is enhanced at the hole doping due to the higher density of states. Another key feature of BLG that we explore is gap opening in a vertical electric field with a strong polarization of electron wave function at van Hove singularities near the gap edges. This polarization, by shifting electron wave function in one BLG closer to or father from the other BLG, gives rise to asymmetric tunneling resonances in the conductance around charge neutrality points, which result in strong sensitivity of the tunneling current to minor changes of the gate voltages. The observed phenomena are reproduced by our theoretical model taking into account electrostatics of the dual-gated structure, quantum capacitance effects, and self-consistent gap openings in both BLGs.

cond-mat.mes-hall↗

arXiv2025

Hybridization of edge modes with substrate phonons

Surface plasmons or phonons propagating on a two-dimensional (2D) material can exhibit coupling with resonant excitations in its substrate, and the resulting coupled modes were extensively studied. Similar coupling of edge modes propagating along a boundary of 2D material with the substrate excitations remains unexplored. This paper aims to bridge this gap by investigating the edge-substrate hybrid modes using the exact Wiener-Hopf analytical approach. We analyze dispersions, decay rate, and confinement of such hybrid modes both without magnetic field (plasmon-phonon and phonon-phonon hybrid modes) and in quantizing magnetic field where edge magnetoplasmons hybridize with substrate phonons. The hybrid modes are predicted to occur in THz and far-IR ranges for several combinations of quasi-2D materials (single- and bilayer graphene, quantum wells, thin-film semiconductors) and substrates with polar-phonon resonances.

physics.optics↗

arXiv2024

Bose-Einstein condensation in canonical ensemble with fixed total momentum

We consider Bose-Einstein condensation of noninteracting homogeneous three-dimensional gas in canonical ensemble when both particle number $N$ and total momentum $\mathbf{P}$ of all particles are fixed. Using the saddle point method, we derive the large-$N$ analytical approximations for partition function, free energy, and statistical distributions of occupation numbers of different single-particle energy levels. At temperatures below the critical point of phase transition, we predict, in some ranges of $\mathbf{P}$, fragmentation of the condensate, when more than one single-particle level is macroscopically occupied. The occupation number distributions have approximately Gaussian shapes for the levels hosting the condensate, and exponential shapes for other, noncondensate levels. Our analysis demonstrates breaking of Galilean invariance of moving finite-temperature many-particle system in the presence of Bose-Einstein condensation and extends the theory of moving and rotating quantum systems to the finite-temperature large-$N$ limit.

cond-mat.quant-gas↗

arXiv2024

Finite-frequency normal and superfluid drag effects in two-component atomic Bose-Einstein condensates

Two-component systems consisting of mutually interacting particles can demonstrate both intracomponent transport effects and intercomponent entrainment (or drag) effects. In the presence of superfluidity, the intracomponent transport is characterized by dissipative conductivity and superfluid weight in the framework of two-fluid model, and intercomponent entrainment gives rise to normal and nondissipative drag effects. We present unified treatment of all these effects for spatially homogeneous two-component atomic Bose-Einstein condensates based on the Bogoliubov theory, focusing specifically on the drag effects. Calculating finite-frequency intra- and intercomponent conductivities with taking into account quasiparticle damping, we derive and numerically check analytical Drude-like approximations applicable at low frequencies, and Lorentz-like approximations applicable at higher frequencies in vicinity of the resonant energy of spin-to-density Bogoliubov quasiparticle conversion. As possible physical realizations of two-component atomic systems, we consider three-dimensional Bose-Bose mixtures and closely spaced two-layered systems of magnetic dipolar atoms.

cond-mat.quant-gas↗

arXiv2024

Drift velocity of edge magnetoplasmons due to magnetic edge channels

Edge magnetoplasmons arise on a boundary of conducting layer in perpendicular magnetic field due to an interplay of electron cyclotron motion and Coulomb repulsion. Lateral electric field, which confines electrons inside the sample, drives their spiraling motion in magnetic field along the edge with the average drift velocity contributing to the total magnetoplasmon velocity. We revisit this classical picture by developing fully quantum theory of drift velocity starting from analysis of magnetic edge channels and their electrodynamic response. We derive the quantum-mechanical expression for the drift velocity, which arises in our theory as a characteristic of such response and can be calculated as harmonic mean of group velocities of edge channels crossing the Fermi level. Using the Wiener-Hopf method to solve analytically the edge mode electrodynamic problem, we demonstrate that the edge channel response effectively enhances the bulk Hall response of the conducting layer and thus increases the edge magnetoplasmon velocity. In the long-wavelength limit of our model, the drift velocity is simply added to the total magnetoplasmon velocity, in agreement with the classical picture.

cond-mat.mes-hall↗

arXiv2023

Coupled system of electrons and exciton-polaritons: Screening, dynamical effects, and superconductivity

Bose-Fermi systems such as mixtures of electrons with excitons or exciton-polaritons are extensively discussed as candidates to host a variety of intriguing phenomena, including polaron formation, drag effects, supersolidity, and superconductivity. In this work, assuming the strong-coupling regime between the semiconductor excitons and cavity photons, we develop the many-body theory approach addressing the interplay of different types of interaction among various species in such a mixture, wherein we take into account dynamical density responses of both the Bose-condensed exciton-polaritons and the two-dimensional electron gas inside an optical microcavity. As was anticipated previously, at high enough polariton densities the lower hybrid mode of the system's excitation spectrum acquires a roton minimum, making the system prone to superconducting pairing in the vicinity of the roton instability. We analyze the possibility of polariton-BEC-mediated superconductivity in the electron gas taking into account full momentum and frequency dependence of the gap, as well as in the Eliashberg approach where the momentum dependence is neglected, and in the Bardeen-Cooper-Schrieffer approach that discards the frequency dependence and dynamical effects. Considering the interaction screening in Thomas-Fermi and in random-phase approximations, we estimate the critical temperatures of superconductivity to be not larger than 0.1 K in the vicinity of instability. As possible realizations of the coupled polariton-electron system, semiconductor quantum wells and two-dimensional transition metal dichalcogenides are considered.

cond-mat.mes-hall↗

arXiv2023

Superconductor-polariton non-dissipative drag in optical microcavity

We consider non-dissipative drag between Bose-condensed exciton polaritons in optical microcavity and embedded superconductors. This effect consists in induction of a non-dissipative electric current in the superconductor by motion of polariton Bose condensate due to electron-polariton interaction, or vice versa. Using many-body theory, we calculate the drag density, characterizing magnitude of this effect, with taking into account dynamical screening of the interaction. Hoping to diminish the interaction screening and microcavity photon absorption, we consider atomically-thin superconductors (both conventional s-wave and copper-oxide d-wave) of planar and nanoribbon shapes. Our estimates show that in realistic conditions the drag effect could be rather weak but observable in accurate experiments in the case of dipolar interlayer excitons in transition metal dichalcogenide bilayers. Use of spatially direct excitons, semiconductor quantum wells as the host for excitons, or thin films of bulk metallic superconductors considerably lowers the drag density.

cond-mat.mes-hall↗

arXiv2022

Superfluid drag between excitonic polaritons and superconducting electron gas

The Andreev-Bashkin effect, or superfluid drag, is predicted in a system of Bose-condensed excitonic polaritons in optical microcavity coupled by electron-exciton interaction with a superconducting layer. Two possible setups with spatially indirect dipole excitons or direct excitons are considered. The drag density characterizing a magnitude of this effect is found by many-body calculations with taking into account dynamical screening of electron-exciton interaction. For the superconducting electronic layer, we assume the recently proposed polaritonic mechanism of Cooper pairing, although the preexisting thin-film superconductor should also demonstrate the effect. According to our calculations, the drag density can reach considerable values in realistic conditions, with excitonic and electronic layers made from GaAs-based quantum wells or two-dimensional transition metal dichalcogenides. The predicted nondissipative drag could be strong enough to be observable as induction of a supercurrent in the electronic layer by a flow of polariton Bose condensate.

cond-mat.mes-hall↗

arXiv2019

Virial theorem, boundary conditions, and pressure for massless Dirac electrons

The virial and the Hellmann--Feynman theorems for massless Dirac electrons in a solid are derived and analyzed using generalized continuity equations and scaling transformations. Boundary conditions imposed on the wave function in a finite sample are shown to break the Hermiticity of the Hamiltonian resulting in additional terms in the theorems in the forms of boundary integrals. The thermodynamic pressure of the electron gas is shown to be composed of the kinetic pressure, which is related to the boundary integral in the virial theorem and arises due to electron reflections from the boundary, and the anomalous pressure, which is specific for electrons in solids. Connections between the kinetic pressure and the properties of the wave function on the boundary are drawn. The general theorems are illustrated by examples of uniform electron gas, and electrons in rectangular and circular graphene samples. The analogous consideration for ordinary massive electrons is presented for comparison.

cond-mat.mes-hall↗

arXiv2019

Edge magnetoplasmons in graphene: Effects of gate screening and dissipation

Magnetoplasmons on graphene edge in quantizing magnetic field are investigated at different Landau level filling factors. To find the mode frequency, the optical conductivity tensor of disordered graphene in magnetic field is calculated in the self-consistent Born approximation, and the nonlocal electromagnetic problem is solved using the Wiener-Hopf method. Magnetoplasmon dispersion relations, velocities and attenuation lengths are studied numerically and analytically with taking into account the screening by metallic gate and the energy dissipation in graphene. The magnetoplasmon velocity decreases in the presence of nearby gate and oscillates as a function of the filling factor because of the dissipation induced frequency suppression occurring when the Fermi level is located near the centers of Landau levels, in agreement with the recent experiments.

cond-mat.mes-hall↗

arXiv2019

Many-body filling-factor dependent renormalization of Fermi velocity in graphene in strong magnetic field

We present the theory of many-body corrections to cyclotron transition energies in graphene in strong magnetic field due to Coulomb interaction, considered in terms of the renormalized Fermi velocity. A particular emphasis is made on the recent experiments where detailed dependencies of this velocity on the Landau level filling factor for individual transitions were measured. Taking into account the many-body exchange, excitonic corrections and interaction screening in the static random-phase approximation, we successfully explained the main features of the experimental data, in particular that the Fermi velocities have plateaus when the 0th Landau level is partially filled and rapidly decrease at higher carrier densities due to enhancement of the screening. We also explained the features of the nonmonotonous filling-factor dependence of the Fermi velocity observed in the earlier cyclotron resonance experiment with disordered graphene by taking into account the disorder-induced Landau level broadening.

cond-mat.mes-hall↗

arXiv2018

Many-body renormalization of Landau levels in graphene due to screened Coulomb interaction

Renormalization of Landau level energies in graphene in strong magnetic field due to Coulomb interaction is studied theoretically, and calculations are compared with two experiments on carrier-density dependent scanning tunneling spectroscopy. An approximate preservation of the square-root dependence of the energies of Landau levels on their numbers and magnetic field in the presence of the interaction is examined. Many-body calculations of the renormalized Fermi velocity with the statically screened interaction taken in the random-phase approximation show good agreement with both experiments. The crucial role of the screening in achieving quantitative agreement is found. The main contribution to the observed rapid logarithmic growth of the renormalized Fermi velocity on approach to the charge neutrality point turned out to be caused not by mere exchange interaction effects, but by weakening of the screening at decreasing carrier density. The importance of a self-consistent treatment of the screening is also demonstrated.

cond-mat.mes-hall↗