arXiv subjects

A. A. Sokolik

Publications and source records attributed to A. A. Sokolik.

15 recordsLinked to original sources

arXiv2025

Terahertz electro-optic effect in Bi$_2$Se$_3$ crystals

We report the observation of the electro-optic effect in Bi$_2$Se$_3$ crystals induced by an intense single-cycle terahertz pulse. The effect reveals itself as a transient change of the polarization state of a femtosecond laser pulse reflected from the crystal that is exposed to the terahertz electric field. The corresponding experimental signal follows the field with a sub-100-fs delay and can be represented as a linear combination of the terahertz electric field and its square. The linear and quadratic components are of comparable magnitude. The latter is almost independent of crystal orientation, while the former demonstrates three-fold rotational symmetry in agreement with the trigonal symmetry of the crystal surface. The electro-optic effect vanishes upon phase transition to the non-topological metal state induced by indium doping and also can be quenched by a femtosecond pre-pulse. We associate this effect with surface Dirac electronic states of Bi$_2$Se$_3$ and discuss its possible mechanisms.

cond-mat.mtrl-sci↗

arXiv2021

Plasmonic modes at inclined edges of anisotropic 2D materials

Confined modes at the edge arbitrarily inclined with respect to optical axes of nonmagnetic anisotropic 2D materials are considered. By developing the exact Wiener-Hopf and approximated Fetter methods we studied edge modes dispersions, field and charge density distributions. The 2D layer is described by the Lorentz-type conductivities in one or both directions, which is realistic for natural anisotropic 2D materials and resonant hyperbolic metasurfaces. We demonstrate that, due to anisotropy, the edge mode exists only at wave vectors exceeding the nonzero threshold value if the edge is tilted with respect to the direction of the resonant conductivity. The dominating contribution to field and charge density spatial profiles is provided by evanescent 2D waves, which are confined both in space near the 2D layer and along the layer near its edge. The degree of field confinement along the layer is determined by wave vector or frequency mismatch between the edge mode and continuum of freely propagating 2D modes. Our analysis is suitable for various types of polaritons (plasmon-, phonon-, exciton-polaritons etc.) at large enough wave vectors. Thanks to superior field confinement in all directions perpendicular to the edge these modes look promising for modern plasmonics and sensorics.

cond-mat.mes-hall↗

arXiv2019

Anisotropic ultrafast optical response of terahertz pumped graphene

We have measured the ultrafast anisotropic optical response of highly doped graphene to an intense single cycle terahertz pulse. The time profile of the terahertz-induced anisotropy signal at 800 nm has minima and maxima repeating those of the pump terahertz electric field modulus. It grows with increasing carrier density and demonstrates a specific nonlinear dependence on the electric field strength. To describe the signal, we have developed a theoretical model that is based on the energy and momentum balance equations and takes into account optical phonons of graphene and substrate. According to the theory, the anisotropic response is caused by the displacement of the electronic momentum distribution from zero momentum induced by the pump electric field in combination with polarization dependence of the matrix elements of interband optical transitions.

cond-mat.mes-hall↗

arXiv2017

Many-body effects of Coulomb interaction on Landau levels in graphene

In strong magnetic fields, massless electrons in graphene populate relativistic Landau levels with the square-root dependence of each level energy on its number and magnetic field. Interaction-induced deviations from this single-particle picture were observed in recent experiments on cyclotron resonance and magneto-Raman scattering. Previous attempts to calculate such deviations theoretically using the unscreened Coulomb interaction resulted in overestimated many-body effects. This work presents many-body calculations of cyclotron and magneto-Raman transitions in single-layer graphene in the presence of Coulomb interaction, which is statically screened in the random-phase approximation. We take into account self-energy and excitonic effects as well as Landau level mixing, and achieve good agreement of our results with the experimental data for graphene on different substrates. Important role of a self-consistent treatment of the screening is found.

cond-mat.mes-hall↗

arXiv2015

Generalized virial theorem for massless electrons in graphene and other Dirac materials

The virial theorem for a system of interacting electrons in a crystal, which is described within the framework of the tight-binding model, is derived. We show that, in particular case of interacting massless electrons in graphene and other Dirac materials, the conventional virial theorem is violated. Starting from the tight-binding model, we derive the generalized virial theorem for Dirac electron systems, which contains an additional term associated with a momentum cutoff at the bottom of the energy band. Additionally, we derive the generalized virial theorem within the Dirac model using the minimization of the variational energy. The obtained theorem is illustrated by many-body calculations of the ground state energy of an electron gas in graphene carried out in Hartree-Fock and self-consistent random-phase approximations. Experimental verification of the theorem in the case of graphene is discussed.

cond-mat.mes-hall↗

arXiv2015

Quantum capacitance and compressibility of graphene: The role of Coulomb interactions

Many-body effects on quantum capacitance, compressibility, renormalized Fermi velocity, kinetic and interaction energies of massless Dirac electrons in graphene, induced by the Coulomb interactions, are analyzed theoretically in the first-order, Hartree-Fock and random phase approximations. Recent experimental data on quantum capacitance and renormalized Fermi velocity are analyzed and compared with the theory. The bare Fermi velocity and the effective dielectric constants are obtained from the experimental data. A combined effect of Coulomb interactions and Gaussian fluctuations of disorder potential is considered.

cond-mat.mes-hall↗

arXiv2012

Electron-hole pairing in topological insulator thin film

We consider pairing of massless Dirac electrons and holes located on opposite surfaces of thin film of "strong" three-dimensional topological insulator. Such pairing was predicted to give rise to topological exciton condensate with unusual properties. We estimate quantitatively achievable critical temperature of the pairing with taking into account self-consistent screening of the Coulomb interaction, disorder and hybridization of electron and hole states caused by a tunneling through the film. Increase of the gap above the hybridization value when the temperature is lowered can be observable signature of the pairing. System parameters required to observe the electron-hole pairing are discussed.

cond-mat.mes-hall↗

arXiv2012

Cooper pairing of electrons and holes in graphene bilayer: Correlation effects

Cooper pairing of spatially separated electrons and holes in graphene bilayer is studied beyond the mean-field approximation. Suppression of the screening at large distances, caused by appearance of the gap, is considered self-consistently. A mutual positive feedback between appearance of the gap and enlargement of the interaction leads to a sharp transition to correlated state with greatly increased gap above some critical value of the coupling strength. At coupling strength below the critical, this correlation effect increases the gap approximately by a factor of two. The maximal coupling strength achievable in experiments is close to the critical value. This indicated importance of correlation effects in closely-spaced graphene bilayers at weak substrate dielectric screening. Another effect beyond mean-field approximation considered is an influence of vertex corrections on the pairing, which is shown to be very weak.

cond-mat.mes-hall↗

arXiv2012

Influence of Landau level mixing on the properties of elementary excitations in graphene in strong magnetic field

Massless Dirac electrons in graphene fill Landau levels with energies scaled as square roots of their numbers. Coulomb interaction between electrons leads to mixing of different Landau levels. The relative strength of this interaction depends only on dielectric susceptibility of surrounding medium and can be large in suspended graphene. We consider influence of Landau level mixing on the properties of magnetoexcitons and magnetoplasmons - elementary electron-hole excitations in graphene in quantizing magnetic field. We show that, at small enough background dielectric screening, the mixing leads to very essential change of magnetoexciton and magnetoplasmon dispersion laws in comparison with the lowest Landau level approximation.

cond-mat.mes-hall↗

arXiv2011

Collective excitations on a surface of topological insulator

We study collective excitations in a helical electron liquid on a surface of three-dimensional topological insulator. Electron in helical liquid obeys Dirac-like equation for massless particless and direction of its spin is strictly determined by its momentum. Due to this spin-momentum locking, collective excitations in the system manifest themselves as coupled charge- and spin-density waves. We develop quantum field-theoretical description of spin-plasmons in helical liquid and study their properties and internal structure. Value of spin polarization arising in the system with excited spin-plasmons is calculated. We also consider the scattering of spin-plasmons on magnetic and nonmagnetic impurities and external potentials, and show that the scattering occurs mainly into two side lobes. Analogies with Dirac electron gas in graphene are discussed.

cond-mat.mes-hall↗

arXiv2011

Spin-plasmons in topological insulator

Collective plasmon excitations in a helical electron liquid on the surface of strong three-dimensional topological insulator are considered. The properties and internal structure of these excitations are studied. Due to spin-momentum locking in helical liquid on a surface of topological insulator, the collective excitations should manifest themselves as coupled charge- and spin-density waves.

cond-mat.mes-hall↗

arXiv2010

Phonon-mediated electron pairing in graphene

The possibility of superconducting pairing of electrons in doped graphene due to in-plane and out-of-plane phonons is studied. Quadratic coupling of electrons with out-of-plane phonons is considered in details, taking into account both deformation potential and bond-stretch contributions. The order parameter of electron-electron pairing can have different structures due to four-component spinor character of electrons wave function. We consider s-wave pairing, diagonal on conduction and valence bands, but having arbitrary structure with respect to valley degree of freedom. The sign and magnitude of contribution of each phonon mode to effective electron-electron interaction turns out to depend on both the symmetry of phonon mode and the structure of the order parameter. Unconventional orbital-spin symmetry of the order parameter is found.

cond-mat.mes-hall↗

arXiv2010

Electron-electron and electron-hole pairing in graphene structures

The superconducting pairing of electrons in doped graphene due to in-plane and out-of-plane phonons is considered. It is shown that the structure of the order parameter in the valley space substantially affects conditions of the pairing. Electron-hole pairing in graphene bilayer in the strong coupling regime is also considered. Taking into account retardation of the screened Coulomb pairing potential shows a significant competition between the electron-hole direct attraction and their repulsion due to virtual plasmons and single-particle excitations.

cond-mat.mes-hall↗

arXiv2009

Ultrarelativistic electron-hole pairing in graphene bilayer

We consider ground state of electron-hole graphene bilayer composed of two independently doped graphene layers when a condensate of spatially separated electron-hole pairs is formed. In the weak coupling regime the pairing affects only conduction band of electron-doped layer and valence band of hole-doped layer, thus the ground state is similar to ordinary BCS condensate. At strong coupling, an ultrarelativistic character of electron dynamics reveals and the bands which are remote from Fermi surfaces (valence band of electron-doped layer and conduction band of hole-doped layer) are also affected by the pairing. The analysis of instability of unpaired state shows that s-wave pairing with band-diagonal condensate structure, described by two gaps, is preferable. A relative phase of the gaps is fixed, however at weak coupling this fixation diminishes allowing gapped and soliton-like excitations. The coupled self-consistent gap equations for these two gaps are solved at zero temperature in the constant-gap approximation and in the approximation of separable potential. It is shown that, if characteristic width of the pairing region is of the order of magnitude of chemical potential, then the value of the gap in the spectrum is not much different from the BCS estimation. However, if the pairing region is wider, then the gap value can be much larger and depends exponentially on its energy width.

cond-mat.mes-hall↗

arXiv2008

Electron-hole pair condensation in graphene bilayer

We consider the pairing of electrons and holes due to their Coulomb attraction in two parallel, independently gated graphene layers, separated by a barrier. At weak coupling, there exist the BCS-like pair-condensed state. Despite the fact that electrons and holes behave like massless Dirac fermions, the problem of BCS-like electron-hole pairing in graphene bilayer turns out to be rather similar to that in usual coupled semiconductor quantum wells. The distinctions are due to Berry phase of electronic wave functions and different screening properties. We estimate values of the gap in one-particle excitation spectrum for different interlayer distances and carrier concentrations. Influence of disorder is discussed. At large enough dielectric susceptibility of surrounding medium, the weak coupling regime holds even at arbitrarily small carrier concentrations. Localized electron-hole pairs are absent in graphene, thus the behavior of the system versus coupling strength is cardinally different from usual BCS-BEC crossover.

cond-mat.str-el↗