arXiv subjects

M. A. Semina

Publications and source records attributed to M. A. Semina.

At least 19 recordsLinked to original sources

arXiv2026

Exciton fine structure in nanocrystals: effect of cuboidal and spheroidal shapes

We present the theory of the band-edge exciton fine structure in nanocrystals (NCs) with spheroidal and cuboidal shapes. The effects of the cubic symmetry of the crystal lattice, the cubic shape of the NC, and NC uniaxial anisotropy on the hole energy states and electron-hole exchange interactions are considered non-perturbatively. Symmetry analysis yields an effective Hamiltonian for the exciton fine structure, parameterized by one constant for hole energy splitting and five independent constants for exchange interaction. Numerical calculations reveal that in uniaxially anisotropic zinc-blende NCs, the sign of the hole ground state splitting depends on the material parameters and on the orientation of the anisotropy axis relative to the crystallographic axes. Beyond the conventional bulk cubically-symmetric contribution to the exchange interaction, which originates from Bloch-function symmetry and is typically negligible, in nanocrystals, we identify the contribution arising from the cubic symmetry of the envelope wavefunction. This cubically symmetric envelope-induced short-range exchange is non-negligible in cuboidal NCs and induces a pronounced splitting of the dark exciton states. We further analyze the influence of uniaxial anisotropy of the exchange constants on both the exciton fine structure and the oscillator strength. Special attention is paid to NCs, where the anisotropy of the exchange constants is comparable to a relatively small hole energy anisotropic splitting.

cond-mat.mes-hall↗

arXiv2026

Non-Newtonian two-dimensional electron fluid in magnetic field

We develop a theory of the non-Newtonian regime of non-linear hydrodynamic magnetotransport of a two-dimensional (2D) viscous electron fluid controlled by the local Joule heating. In this mechanism, the electron shear viscosity in magnetic field becomes a non-monotonic function of the gradient of the hydrodynamic velocity due to the flow-induced increase of the electron temperature. We derive and solve the corresponding non-linear hydrodynamic equations for a velocity profile in a Poiseuille-like flow geometry. We demonstrate that the calculated magnetoresistance of such flow well explains the differential magnetoresistance observed for 2D electrons in various samples of ultra-high-quality GaAs quantum wells at high currents. We conclude that the non-linear regime of 2D electron fluid flows is such systems is realized via formation of a 2D non-Newtonian electron fluid, related to non-linearity in viscosity, but not via convective ``kinematic-induced'' effects, as it takes place in ordinary uncharged fluids.

cond-mat.mes-hall↗

arXiv2025

Electron and hole $g$ factors in semiconductors and nanostructures (Review)

We present a review of experimental and theoretical studies of the spin response of charge carriers to an external magnetic field in bulk semiconductors and semiconductor nanostructures. The linear response is quantitatively characterized by the magnitude of the electron or hole g factor. Various experimental methods for measuring the electron g factor are considered, beginning with historical works and including modern research. A detailed analysis of theoretical methods for calculating the electron and hole g factors in bulk semiconductors and nanostructures of various shapes also includes fundamental work from previous years and the present time.

cond-mat.mtrl-sci↗

arXiv2025

Analytical model for non-linear magnetotransport in viscous electron fluid

We develop an analytical theoretical model for non-linear hydrodynamic magnetotransport of two-dimensional (2D) electron fluid with strong pair correlations in the electron dynamics. Within classical kinetics of 2D electrons, such correlations are described as subsequent ``extended'' collisions of the same electrons, temporarily joined in pairs. Corresponding correlation-induced retarded terms in the fluid dynamic equations can be described for slow flows as the dependence of the electron fluid viscosity on the flow velocity gradient, that is Non-Newtonian behavior of the fluid. We analytically calculate flow profiles in long samples in a stationary highly non-linear regime and the corresponding magnetoresistance. Pair correlations lead to a characteristic non-monotonic dependence of the differential resistance on magnetic field. We compare our results with experimental data on non-linear magnetotransport of high-purity GaAs quantum wells; we conclude that our model can be responsible for a part of the observed features of the differential magnetoresistance.

cond-mat.mes-hall↗

arXiv2025

Nonreciprocal magnetic-field-induced second harmonic generation of exciton polaritons in ZnSe

We report on the optical second harmonic generation (SHG) on the 1S exciton-polariton resonance in bulk ZnSe that is subject to an external magnetic field applied perpendicular to the light wave vector $\mathbf k$ (Voigt geometry). For the symmetry allowed geometry with the $\mathbf{k}\parallel[111]$ crystal axes, the nonreciprocal dependence of the SHG intensity on the magnetic field direction is found. It is explained by an interference of the crystallographic and magnetic-field-induced SHG signals. Relative phases of these signals are evaluated from the rotational anisotropy diagrams. Phenomenological and microscopic models of the effect are developed. To the best of our knowledge, this is the first experimental observation of the nonreciprocal SHG in semiconductor crystals, and the first one for exciton-polaritons.

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arXiv2025

Excitons and trions in CrSBr bilayers

We study theoretically the neutral and charged excitons in two-dimensional semiconductors with anisotropic dispersion of charge carriers. Such a situation is realized in CrSBr-based van der Waals heterostructures. We calculate the binding energies of excitons and trions and explore their dependence on the mass ratio, dielectric screening, and interlayer distance in bilayer structures. We also address the effects of exciton-light coupling, including the radiative decay and long-range electron-hole exchange interaction, and briefly analyze correlations between the excitons and the Fermi sea of resident electrons. The estimates for CrSBr bilayers are in reasonable agreement with recent experiments.

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arXiv2023

Shear Bernstein modes in a two-dimensional electron liquid

Bernstein modes are formed as a result of non-local coupling of collective excitations and cyclotron harmonics in magnetized plasma. In degenerate solid state plasma they are typically associated with magnetoplasmons. A different type of Bernstein modes arises in two-dimensional electron liquid at sufficiently strong quasiparticle interaction. We consider Bernstein modes originating from coupling between quasiparticle cyclotron harmonics and shear magnetosound waves. The latter may be responsible for the giant peak in radio-frequency photoresistance observed in high-quality GaAs quantum wells. Using Landau-Silin kinetic equation with an arbitrary strength of the interparticle Landau interaction, we trace the reconstruction of Bernstein mode spectrum in high-quality 2D electron systems across the crossover between weakly interacting degenerate electron gas and the correlated electron liquid. Sensitivity of Bernstein modes to the strength of quasiparticle interaction allows one to use them for spectroscopy of Landau interaction function in the electron Fermi liquids.

cond-mat.mes-hall↗

arXiv2023

Cubic anisotropy of hole Zeeman splitting in semiconductor nanocrystals

We study theoretically cubic anisotropy of Zeeman splitting of a hole localized in semiconductor nanocrystal. This anisotropy originates from three contributions: crystallographic cubically-symmetric spin and kinetic energy terms in the bulk Luttinger Hamiltonian and the spatial wave function distribution in a cube-shaped nanocrystal. From symmetry considerations, an effective Zeeman Hamiltonian for the hole lowest even state is introduced, containing a spherically symmetric and a cubically symmetric term. The values of these terms are calculated numerically for spherical and cube-shaped nanocrystals as functions of the Luttinger Hamiltonian parameters. We demonstrate that the cubic shape of the nanocrystal and the cubic anisotropy of hole kinetic energy (so called valence band warping) significantly affect effective $g$ factors of hole states. In both cases, the effect comes from the cubic symmetry of the hole wave functions in zero magnetic field. Estimations for the effective $g$ factor values in several semiconductors with zinc-blende crystal lattices are made. Possible experimental manifestations and potential methods of measurement of the cubic anisotropy of the hole Zeeman splitting are suggested.

cond-mat.mes-hall↗

arXiv2023

Tailoring the electron and hole Landé factors in lead halide perovskite nanocrystals by quantum confinement and halide exchange

The tunability of the optical properties of lead halide perovskite nanocrystals makes them highly appealing for applications. Both, halide anion exchange and quantum confinement pave the way for tailoring their band gap energy. For spintronics applications, the Landé g-factors of electrons and hole are of great importance. By means of the empirical tight-binding and $\textbf{k}\cdot\textbf{p}$ methods, we calculate them for nanocrystals of the class of all-inorganic lead halide perovskites CsPb$X_3$ ($X = \text{I},\,\text{Br},\,\text{Cl}$). The hole g-factor as function of the band gap follows the universal dependence found for bulk perovskites, while for the electrons a considerable modification is predicted. Based on the $\textbf{k}\cdot\textbf{p}$ analysis we conclude that this difference arises from the interaction of the bottom conduction band with the spin-orbit split electron states. The model predictions are confirmed by experimental data for the electron and hole g-factors in CsPbI3 nanocrystals placed in a glass matrix, measured by time-resolved Faraday ellipticity in a magnetic field at cryogenic temperatures.

cond-mat.mes-hall↗

arXiv2023

Control of the Bright-Dark Exciton Splitting using Lamb Shift in a 2D Semiconductor

We have investigated the exciton fine structure in atomically thin WSe2 -based van der Waals heterostructures where the density of optical modes at the location of the semiconductor monolayer can be tuned. The energy splitting $Δ$ between the bright and dark exciton has been measured by photoluminescence spectroscopy. We demonstrate that $Δ$ can be tuned by a few meV, as a result of a significant Lamb shift of the optically active exciton which arises from emission and absorption of virtual photons triggered by the vacuum fluctuations of the electromagnetic field. We also measured strong variations of the bright exciton radiative linewidth, as a result of the Purcell effect. All these experimental results illustrate the strong sensitivity of the excitons to local vacuum field. We found a very good agreement with a model that demonstrates the equivalence, for our system, of a classical electrodynamical transfer matrix formalism and quantum-electrodynamical approach. The bright-dark splitting control demonstrated here should apply to any semiconductor structures.

cond-mat.mes-hall↗

arXiv2023

Excitons and trions with negative effective masses in two-dimensional semiconductors

We study theoretically fundamental Coulomb-correlated complexes: neutral and charged excitons, also known as trions, in transition metal dichalogenides monolayers. We focus on the situation where one of the electrons occupies excited, high-lying, conduction band characterized by a negative effective mass. We develop the theory of such high-lying excitons and trions with negative effective mass and demonstrate the key role of the non-parabolicity of the high-lying conduction band dispersion in formation of the bound exciton and trion states. We present simple, accurate and physically justified trial wavefunctions for calculating the binding energies of Coulomb-bound complexes and compare the results of variational calculations with those of a fully numerical approach. Within the developed model we discuss recent experimental results on observation of high-lying negative effective mass trions [K.-Q. Lin et al., Nat. Commun. 13, 6980 (2022)].

cond-mat.mes-hall↗

arXiv2022

Hall effect in Poiseuille flow of two-dimensional electron fluid

The hydrodynamic regime of charge transport has been recently realized in high-quality conductors. In the hydrodynamic as well as in the Ohmic regimes the main part of the Hall resistance of a long sample is determined by the balance between the Lorentz force and the electric force, acting on conduction electrons. Experimentally observed deviations of the Hall resistance in hydrodynamic samples from such the ''standard'' value are usually associated with the Hall viscosity term in the Navier-Stokes equation. In this work we theoretically study the Hall effect in a Poiseuille flow of a two-dimensional electron fluid. We show that the near-edge semiballistic layers with the width of the order of the inter-particle mean free path, which inevitably appear near sample edges, give the contribution to the Hall resistance which is comparable with the bulk contribution from the Hall viscosity. In this way, the measured deviations of the Hall resistance from the ''standard'' one in hydrodynamic samples by the usual contact techniques should be associated with both the Hall viscosity in the bulk and the semiballistic effects in the near-edge layers

cond-mat.mes-hall↗

arXiv2022

Rotational viscosity in spin resonance of hydrodynamic electrons

In novel ultra-pure materials electrons can form a viscous fluid, which is fundamentally different by its dynamics from the electron gas in ordinary conductors with significant density of defects. The shape of the non-stationary flow of such electron fluid is similar to the alternating flow of blood in large-radius arteries [J. R. Womersley, J. Physiol. 127, 552 (1955)]. The rotational viscosity effect is responsible for interconnection between the dynamics of electron spins and flow inhomogeneities. In particular, it induces the spin polarization of electrons in a curled flow via an internal spin-orbit torque acting on electron spins. Here we show that this effect in an electron fluid placed in a magnetic field leads to a correction to the ac sample impedance, which has a resonance at the Larmor frequency of electrons. In this way, via the electrically detected spin resonance the Womersley flow of an electron fluid can be visualized and the rotational viscosity can be measured.

cond-mat.mes-hall↗

arXiv2022

Valley polarization fluctuations, bistability, and switching in two-dimensional semiconductors

We study theoretically nonlinear valley polarization dynamics of excitons in atom-thin semiconductors. The presence of significant polarization slows down valley relaxation due to an effective magnetic field resulting from exciton-exciton interactions. We address temporal dynamics of valley polarized excitons and study the steady states of the polarized exciton gas. We demonstrate bistability of the valley polarization where two steady states with low and high valley polarization are formed. We study the effects of fluctuations and noise in such system. We evaluate valley polarization autocorrelation functions and demonstrate that for a high-polarization regime the fluctuations are characterized by high amplitude and long relaxation time. We study the switching between the low- and high-valley polarized states caused by the noise in the system and demonstrate that the state with high valley polarization is preferential in a wide range of pumping rates.

cond-mat.mes-hall↗

arXiv2022

Flexural deformations and collapse of bilayer two-dimensional crystals by interlayer exciton

We develop a consistent theory of the interlayer exciton-polaron formed in atomically-thin bilayers. Coulomb attraction between an electron and a hole situated in the different layers results in their flexural deformation and provides an efficient mechanism of the exciton coupling with flexural phonons. We study the effect of layers tension on the polaron binding energy and effective mass leading to suppression of polaron formation by the tension both in the weak and strong coupling regimes. We also consider the role of the nonlinearity related to the interaction between the out- and in-plane lattice displacements and obtain the criterion of the layer sticking, where the exciton collapses, due to the Coulomb attraction between the charge carriers.

cond-mat.mes-hall↗

arXiv2021

The Landé factors of electrons and holes in lead halide perovskites: universal dependence on the band gap

The Landé or $g$-factors of charge carriers are decisive for the spin-dependent phenomena in solids and provide also information about the underlying electronic band structure. We present a comprehensive set of experimental data for values and anisotropies of the electron and hole Landé factors in hybrid organic-inorganic (MAPbI$_3$, MAPb(Br$_{0.5}$Cl$_{0.5}$)$_3$, MAPb(Br$_{0.05}$Cl$_{0.95}$)$_3$, FAPbBr$_3$, FA$_{0.9}$Cs$_{0.1}$PbI$_{2.8}$Br$_{0.2}$) and all-inorganic (CsPbBr$_3$) lead halide perovskites, determined by pump-probe Kerr rotation and spin-flip Raman scattering in magnetic fields up to 10~T at cryogenic temperatures. Further, we use first-principles DFT calculations in combination with tight-binding and $\mathbf k \cdot \mathbf p$ approaches to calculate microscopically the Landé factors. The results demonstrate their universal dependence on the band gap energy across the different perovskite material classes, which can be summarized in a universal semi-phenomenological expression, in good agreement with experiment.

cond-mat.mtrl-sci↗

arXiv2021

Ballistic flow of two-dimensional electrons in a magnetic field

In conductors with a very small density of defects, electrons at low temperatures collide predominantly with the edges of a sample. Therefore, the ballistic regime of charge and heat transport is realized. The application of a perpendicular magnetic field substantially modifies the character of ballistic transport. For the case of two-dimensional (2D) electrons in the magnetic fields corresponding to the diameter of the cyclotron trajectories smaller than the sample width a hydrodynamic transport regime is formed. In the latter regime, the flow is mainly controlled by rare electron-electron collisions, which determine the viscosity effect. In this work, we study the ballistic flow of 2D electrons in long samples in magnetic fields up to the critical field of the transition to the hydrodynamic regime. From the solution of the kinetic equation, we obtain analytical formulas for the profiles of the current density and the Hall electric field far and near the ballistic-hydrodynamic transition as well as for the longitudinal and the Hall resistances in these ranges. Our theoretical results, apparently, describe the observed longitudinal resistance of pure graphene samples in the diapason of magnetic fields below the ballistic-hydrodynamic transition.

cond-mat.mes-hall↗

arXiv2021

Ballistic-hydrodynamic phase transition in flow of two-dimensional electrons

Phase transitions are characterized by a sharp change in the type of dynamics of microparticles, and their description usually requires quantum mechanics. Recently, a peculiar type of conductors was discovered in which two-dimensional (2D) electrons form a viscous fluid. In this work we reveal that such electron fluid in high-quality samples can be formed from ballistic electrons via a phase transition. For this purpose, we theoretically study the evolution of a ballistic flow of 2D weakly interacting electrons with an increase of magnetic field and trace an emergence of a fluid fraction at a certain critical field. Such restructuring of the flow manifests itself in a kink in magnetic-field dependencies of the longitudinal and the Hall resistances. It is remarkable that the studied phase transition has a classical-mechanical origin and is determined by both the ballistic size effects and the electron-electron scattering. Our analysis shows that this effect was apparently observed in the recent transport experiments on 2D electrons in graphene and high-mobility GaAs quantum wells.

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