arXiv subjects

M. M. Mahmoodian

Publications and source records attributed to M. M. Mahmoodian.

At least 19 recordsLinked to original sources

arXiv2023

Orbital momentum excitation by interband optical transitions in a 2D system illuminated by twisted light

Illumination of a two-dimensional system by a twisted light beam is considered in order to find specific effects caused by twisting. Direct interband transitions between the valence and conduction bands are supposed. The generation rates of the electron orbital momentum is found. A kinetic equation for an orbital momentum distribution function is formulated and solved. The mean electron orbital momentum is found.

cond-mat.mes-hall↗

arXiv2022

Spin structure and spin magnetic susceptibility of two-dimensional Wigner clusters

Spin states of two-dimensional Wigner clusters are considered at low temperatures, when all electrons are in ground coordinate states. The spin subsystem behavior is determined by antiferromagnetic exchange integrals. The spin states in such a system in the presence of a magnetic field are described in terms of the Ising model. The spin structure, correlation function, and magnetic susceptibility of the cluster are found by computer simulations. It is shown that the spin susceptibility experiences oscillations with respect to the magnetic field, owing to the magnetoinduced spin subsystem rearrangements.

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arXiv2021

Electron states on the smooth edge of 2D topological insulator: elastic backscattering and light absorption

The 2D TI edge states are considered within the Volkov-Pankratov (VP) Hamiltonian. A smooth transition between TI and OI is assumed. The edge states are formed in the total gap of homogeneous 2D material. A pair of these states are of linear dispersion, others have gapped Dirac spectra. The optical selection rules are found. The optical transitions between the neighboring edge states appear in the global 2D gap for the in-plane light electric field directed across the edge. The electrons in linear edge states have no backscattering, that is indicative of the fact of topological protection. However, when linear edge states get to the energy domain of Dirac edge states, the backscattering becomes permitted. The elastic backscattering rate is found. The Drude-like conductivity is found when the Fermi level gets into the energy domain of the coexistence of linear and Dirac edge states. The localization edge conductance of a finite sample at zero temperature is determined.

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arXiv2021

Quantum Dot in a Hybrid Structure with Dipolar Excitons

Electron states in a quantum dot (QD) located near a 2D system of dipolar excitons are perturbed by fluctuations of the exciton density caused by the electron-exciton interaction. This results in the frequency changes of electron transitions in a QD. The frequency depends on the exciton density, as well as on the exciton gas phase state. In the present work, the shifts of the two lowest QD energy levels are found both in the normal state of the exciton system and for the Bose-Einstein condensation (BEC) regime.

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arXiv2020

Conductivity of a two-dimensional HgTe layer near the critical width: The role of developed edge states network and random mixture of $p$- and $n$-domains

The conductivity of a two-dimensional HgTe quantum well with a width $\sim$6.3~nm, close to the transition from ordinary to topological insulating phases, is studied. The Fermi level is supposed to get to the overall energy gap. The consideration is based on the percolation theory. We have found that the width fluctuations convert the system to a random mixture of domains with positive and negative energy gaps with internal edge states formed near zero gap lines. In the case with no potential fluctuations, the conductance of a finite sample is provided by a random edge states network. The zero-temperature conductivity of an infinite sample is determined by the free motion of electrons along the zero-gap lines and tunneling between them. The conductance of a single $p$-$n$ junction, which is crossed by the edge state, is found. The result is applied to the situation when potential fluctuations transform the system to a mixture of $p$- and $n$-domains. It is stated that the tunneling across $p$-$n$ junctions forbids the low-temperature conductivity of a random system, but the latter is restored due to the random edge states crossing the junctions.

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arXiv2019

Scattering of Electrons between Edge and Two-Dimensional States of a Two-Dimensional Topological Insulator and the Conductivity of the Topological Insulator Strip in a Metallic State

The lifetime of electrons on edge states of a two-dimensional topological insulator against the background of an allowed two-dimensional band has been determined. It has been shown that this time in the case of scattering on Coulomb impurities can be significantly larger than the mean free time of two-dimensional electrons. As a result, the conductivity of the metallic two-dimensional topological insulator strip can be determined primarily by edge states.

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arXiv2019

Bound electron pair in a MOS-structure

In developing our previous contribution (arXiv:1804.00889) we have numerically found the bound state energy and correspondent wave function of the two electrons confined to move in a quantum well placed close to the gate electrode. Spin-orbit interaction (SOI) and image charge forces result in effective attraction between electrons. We considered also the effect of gate voltage applied to the structure and discovered that this can essentially increase the bound energy of the pair so that it remains stable even at room temperature.

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arXiv2018

Microwave absorption in a 2D topological insulators with a developed network of edge states

The 2D HgTe quantum well is analyzed based on the assumption that the width fluctuations convert the system to a random mixture of domains with positive and negative energy gaps. The borders between ordinary and topological insulator phases form a network of the edge states covering the overall sample. The optical transitions within the edge states yield the 2D absorption. The qualitative consideration is based on the model of optical intraedge transitions in curved edge states together with the percolation arguments.

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arXiv2018

Formation of the Bielectron in a 2D System due to Spin-Orbit Interaction and Image Forces

It is shown that two electrons located in a quantum well near a metal electrode attract each other due to the spin-orbit interaction (SOI) of the Bychkov-Rashba type and the electrostatic image forces. Using the example of a simple model, it is shown that, with quite attainable values of the characteristic parameters of the system, the effective attraction caused by SOI prevails over the Coulomb repulsion, and the formation of a bielectron becomes possible.

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arXiv2017

Edge absorption and pure spin current in 2D topological insulator in the Volkov-Pankratov model

The light absorption due to the transitions between the edge and two-dimensional (2D) states of a 2D topological insulator (TI) is considered in the Volkov-Pankratov model. It is shown that the transitions are allowed only for the in-plane electric field orthogonal to the edge of the TI. It is found that the absorption is accompanied by the pure spin photocurrent along the TI edge. The possibility of the spin current measurement using polarized luminescence from 2D TI quantum dots is discussed.

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arXiv2017

Linearity of the edge states energy spectrum in the 2D topological insulator

Linearity of the topological insulator edge state spectrum plays the crucial role for various transport phenomena. The previous studies found that this linearity exists near the spectrum crossing point, but did not determine how perfect the linearity is. The purpose of the present study is to answer this question in various edge states models. We examine Volkov and Pankratov (VP) model [1] for the Dirac Hamiltonian and the model of [2,3] (BHZ1) for the Bernevig, Hughes and Zhang (BHZ) Hamiltonian [4] with zero boundary conditions. It is found that both models yield ideally linear edge states. In the BHZ1 model the linearity is conserved up to the spectrum ending points corresponding to the tangency of the edge spectrum with the boundary of 2D states. In contrast, the model of[5] (BHZ2) with mixed boundary conditions for BHZ Hamiltonian and the 2D tight-binding (TB) model from [4] yield weak non-linearity.

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arXiv2016

Edge excitons in a 2D topological insulator in the magnetic field

Exciton edge states and the microwave edge exciton absorption of a 2D topological insulator subject to the in-plane magnetic field are studied. The magnetic field forms a narrow gap in electron edge states that allows the existence of edge exciton. The exciton binding energy is found to be much smaller than the energy of a 1D Coulomb state. Phototransitions exist on the exciton states with even numbers, while odd exciton states are dark.

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arXiv2013

Moving zero-gap Wannier-Mott excitons in graphene

We demonstrate the possibility of existence of indirect moving Wannier-Mott excitons in graphene. Electron-hole binding is conditioned by the trigonal warping of conic energy spectrum. The binding energies are found for the lowest exciton states. These energies essentially depend on the value and direction of exciton momentum and vanish when the exciton momentum tends to the conic points. The ways to observe the exciton states are discussed. The opportunity of experimental observation of zero-gap excitons by means of external electron scattering is examined.

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arXiv2008

High-frequency blockade and related phenomena

We study systems with local vibrating potentials, one-dimensional single and double wells and the tight-binding 1D model with single vibrating site. In general, these systems transmit, or reflect particles inelastically, with absorption or emission of several frequency quanta. Nevertheless, we have found that at some conditions these systems can perfectly and elastically reflect electrons. This high-frequency "blockade" give rise to unique possibility of near-ideal localization of electron with the energy lying on the background of continuous energy spectrum. We discuss different consequences of this statement.

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arXiv2008

High-frequency blockade in a tight-binding one-dimensional lattice with single vibrating atomic state

One-dimensional tight-binding lattice, single site of which possesses harmonically vibrating level is studied. The states of non-interacting electrons incident with fixed energy from infinity are considered. It is shown that at definite conditions the site reflects electrons {\it absolutely and elastically} (high-frequency blockade states). The problem is treated both numerically and (in the case of narrow band) analytically. The results are compared with the free-electron 1D problem with vibrating $δ$-functional potential. Together with the blockade states the local and reflectionless states are examined. Possible realization of the system as a lattice of quantum dots is discussed.

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arXiv2006

Theory of one-dimensional double-barrier quantum pump in two-frequency signal regime

A one-dimensional system with two $δ$-like barriers or wells bi-chromaticaly oscillating at frequencies $ω$ and $2ω$ is considered. The alternating signal leads to the direct current across the structure (even in a symmetric system). The properties of this quantum pump are studied in a wide range of the system parameters.

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arXiv2005

Properties of 1D two-barrier quantum pump with harmonically oscillating barriers

We study a one-dimensional quantum pump composed of two oscillating delta-functional barriers. The linear and non-linear regimes are considered. The harmonic signal applied to any or both barriers causes the stationary current. The direction and value of the current depend on the frequency, distance between barriers, value of stationary and oscillating parts of barrier potential and the phase shift between alternating voltages.

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arXiv2003

Distribution of equilibrium edge currents

We have studied the distribution of equilibrium edge current density in 2D system in a strong (quantizing) magnetic field. The case of half plane in normal magnetic field has been considered. The transition from classical strong magnetic field to ultraquantum limit has been investigated. We have shown that the edge current density oscillates and decays with distance from the edge. The oscillations have been attributed to the Fermi wavelength of electrons. The additional component of the current smoothly depending on the distance but sensitive to the occupation of Landau levels has been found. The temperature suppression of oscillations has been studied.

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