arXiv subjects

O. G. Balev

Publications and source records attributed to O. G. Balev.

At least 19 recordsLinked to original sources

arXiv2012

Edge magnetoplasmons in a wide armchair graphene ribbon with a weak superlattice potential: finite frequency gaps and zero group velocity

We show strong effects of a weak and smooth, on the magnetic length, superlattice potential upon edge magnetoplasmons (EMPs) at the armchair edge, with a smooth steplike electrostatic lateral confining potential, of a wide graphene channel in the $ν=2$ quantum Hall effect regime. The superlattice potential leads to essential enlargement of a number of EMPs, descend from two fundamental EMPs in the absence of superlattice. For the wave vector $k_{x}$ within the first Brillouin zone, the EMPs show as the regions of acoustical or quasi-acoustical dispersion, with a finite value of group velocity, so the regions with frequency gaps, where a group velocity is nullified at some $k_{x}$. We obtain that for $k_{x} \to 0$ only for two EMPs the frequency tends to zero as for other EMPs it obtains finite values. Strong dependence of dispersion relations of the EMPs from the period of the superlattice $a_{0}$ and the distance $d$ from a metallic gate is shown; in particular, for typical size of a gap, for characteristic value of the frequency and $k_{x}$ at which the group velocity is reduced to zero. At the frequency that corresponds to zero group velocity of pertinent fundamental EMP branch the response of the system should present a strong resonance.

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arXiv2011

Edge magnetoplasmons in wide armchair graphene ribbons

We show that near an armchair edge of a wide graphene channel, and in the presence of a smooth step-like electrostatic lateral confining potential, the chirality, spectrum, spatial structure, and number of the fundamental edge magnetoplasmons (EMPs), in the $ν=2$ regime of the quantum Hall effect, depend strongly on the position of the Fermi level $E_{F}$. (i) When $E_{F}$ is small enough and intersects four degenerate states of the zero Landau level (LL) at one location and two degenerate states of this level at a different one, two fundamental, counter propagating EMPs exist with opposite chirality. This is in contrast with EMPs in conventional two-dimensional electron systems in which only one fundamental EMP exists. For the same wave vector these EMPs have different moduli of phase velocities and an essential spatial overlap. These EMPs can be on resonance in a wide range of frequencies, for micron or submicron lengths along the edge. (ii) When $E_{F}$ is sufficiently high and intersects only two degenerate states of the zero LL only one fundamental EMP exists with the usual chirality.

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arXiv2010

Spectral and polarization dependencies of luminescence by hot carriers in graphene

The luminescence caused by the interband transitions of hot carriers in graphene is considered theoretically. The dependencies of emission in mid- and near-IR spectral regions versus energy and concentration of hot carriers are analyzed; they are determined both by an applied electric field and a gate voltage. The polarization dependency is determined by the angle between the propagation direction and the normal to the graphene sheet. The characteristics of radiation from large-scale-area samples of epitaxial graphene and from microstructures of exfoliated graphene are considered. The averaged over angles efficiency of emission is also presented.

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arXiv2009

Hot carriers in an intrinsic graphene

Heating of carriers in an intrinsic graphene under dc electric field is considered taking into account the intraband energy relaxation due to acoustic phonon scattering and the interband generation-recombination transitions due to thermal radiation. The distribution of nonequilibrium carriers is obtained for the cases when the intercarrier scattering is unessential and when the carrier-carrier Coulomb scattering effectively establishes the quasiequilibrium distribution with the temperature and the density of carriers that are determined by the balance equations. Because of an interplay between weak energy relaxation and generation-recombination processes a very low threshold of nonlinear response takes place. The nonlinear current-voltage characteristics are calculated for the case of the momentum relaxation caused by the elastic scattering. Obtained current-voltage characteristics show low threshold of nonlinear behavior and appearance of the second ohmic region, for strong fields.

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arXiv2007

Fractionally Quantized Hall Effect: Liquid-Crystal Ground-State and it Excitations

It is shown that the ground-state and the lowest excited-states of two-dimensional electron system (2DES), with ion jellium background, correspond to partial crystal-like (with the period $L_{x}^{\square}=\sqrt{2 m π} \ell_{0}$) correlation order among $N$ electrons of the main region (MR; $L_{x} \times L_{y}$). Many-body variational ground-state wave function of 2DES is presented at the fractional and the integral filling factors $ν=1/m$; $m=2\ell+1$ and $\ell=0, 1, 2,...$. The ground-state manifests the broken symmetry liquid-crystal state with 2DES density that is periodic along the $y-$ direction, with the period $L_{x}^{\square}/m$, and independent of $x$. At $m=3, 5$, the ground-state has essentially lower energy per electron than the Laughlin, uniform liquid, ground-state; the same holds at $m=1$. At $m \geq 3$, the compound form of the many-body ground-state wave function leads to the compound structure of each electron within the main strip (MS; $L_{x}^{\square} \times L_{y}$). Obtained compound exciton and compound spin-exciton states show finite excitation gaps. The excited compound electron (hole) is composed, within MS, from $m$ strongly correlated quasielectrons (quasiholes) of the total charge $e/m$ ($-e/m$) each. The activation gap is obtained: it is given by the excitation gap of relevant compound exciton, at $m \geq 3$, and by the gap of pertinent compound spin-exciton, at $m=1$. Quantized Hall conductance $σ_{H}=e^{2}/(2 m π\hbar)$ is obtained. The theory is in good agreement with experiments.

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arXiv2007

Ground-state of fractional and integral quantum Hall systems at $ν\leq 1$ and it excitations

Many-body variational ground-state wave function of two-dimensional electron system (2DES), localized in the main strip (MS)$L_{x}^{\square} \times L_{y}$ of the finite width $L_{x}^{\square}=\sqrt{2 πm} \ell_{0}$ (and the periodic boundary condition (PBC) imposed along $x-$direction), is presented at the fractional and the integral filling factors $ν=1/m$ for two different ion backgrounds: microscopical uniform ion background (UIB) and classical ion jellium background (IJB); $\ell_{0}$ is the magnetic length, $m=2\ell+1$ and $\ell=0, 1, 2,...$. It is shown that the ground-state and the lowest excited-state can correspond to partial crystal-like correlation order among $N$ electrons of the main region (MR) $L_{x} \times L_{y}$; then the study of 2DES of $N$ electrons within MR is exactly reduced to the treatment of 2DES of $\tilde{N}=N L_{x}^{\square}/L_{x}$ electrons localized within MS, with PBC along $x$. The ground-state manifests the broken symmetry liquid-crystal state with 2DES density that is periodic along the $y-$ direction, with the period $L_{x}^{\square}/m$, and independent of $x$. For IJB, at $m=3, 5$, the ground-state has essentially lower energy per electron than the Laughlin, uniform liquid, ground-state (the Laughlin model uses IJB); the same holds at $m=1$. Obtained compound exciton and compound spin-exciton states show finite excitation gaps. The excited compound electron (hole) is composed, within MS, from $m$ strongly correlated quasielectrons (quasiholes) of the charge $e/m$ ($-e/m$. Quantized Hall conductance $σ_{H}=e^{2}/(2 m π\hbar)$ is obtained. The theory is in good agreement with experiments.

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arXiv2005

Liquid-Crystal State of $ν=1/m$ Quantum Hall Effects

At filling factor $ν=1/m$, $m$ odd integer, I present variational ground-state and excited-state wave functions, of two-dimensional electron system with homogeneous ion background, that show the condensation into a liquid-crystal state. For $m=1, 3, 5$, the ground-state energy per electron is substantially lower than the Laughlin one, for uniform liquid state.

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arXiv2005

Many-body interactions in a quantum wire in the integer quantum Hall regime: suppression of exchange-enhanced g factor

The collapse of Hall gaps in the integer quantum Hall liquid in a quantum wire is investigated. Motivated by recent experiment [Pallecchi et al. PRB 65, 125303 (2002)] previous approaches are extended to treat confinement effects and the exchanged enhanced g-factor in quantum wires. Two scenarios for the collapse of the $ν=1$ state are discussed. In the first one the $ν=1$ state becomes unstable at $B_{cr}^{(1)}$, due to the exchange interaction and correlation effects, coming from the edge-states screening. In the second scenario, a transition to the $ν=2$ state occurs at $B_{cr}^{(2)}$, with a smaller effective channel width, caused by the redistribution of the charge density. This effect turns the Hartree interaction essential in calculating the total energy and changes $B_{cr}^{(2)}$ drastically. In both scenarios, the exchange enhanced g-factor is suppressed for magnetic fields lower than $B_{cr}$. Phase diagrams for the Hall gap collapse are determined. The critical fields, activation energy, and optical $g$-factor obtained are compared with experiments. Within the accuracy of the available data, the first scenario is most probable to be realized.

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arXiv2001

Temperature effects on edge-state properties in the integer quantum Hall regime

The edge and bulk structure of Landau levels (LLs) in a wide channel at the $ ν=1$ quantum Hall regime is calculated for not-too-low temperatures, $\hbar ω_{c} \gg k_{B}T\gg \hbar v_{g}/2\ell_{0}$, where $v_{g}$ is the group velocity of the edge states and $\ell_{0}=\sqrt{\hbar c/|e|B}$ is the magnetic length. Edge-states correlations essentially modify the spatial behavior of the lowest spin-up LL, which is occupied, compared to the lowest spin-down LL, which is empty. The influence of many-body interactions on the spatially inhomogeneous spin-splitting between the two lowest LLs is studied within the generalized local density approximation. Temperature effects on the enhanced spin-splitting, the position of the Fermi level within the exchange enhanced gap and the renormalization of edge-states group velocity by edge states screening are considered. It is shown that the maximum activation energy $G$ in the bulk of the channel is determined by the gap between the Fermi level and the bottom of the spin-down LL, because the gap between the Fermi level and the spin-up LL is much larger. For the maximum value of $G$, it is shown that the renormalized group velocity $v_{g}\propto T$ for $T\to 0$ and, in particular, the condition of not-too-low $T$ can be satisfied for $4.2\agt T\agt0.3$ K. In other words, the regime of not-too-low temperatures regime can be achieved even for rather low $T$.

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arXiv2001

Electron correlation effects in a wide channel from the $ν=1$ quantum Hall edge states

The spatial behavior of Landau levels (LLs) for the $nu=1$ quantum Hall regime at the edge of a wide channel is studied in a self-consistent way by using a generalized local density approximation proposed here. Both exchange interaction and strong electron correlations, due to edge states, are taken into account. They essentially modify the spatial behavior of the occupied lowest spin-up LL in comparison with that of the lowest spin-down LL, which is totally empty. The contrast in the spatial behavior can be attributed to a different effective one-electron lateral confining potentials for the spin-split LLs. Many-body effects on the spatially inhomogeneous spin-splitting are calculated within the screened Hartree-Fock approximation. It is shown that, far from the edges, the maximum activation energy is dominated by the gap between the Fermi level and the bottom of the spin-down LL, because the gap between the Fermi level and the spin-up LL is much larger. In other words, the maximum activation energy in the bulk of the channel corresponds to a highly asymmetric position of the Fermi level within the gap between spin-down and spin-up LLs in the bulk. We have also studied the renormalization of the edge-state group velocity due to electron correlations. The results of the present theory are in line with those suggested and reported by experiments on high quality samples.

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arXiv2000

Inhomogeneous broadening of tunneling conductance in double quantum wells

The lineshape of the tunneling conductance in double quantum wells with a large-scale roughness of heterointerfaces is investigated. Large-scale variations of coupled energy levels and scattering due to the short-range potential are taken into account. The interplay between the inhomogeneous broadening, induced by the non-screened part of large-scale potential, and the homogeneous broadening due to the scattering by short-range potentials is considered. It is shown that the large inhomogeneous broadening can be strongly modified by nonlocal effects involved in the proposed mechanism of inhomogeneity. Related change of lineshape of the resonant tunneling conductance between Gaussian and Lorentzian peaks is described. The theoretical results agree quite well with experimental data.

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arXiv2000

Two-subband electron transport in nonideal quantum wells

Electron transport in nonideal quantum wells (QW) with large-scale variations of energy levels is studied when two subbands are occupied. Although the mean fluctuations of these two levels are screened by the in-plane redistribution of electrons, the energies of both levels remain nonuniform over the plane. The effect of random inhomogeneities on the classical transport is studied within the framework of a local response approach for weak disorder. Both short-range and small-angle scattering mechanisms are considered. Magnetotransport characteristics and the modulation of the effective conductivity by transverse voltage are evaluated for different kinds of confinement potentials (hard wall QW, parabolic QW, and stepped QW).

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arXiv1999

Edge magnetoplasmons in periodically modulated structures

We present a microscopic treatment of edge magnetoplasmons (EMP's) within the random-phase approximation for strong magnetic fields, low temperatures, and filling factor $ν=1(2)$, when a weak short-period superlattice potential is imposed along the Hall bar. The modulation potential modifies both the spatial structure and the dispersion relation of the fundamental EMP and leads to the appearance of a novel gapless mode of the fundamental EMP. For sufficiently weak modulation strengths the phase velocity of this novel mode is almost the same as the group velocity of the edge states but it should be quite smaller for stronger modulation. We discuss in detail the spatial structure of the charge density of the renormalized and the novel fundamental EMP's.

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arXiv1999

Temperature Effects on Edge Magnetoplasmons in the Quantum Hall Regime

A microscopic treatment of edge magnetoplasmons (EMPs) is presented for the case of not-too-low temperatures in which the inequality $k_{B}T\gg \hbar v_{g}/\ell_{0}$, where $v_{g}$ is the group velocity of the edge states and $\ell_{0}$ is the magnetic length, is fulfilled, and for filling factors $ν=1(2)$. We have obtained independent EMP modes spatially symmetric and antisymmetric with respect to the edge. We describe in detail the spatial structure and dispersion relations of the new edge waves (edge helicons, dipole, quadrupole and octupole EMPs), which have the characteristic length $\ell_{T}=\ell_{0}^{2}k_{B}T/\hbar v_{g}$. We have found that, in contrast to well-known results for a spatially homogeneous dissipation within the channel, the damping of the fundamental EMP at not-too-low temperatures is not quantized and has a $T^{-1}$ dependence.

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arXiv1998

Edge helicons and repulsion of fundamental edge magnetoplasmons in the quantum Hall regime

A quasi-microscopic treatment of edge magnetoplasmons (EMP) is presented for very low temperatures and confining potentials smooth on the scale of the magnetic length $\ell_{0}$ but sufficiently steep at the edges such that Landau level (LL) flattening can be discarded. The profile of the unperturbed electron density is sharp and the dissipation taken into account comes only from electron intra-edge and intra-LL transitions due to scattering by acoustic phonons. For wide channels and filling factors $ν=1$ and 2, there exist independent EMP modes spatially symmetric and antisymmetric with respect to the edge. Some of these modes, named edge helicons, can propagate nearly undamped even when the dissipation is strong. Their density profile changes qualitatively during propagation and is given by a rotation of a complex vector function. For $ν>2,$ the Coulomb coupling between the LLs leads to a repulsion of the uncoupled fundamental LL modes: the new modes have very different group velocities and are nearly undamped. The theory accounts well for the experimentally observed plateau structure of the delay times as well as for the EMP's period and decay rates.

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arXiv1998

Repulsion of Single-well Fundamental Edge Magnetoplasmons in Double Quantum Wells

A {\it microscopic} treatment of fundamental edge magnetoplasmons (EMPs) along the edge of a double quantum well (DQW) is presented for strong magnetic fields, low temperatures, and total filling factor ν=2. It is valid for lateral confining potentials that Landau level (LL) flattening can be neglected. The cyclotron and Zeeman energies are assumed larger than the DQW energy splitting \sqrt{Δ^2 +4T^2}, where Δis the splitting of the isolated wells and T the tunneling matrix element. %hen calculated unperturbed density profile is sharp at the edge. Using a random-phase approximation (RPA), which includes local and nonlocal contributions to the current density, it is shown that for negligible tunnel coupling 2T << Δthe inter-well Coulomb coupling leads to two DQW fundamental EMPs which are strongly renormalized in comparison with the decoupled, single-well fundamental EMP. These DQW modes can be modified further upon varying the inter-well distance d, along the z axis, and/or the separation of the wells' edges Δy along the y axis. The charge profile of the {\it fast} and {\it slow} DQW mode varies, respectively, in an {\it acoustic} and {\it optical} manner along the y axis and is not smooth on the \ell_{0} scale. For strong tunneling Δ\alt 2T these DQW modes are essentially modified when Δis changed by applying a transverse electric field to the DQW.

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arXiv1998

Random-phase Approximation Treatment Of Edge Magnetoplasmons: Edge-state Screening And Nonlocality

A random-phase approximation (RPA) treatment of edge magnetoplasmons (EMP) is presented for strong magnetic fields, low temperatures, and integer filling factors ν. It is valid for negligible dissipation and lateral confining potentials smooth on the scale of the magnetic length \ell_{0} but sufficiently steep that the Landau-level (LL) flattening can be neglected. LL coupling, screening by edge states, and nonlocal contributions to the current density are taken into account. In addition to the fundamental mode with typical dispersion relation ω\sim q_x \ln(q_{x}), fundamental modes with {\it acoustic} dispersion relation ω\sim q_x are obtained for ν>2. For ν=1,2 a {\bf dipole} mode exists, with dispersion relation ω\sim q_x^3, that is directly related to nonlocal responses.

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arXiv1998

Collective Edge Excitations In The Quantum Hall Regime: Edge Helicons And Landau-level Structure

Based on a microscopic evaluation of the local current density, a treatment of edge magnetoplasmons (EMP) is presented for confining potentials that allow Landau level (LL) flattening to be neglected. Mode damping due to electron-phonon interaction is evaluated. For nu=1, 2 there exist independent modes spatially symmetric or antisymmetric with respect to the edge. Certain modes, changing shape during propagation, are nearly undamped even for very strong dissipation and are termed edge helicons. For nu > 2 inter-LL Coulomb coupling leads to a strong repulsion of the decoupled LL fundamental modes. The theory agrees well with recent experiments.

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