Searcharxiv⌕ Search

arXiv subjects

E. Kogan

Publications and source records attributed to E. Kogan.

At least 37 records · Page 2Linked to original sources
arXiv2014

Energy bands in graphene: Comparison between the tight-binding model and {\it ab initio} calculations

We compare the classification of the electron bands in graphene, obtained by group theory algebra in the framework of tight-binding model (TBM), with that calculated in the density-functional theory (DFT) framework. Identification in the DFT band-structure of all eight energy bands (four valence and four conduction bands) corresponding to the TBM-derived energy bands is performed and corresponding analysis is presented. The four occupied (three $σ$- and one $π$-like) and three unoccupied (two $σ$- and one $π$-like) bands given by DFT closely correspond to those predicted by TBM, both by their symmetry and their dispersion law. However, the two lowest lying at the $Γ$-point unoccupied bands (one of them of a $σ$-like type and the other of a $π$-like one), are not of TBM type. According both to their symmetry and to the electron density these bands are plane waves orthogonal to the TBM valence bands; dispersion of these states can be determined unambiguously up to the Brillouin zone borders. On the other hand, the fourth unoccupied band given by the TBM, can be identified among those given by the DFT band calculations; it is situated rather high with respect to energy. The interaction of this band with the free-electron states is so strong, that it exists only in a part of $k$-space.

cond-mat.mes-hall↗
arXiv2013

Space Group Symmetry Classification of Energy Bands in Graphene and Destruction of Dirac Points by Supercell Potential

In the previous publications (E. Kogan and V. U. Nazarov, Phys. Rev. B {\bf 85}, 115418 (2012) and E. Kogan, Graphene {\bf 2}, 74 (2013)) we presented point group symmetry classification of energy bands in graphene. In the present note we generalize classification by considering the space group symmetry. Also in the framework of group theory we describe destruction of the Dirac points in graphene by a supercell potential.

cond-mat.mes-hall↗
arXiv2013

Next-nearest-neighbor Tight-binding Model of Plasmons in Graphene

In this paper we investigate the influence of the next-nearest-neighbor coupling of tight-binding model of graphene on the spectrum of plasmon excitations. The nearest-neighbor tight-binding model was previously used to calculate plasmon spectrum in the next paper [1]. We expand the previous results of the paper by the next-nearest-neighbor tight-binding model. Both methods are based on the numerical calculation of the dielectric function of graphene and loss function. Here we compare plasmon spectrum of the next-nearest and nearest-neighbor tight-binding models and find differences between plasmon dispersion of two models.

cond-mat.mes-hall↗
arXiv2013

Comment on the "Create Dirac Cones in Your Favorite Materials", by Chia-Hui Lin and Wei Ku (arXiv:1303.4822)

Recently a paper by Lin and Ku \cite{lin} was posted, where the authors propose a very interesting idea to engineer the Dirac points in material which originally did not have such points by inducing a CDW state through introduction of impurities like vacancy, substitution, or intercalation. In this comment we would like to explain the appearance of the Dirac points in such engineered structures by symmetry arguments.

cond-mat.mes-hall↗
arXiv2012

RKKY interaction in gapped or doped graphene

In our previous work (E. Kogan, Phys. Rev. B {\bf 84}, 115119 (2011)) we calculated RKKY interaction between two magnetic impurities in pristine graphene using the Green's functions (GF) in the coordinate -- imaginary time representation. Now we show that the calculations of the GF in this representation can be simplified by using the Feynman's trick, which allows to easily calculate RKKY interaction in gapped graphene. We also present calculations of the RKKY interaction in gapped or doped graphene using the coordinate -- imaginary frequency representation. Both representations, corresponding to calculation of the bubble diagram in Euclidean space, have an important advantage over those corresponding to calculation in Minkowskii space, which are very briefly reviewed in the Appendix to the present work. The former, in distinction to the latter, operate only with the convergent integrals from the start to the end of the calculation.

cond-mat.mes-hall↗
arXiv2012

All approaches to RKKY interaction in graphene are equal, but some approaches are more equal than others

We show explicitly that two approaches to calculations of RKKY interaction between two magnetic impurities in graphene based on Green's functions: one using coordinate--imaginary time representation, and the other using momentum--frequency representation, give exactly the same results and none of them has to operate with diverging integrals and implement the cut-off procedure. Together with this, we claim that the first approach is more convenient for the calculations than the second one.

cond-mat.mes-hall↗
arXiv2012

While calculating RKKY interaction in graphene no theorist should do a cut-off without cause

In our previous work (E. Kogan, Phys. Rev. B 84, 115119 (2011)) we presented calculation of RKKY interaction between two magnetic impurities in graphene based on Matsubara Green's functions (MGF) in the coordinate -- imaginary time representation. Now we present the calculation based on MGF in the coordinate -- frequency representation. We claim that both approaches have an important advantage over those based on zero temperature Green's functions (ZTGF), which are very briefly reviewed in the beginning of the present work. The MGF approaches, in distinction to the ZTGF approaches, operate only with the convergent integrals from the start to the end of the calculation. The coordinate -- frequency representation for the MGF turns out to be as convenient as the coordinate -- imaginary time representation and allows to easily consider the cases of doped and gapped graphene.

cond-mat.mes-hall↗
arXiv2012

Symmetry classification of energy bands in graphene and silicene

We present the results of the symmetry classification of the electron energy bands in graphene and silicene using group theory algebra and the tight--binding approximation. The analysis is performed both in the absence and in the presence of the spin-orbit coupling. We also discuss the bands merging in the Brillouin zone symmetry points and the conditions for the latter to become Dirac points.

cond-mat.mes-hall↗
arXiv2012

Symmetry classification of energy bands in graphene

We present the results of the first principle calculations of the energy bands in graphene and their symmetry classification. The valence bands and four lowest conduction bands are classified according to their symmetry at the points $Γ$ and $K$. Merging of the bands is interpreted in the framework of the group theory.

cond-mat.mes-hall↗
arXiv2012

Why Dirac points in graphene are where they are?

We present a simple group theory explanation of the fact that the energy bands merge in the corners of the Brillouin zone for graphene and for two particular cases of Kagome lattice for arbitrary tight--binding Hamiltonian. We connect the linearity of the spectrum in the vicinity of these points for monolayer graphene, bilayer graphene for AA stacking and Kagome lattice with the properties of the conical points of the surface, known from geometry.

cond-mat.mes-hall↗
arXiv2011

On the theory of indirect exchange in EuO

We present the calculations of the Curie temperature and magnetization of doped EuO both in the absence and in the presence of external magnetic field. The calculations were performed both for the free electrons model and for the model with finite electron band width. Both models give similar results for the magnetization, close to Brillouin function.

cond-mat.mtrl-sci↗
arXiv2010

Frequency splitting of intervalley phonons in graphene

We study the thermal distribution of intervalley phonons in a graphene sheet. These phonons have two components with the same frequency. The degeneracy of the two modes is preserved by weak electron-phonon coupling. A sufficiently strong electron-phonon coupling, however, can result in a splitting into an optical and an acoustic phonon branch, which creates a fluctuating gap in the electronic spectrum. We describe these effects by treating the phonon distribution within a saddle-point approximation. Fluctuations around the saddle point indicate a Berezinskii-Kosterlitz-Thouless transition of the acoustic branch. This transition might be observable in the polarization of Raman scattered light.

cond-mat.mtrl-sci↗
arXiv2010

Ising instability of a Holstein phonon mode in graphene

We study the thermal distribution of phonons in a graphene sheet. Due to the two electronic bands there are two out-of-plane phonon modes with respect to the two sublattices. One of these modes undergoes an Ising transition by spontaneously breaking the sublattice symmetry. We calculate the critical point, the renormalization of the phonon frequency and the average lattice distortion. This transition might be observable in Raman scattering and in transport properties.

cond-mat.mtrl-sci↗
arXiv2009

Itinerant Ferromagnetism in the electronic localization limit

We present Hall effect, $R_{xy}(H)$, and magnetoresistance, $R_{xx}(H)$, measurements of ultrathin films of Ni, Co and Fe with thicknesses varying between 0.2-8 nm and resistances between 1 M$Ω$ - 100 $Ω.$ Both measurements show that films having resistance above a critical value, $R_{C}$, (thickness below a critical value, $d_{C}$) show no signs for ferromagnetism. Ferromagnetism appears only for films with $R<R_{C}$, where $R_{C}$ is material dependent. We raise the possibility that the reason for the absence of spontaneous magnetization is suppression of itinerant ferromagnetism by electronic disorder in the strong localization regime.

cond-mat.str-el↗
arXiv2008

Quasi-localization and quasi-mobility edge for light atoms mixed with heavy ones

A mixture of light and heavy atoms is considered. We study the kinetics of the light atoms, scattered by the heavy ones, the latter undergoing slow diffusive motion. In three-dimensional space we claim the existence of a crossover region (in energy), which separates the states of the light atoms with fast diffusion and the states with slow diffusion; the latter is determined by the dephasing time. For the two dimensional case we have a transition between weak localization, observed when the dephasing length is less than the localization length (calculated for static scatterers), and strong localization observed in the opposite case.

cond-mat.dis-nn↗
arXiv2007

On the analytic structure of Green's function for the Fano - Anderson model

We study analytic structure of the Green's function (GF) for the exactly solvable Fano-Anderson model. We analyze the GF poles, branch points and Riemann surface, and show how the Fermi's Golden Rule, valid in perturbative regime for not to large time, appears in this context. The knowledge of analytic structure of the GF in frequency representation opens opportunities for obtaining formulas for the GF in time representation alternative to the standard one using the spectral density.

quant-ph↗