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Margulis Vl.A.,Mordovian Ogarev State University | Muryumin E.E.,Mordovian Ogarev State University
Physica B: Condensed Matter | Year: 2010

We present a detailed theoretical study of the collective π -electronic excitations associated with electron-density fluctuations (π -plasmons) in individual single-walled carbon nanotubes (SWCNTs) possessing chiral symmetry. Unlike previous theoretical treatments of this problem, based on a cylindrical-symmetry model, we use a different approach, initiated by White et al. [Phys. Rev. B 47 (1993) 5485], which properly takes account of genuine helical and rotational symmetries of the chiral SWCNTs. Taking into account the Coulomb interaction between π -electrons within the random-phase-approximation, we derive an explicit expression for the dynamic-dielectric-response function of the SWCNTs in helical representation and apply it to the numerical calculation of the dispersion and damping of the π -plasmons in a number of representative SWCNTs with chiral symmetry. Multiple (up to eight) well-defined inter- π -band-plasmon modes, with wave vectors lying in the direction of the helical line of such nanotubes, are shown to exist for each value of the quantum angular momentum L independent of whether the nanotubes are metallic or semiconducting. These π -plasmon modes, which we refer to as helical π -plasmons, are found to have several unexpected dispersive features, making them distinctly different from those obtained previously within the above-mentioned cylindrical-symmetry model. In particular, none of the helical π -plasmon branches starts from a zero value of the helical wave-number qH, and in all the calculated spectra of the helical π -plasmons, there exists the highest-lying branch of the dispersive π -plasmon modes, which either extends up to the energy of about 15 eV or makes its start with this energy. This highest-lying branch in the spectrum of the π -plasmons is found to have the largest spectral weight in the electron-energy losses, leading to the appearance of the dominant 15-eV-resonant peak in the low-loss spectra of the nanotubes under consideration. This possibly suggests an explanation, on the microscopic level, to the origin of the intensive resonant feature at ∼ 15 eV, observed in recent electron-energy-loss-spectroscopy experiments on isolated SWCNTs. © 2010 Elsevier B.V. All rights reserved.

Tomilin O.B.,Mordovian Ogarev State University | Stankevich I.V.,Russian Academy of Sciences | Muryumin E.E.,Mordovian Ogarev State University | Rodionova E.V.,Mordovian Ogarev State University
Carbon | Year: 2012

An examination of the peculiarities of ρ-electron conjugation of carbon atoms in spherical (fullerenes) and cylindrical (single-walled carbon nanotubes) molecular systems was performed using an analysis of topological isomerism of [N]-annulenes. It was demonstrated that a ρ-electron conjugation has a stabilizing effect with respect to a π-electron conjugation; however, the value of this effect is minor. A non-uniform distribution of electron density in the molecules leads to a considerable alternation of C-C bond lengths and the absence of ring currents in external magnetic field. All the conclusions regarding the peculiarities of ρ-electron conjugation were confirmed by the model and by calculations for the real molecular systems using an ab initio Hartree-Fock method in the 3-21G basis and comparison of its results with experimental data. © 2012 Elsevier Ltd. All rights reserved.

Margulis Vl.A.,Mordovian Ogarev State University | Muryumin E.E.,Mordovian Ogarev State University | Gaiduk E.A.,Mordovian Ogarev State University
Physica B: Condensed Matter | Year: 2011

We present a theoretical study of the Rayleigh (elastic light) scattering (RS) from π electrons in boron-nitride single-walled nanotubes (BN-SWNTs) with a zigzag achiral structure. The π-electronic band structure of the nanotubes is treated within a simple analytical model based on the tight-binding and zone-folding approximations. We provide explicit formulas for the real and imaginary parts of the linear optical susceptibility χ( 1)(ω), both of which contribute to the RS cross section σ(ω). The σ(ω) spectra we have calculated for several representative BN-SWNTs (l,0) of fairly large diameters (>1.3nm) show a series of very pronounced peak structures which occur at energies corresponding to resonant π-electron transitions between the valence- and conduction-energy-subband edges, where van Hoves singularities of the joint density of (electronic) states of the nanotubes are located. It is found that the RS spectra are indicative of the underlying electronic structure of the BN-SWNTs (l,0), exhibiting radically different global resonant profiles for the nanotubes with odd l and for those with even l, and thus suggesting a way of discriminating between them in experiment. Experimental implications of a number of other findings, emerging from the present theoretical calculation of the RS spectra, are also discussed. © 2011 Elsevier B.V.

Margulis V.A.,Mordovian Ogarev State University | Muryumin E.E.,Mordovian Ogarev State University | Gaiduk E.A.,Mordovian Ogarev State University
Physica B: Condensed Matter | Year: 2012

We present an analytic calculation of the optical conductivity of atomically thin hexagonal boron nitride (h-BN) layers in the spectral range where this quantity is expected to be dominated by π-electron transitions. The approach, which we rely on in this study, is entirely within an independent-particle description of optical response and is based on the tight-binding representation for the π-electron energy bands of a single h-BN sheet. The most significant result of our calculation is an explicit closed form, in terms of elementary functions, we have found for the real part of the optical conductivity of the h-BN sheet. This allows the optical absorbance of a few-layer h-BN films to be easily evaluated within the independent-layer approximation. The absorbance is found to be spectrally quite flat (at a level close to zero) up to the threshold frequency (in ultraviolet) corresponding to the onset of resonant interband transitions. Slightly above the threshold, the absorbance exhibits a pronounced sharp symmetric peak reaching ∼20% in magnitude. This feature as well as the overall spectral profile of the calculated optical absorbance is in good qualitative agreement with experiment. © 2012 Elsevier B.V.

Margulis V.A.,Mordovian Ogarev State University | Muryumin E.E.,Mordovian Ogarev State University | Gaiduk E.A.,Mordovian Ogarev State University
Physics Letters, Section A: General, Atomic and Solid State Physics | Year: 2016

In connection with the controversial question about the frequency dependence of the optical third-harmonic generation (THG) from doped graphene, which has recently been discussed in the literature, we develop an analytical theory for the THG susceptibility of doped graphene by using the original Genkin-Mednis nonlinear-conductivity-theory formalism including mixed intra- and interband terms. The theory is free of any nonphysical divergences at zero frequency, and it predicts the main resonant peak in the THG spectrum to be located at the photon energy Latin small letter h with strokeω equal to two thirds of the Fermi energy EF of charge carriers in doped graphene. © 2015 Elsevier B.V.

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