Shyamalendu M Bose

We present a theory for the calculation of the low energy intraband plasmon frequencies and the electron energy loss (EEL) spectra of single layer and multilayer graphene sheets. Our calculation shows that the number of plasmons that can be excited is equal to the number of graphene layers in the sample. One of these is the dominant in-phase plasmon having a square root dependence on the wave number at low wave vectors, whereas the others are out-of-phase plasmons having near linear dependences on the wave number. The EEL spectra of a single layer graphene shows a single peak at the plasmon frequency, which has been observed experimentally. The EEL spectra of all multilayer graphenes have two peaks, one corresponding to the dominant in-phase plasmon and the other due to the out of phase plasmons. We predict that careful measurement of the EEL of multilayer graphene will show both peaks due to the low energy intraband plasmons.

Electronic conduction through quantum dots undergoing Jahn-Teller distortion is studied utilizing a model presented recently in connection with investigation of possible magnetovoltaic effect in this system. The quantum dot connected to two metallic leads is described by the single impurity Anderson model (SIAM) Hamiltonian along with two additional terms describing the Jahn-Teller distortion and an applied magnetic field. The self-consistent calculation shows that the Jahn-Teller (J-T) order parameter which is a measure of the splitting of the degenerate dot level is maximum at zero temperature and smoothly goes to zero at the structural transition temperature, T-s. The conductance is greatly suppressed by the J-T distortion at low temperatures, slowly increases and attains a maximum at T-s, above which it shows a slow decrease. When plotted as a function of the energy of the dot level, the conductance shows two peaks corresponding to the two split J-T levels at temperatures below T-s, which further develops into a four peak structure in the presence of a magnetic field. (C) 2013 Elsevier B.V. All rights reserved.

A model is proposed to study the electrical transport properties of a quantum dot capable of undergoing a structural phase transition driven by the Jahn-Teller mechanism. The dot attached to two metallic leads is described by the single impurity Anderson model Hamiltonian to which a term describing the Jahn-Teller distortion and another term to include the possibility of the presence of a magnetic field are added. Calculation of the electrical conductance and Jahn-Teller order parameter reveals several interesting features the most striking of which is the new phenomenon of "magnetovoltaic effect." (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4737009]

The Raman spectra of two G bands and a radial breathing mode RBM of unfilled and filled single-wall semiconducting and metallic carbon nanotubes have been investigated theoretically in the presence of electronphonon and phonon-phonon interactions. Excitation of low frequency optical plasmons in the metallic nanotube is responsible for the peak known as the Breit-Wigner-Fano BWF line shape in the G-band Raman spectra. In a filled nanotube, there is an additional peak due to excitation of the phonon of the filling atom or molecule. Positions, shapes, and relative strengths of these Raman peaks depend on the phonon frequencies of the nanotube and that of the filling atoms, and strengths and forms of the plasmon-phonon and phonon-phonon interactions. For example, filling atoms with phonon frequency close to the RBM frequency of the nanotube may broaden and lower the RBM Raman peak to such an extent that it may become barely visible. Hybridization between the G bands and the filling atom phonon is also strong when these two frequencies are close to each other, and it has important effects on the G band and the BWF line shapes. When the phonon frequency of the filling atom is far from the RBM and G-band frequencies, it gives rise to a separate peak with modest effects on the RBM and G-band spectra. The Raman spectra of semiconducting unfilled and filled nanotubes have similar behaviors as those of metallic nanotubes, except that normally they have Lorentzian line shapes and do not show a BWF line shape. However, if a semiconducting nanotube is filled with donor atoms, it is predicted that the BWF-type line shape may be observed near the RBM, or the G band, or the filling atom Raman peak.

The electronic properties of one-dimensional clusters of N atoms or molecules have been studied. The model used is similar to the Kronig–Penney model with the potential offered by each ion being approximated by an attractive δ-function. The energy eigenvalues, the eigenstates and the density of states are calculated exactly for a linear cluster of N atoms or molecules. The dependence of these quantities on the various parameters of the problem show interesting behavior. Effects of random distribution of the positions of the atoms and random distribution of the strengths of the potential have also been studied. The results obtained in this paper can have direct applications for linear chains of atoms produced on metal surfaces or for artificially created chains of atoms using scanning tunneling microscope or in studying molecular conduction of electrons across one-dimensional barriers.

Recent investigations of superconductivity in carbon nanotubes have shown that a single-wall zig-zag nanotube can become superconducting at around 15 K. Theoretical studies of superconductivity in nanotubes using the traditional phonon exchange model, however, give a superconducting transition temperature T c less than 1 K. To explain the observed higher critical temperature we explore the possibility of the plasmon exchange mechanism for superconductivity in nanotubes. We first calculate the effective interaction between electrons in a nanotube mediated by plasmon exchange and show that this interaction can become attractive. Using this attractive interaction in the modified Eliashberg theory for strong coupling superconductors, we then calculate the critical temperature T c in a single-wall nanotube. Our theoretical results can explain the observed T c in a single-wall nanotube. In particular, we find that T c is sensitively dependent on the dielectric constant of the medium, the effective mass of the electrons and the radius of the nanotube. We then consider superconductivity in a bundle of single-wall nanotubes and find that bundling of nanotubes does not change the critical temperature significantly. Going beyond carbon nanotubes we show that in a metallic hollow nanowire T c has some sort of oscillatory behaviour as a function of the surface number density of electrons.

The Raman spectra of a metallic carbon nanotube filled with atoms or molecules have been investigated theoretically. It is found that there will be a three way splitting of the main Raman lines due to the interaction of the nanotube phonon with the collective excitations (plasmons) of the conduction electrons of the nanotube as well as its coupling with the phonon of the filling material. The positions and relative strengths of these Raman peaks depend on the strength of the electron–phonon interaction, phonon frequency of the filling atom and the strength of interaction of the nanotube phonon and the phonon of the filling atoms. Careful experimental studies of the Raman spectra of filled nanotubes should show these three peaks. It is also shown that in a semiconducting nanotube the Raman line will split into two and should be observed experimentally.

A model is proposed to calculate the thermoelectric figure of merit of a framework crystal containing rattler atoms in cages. Such systems are expected to behave like a Phonon Glass and Electron Crystal (PGEC). The model resembles an effective Anderson model for a correlated system. The dispersion of the electronic energies and of the phonon frequencies in the system is calculated exactly. These results are used to evaluate the electronic and the thermal transport coefficients, which in turn give the temperature dependence of the thermoelectric figure of merit. Explicit calculation of the thermoelectric figure of merit for a one-dimensional case shows that the lattice thermal conductivity plays an important role in providing a peak structure to its temperature dependence. The results are presented for three different electronic dispersions and it is found that the room temperature figure of merit attains the highest value for the tight binding case. The calculation provides guidelines for designing a better thermoelectric material for use as a refrigerator.