Search

In the presence of an external vertical electric field and strain, it is evident that 1T ′ -MoS 2 exhibits tilted Dirac bands which are valley-spin-polarized. Additionally, this material experiences a topological phase change between a topological insulator and band insulator for a critical value of the electric field. Using linear response theory, we calculated the polarization function which is in turn employed to obtain the dielectric function. This latter quantity is subsequently utilized in calculations to determine the plasmons dispersion relation, their decay rate and impurity screening corresponding to various levels of doping, the critical applied vertical electric field strengths and the spin-orbit coupling gap in 1T ′ -MoS 2 with tilted Dirac bands. • Polarization function for tilted Dirac fermion materials. • Anisotropy of transverse magnetic modes. • Analytic expressions for the plasmon excitations. • Static shielding of impurities. • Landau damping of plasmons and the calculated decay rates. [ABSTRACT FROM AUTHOR] more...

Abstract: We present a formalism for calculating the absorption coefficient of a pair of coaxial tubules. A spatially nonlocal, dynamical self-consistent field theory is obtained by calculating the electrostatic potential produced by the charge density fluctuations as well as the external electric field. There are peaks in the absorption spectrum arising from plasma excitations corresponding either to plasmon or particle-hole modes. In this Letter, we numerically calculate the plasmon contribution to the absorption spectrum when an external electric field is applied. The number of peaks depends on the radius of the inner as well as outer tubule. The height of each peak is determined by the plasmon wavelength and energy. For a chosen wave number, the most energetic plasmon has the highest peak corresponding to the largest oscillator strength of the excited modes. Some of the low-frequency plasmon modes have such weak coupling to an external electric field that they are not seen on the same scale as the modes with larger energy of excitation. We plot the peak positions of the plasmon excitations for a pair of coaxial tubules. The coupled modes on the two tubules are split by the Coulomb interaction. The energies of the two highest plasmon branches increase with the radius of the outer tubule. On the contrary, the lowest modes decrease in energy as this radius is increased. No effects due to inter-tubule hopping are included in these calculations. [Copyright &y& Elsevier] more...

Abstract: In this work we analyze the energy loss of a fast charged particle moving parallel to a two-dimensional (2D) graphene sheet. The response dynamics of the 2D graphene sheet are modeled using the random phase approximation in the degenerate limit (zero temperature). We determine the energy loss and stopping power for motion parallel to the graphene sheet as a function of the distance of the charged particle from the 2D sheet and of its velocity. [ABSTRACT FROM AUTHOR] more...

Abstract: An experimental and theoretical study of electronic excitations in MgB2 covering the domain of large energy and momentum transfers is reported. Energy-loss spectra for several values of momentum transfers were measured in a polycrystalline sample by means of inelastic X-ray scattering spectroscopy. Ab initio calculations of the dielectric function as well as the energy-loss function were performed in the frame of the time-dependent local density approximation with inclusion of crystal local-field effects. We obtained very good agreement between the experimental and the theoretical energy dispersion of the peak maximum of the loss function. We found that crystal local-field effects are responsible for this agreement at large momenta. Fine structure observed in the measured spectra was interpreted in terms of strong interband transitions predicted by the calculations in the and directions. The theoretical dispersion of these features is in good accordance with the experimental data. Further spectral features in the measured spectra due to Mg 2s and 2p core electron excitations are also discussed. [Copyright &y& Elsevier] more...