Bulk band inversion and surface Dirac cones in LaSb and LaBi : Prediction of a new topological heterostructure

We perform \textit{ab initio} investigations of the bulk and surface band structures of LaSb and LaBi and resolve the existing disagreements about the topological property of LaSb, considering LaBi as a reference. We examine the bulk band structure for band inversion, along with the stability of surface Dirac cones (if any) to time-reversal-preserving perturbations, as a strong diagnostic test for determining the topological character of LaSb, LaBi and LaSb-LaBi multilayer. A detailed \textit{ab initio} investigation of a multilayer consisting of alternating unit cells of LaSb and LaBi shows the presence of band inversion in the bulk and a massless Dirac cone on the (001) surface, which remains stable under the influence of time-reversal-preserving perturbations, thus confirming the topologically non-trivial nature of the multilayer in which the electronic properties can be tailored as per requirement. A detailed $\mathbb{Z}_2$ invariant calculation is performed to arrive at a holistic conclusion

Role of long-range coupling on the properties of single polarons in models with dual electron-phonon couplings

We use the Variational Exact Diagonalization to investigate the single polaron properties for four different dual models, combining a short-range off-diagonal (Peierls) plus a longer-range diagonal (Holstein or breathing-mode) coupling. This allows us to investigate the sensitivity of various polaron properties both to the range of the diagonal coupling and to the specific diagonal coupling chosen. We find strong sensitivity to the range for all duals models as the adiabatic limit is approached, however considerable sensitivity is observed for some quantities even for large phonon frequencies. Also, the strong dependence of the results on the specific form of the diagonal coupling is observed everywhere in the parameter space. Taken together, these results suggest that careful consideration must be given to the specific coupling and its proper range when quantitative comparisons with experiments are sought.Comment: 9 pages, 12 figures, submitted to Physical Review

Half Metallicity in Pr0.75_{0.75}Sr0.25_{0.25}MnO3_3: A first Principle study

In this communication we present a first principle study of Pr$_{1-x}$Sr$_{x}$MnO$_3$ with $x = 0.25$. While the parent compounds of this system are antiferromagnetic insulators with different structural and magnetic ground states, the $x = 0.25$ is in the colossal magnetoresistance regime of the Pr$_{1-x}$Sr$_{x}$MnO$_3$ phase diagram [C. Martin, A. Maignan, M. Hervieu, B. Raveau, Phys. Rev. B 60 (1999) 12191]. Our band structure calculations for the end-point compounds matches well with the existing theoretical and experimental results [C. Martin, A. Maignan, M. Hervieu, B. Raveau, Phys. Rev. B 60 (1999) 12191; Rune Sondena, P. Ravindran, Svein Stolen, Tor Grande, Michael Hanfland, Phys. Rev. B 74 (2006) 144102]. Interestingly, our calculations show that the Pr$_{0.75}$Sr$_{0.25}$MnO$_3$ has a half-metallic character with a huge band gap of 2.8 eV in the minority band. We believe this result would fuel further interest in some of these special compositions of colossal magnetoresistive manganites as they could be potential candidates for spintronic devices. We discuss the half-metallicity of the Pr$_{0.75}$Sr$_{0.25}$MnO$_3$ in the light of changes in the orbital hybridization as a result of Sr doping in PrMnO$_3$. Further, we highlight the importance of half-metallicity for a consolidated understanding of colossal magnetoresistance effect.Comment: 8 pages, 3 figure

Study of the one dimensional Holstein model using the augmented space approach

A new formalism using the ideas of the augmented space recursion (introduced by one of us) has been proposed to study the ground state properties of ordered and disordered one-dimensional Holstein model. For ordered case our method works equally well in all parametric regime and matches with the existing exact diagonalization and DMRG results. On the other hand the quenched substitutionally disordered model works in low and intermediate regime of electron phonon coupling. Effect of phononic and substitutional disorder are treated on equal footing.Comment: Accepted for publication in Physica