Tanmoy Das

Topological insulators in the quaternary chalcogenide compounds and ternary famatinite compounds

We present first-principles calculations to predict several three- dimensional (3D) topological insulators in quaternary chalcogenide compounds of compositions I2-II-IV-VI4 and ternary famatinite compounds of composi- tions I3-V-VI4. Among the large number of members of these two families, we give examples of naturally occurring compounds that are mainly Cu-based chalcogenides. We show that these materials are candidates for 3D topological insulators or can be tuned to obtain topologically interesting phases by manipu- lating the atomic number of the various cations and anions. A band inversion can occur at a single point 0 with large inversion strength, in addition to the opening of a bulk bandgap throughout the Brillouin zone. We discuss how the two inves- tigated families of compounds are related to each other by cross-substitution of cations in the underlying tetragonal structure.

Optical model-solution to the competition between a pseudogap phase and a charge-transfer-gap phase in high-temperature cuprate superconductors

We present a theoretical framework for a quantitative understanding of the full doping dependence of the optical spectra of the cuprates. In accord with experimental observations, the computed spectra show how the high-energy charge-transfer (CT) gap features persist in the overdoped regime even after the midinfrared (MIR) peak originating from the pseudogap has collapsed in a quantum critical point. In this way, we reconcile the opposing tendencies of the MIR and CT peaks to shift in opposite directions in the optical spectra with increasing doping. The competition between the pseudogap and the CT gap also results in rapid loss of spectral weight in the high energy region with doping.

Nodeless d-wave superconducting pairing due to residual antiferromagnetism in underdoped Pr2-xCexCuO4-δ

We investigate the doping dependence of the penetration depth versus temperature in electron-doped Pr(2-x)Ce(x)CuO(4-delta) using a model which assumes the uniform coexistence of (mean-field) antiferromagnetism and superconductivity. Despite the presence of a d(x2-y2) pairing gap in the underlying spectrum, we find nodeless behavior of the low-T penetration depth in the underdoped case, in accord with experimental results. As doping increases, a linear-in-T behavior of the penetration depth, characteristic of d-wave pairing, emerges as the lower magnetic band crosses the Fermi level and creates a nodal Fermi surface pocket.

An isolated Dirac cone on the surface of ternary tetradymite-like topological insulators

We have extended the search for topological insulators to the ternary tetradymite-like compounds M2X2Y (M = Bi or Sb; X and Y = S, Se or Te), which are variations of the well-known binary compounds Bi2Se3 and Bi2Te3. Ourfirst-principles computations suggest thatfive existing compounds are strong topological insulators with a single Dirac cone on the surface. In particular, stoichiometric Bi2Se2S, Sb2Te2Se and Sb2Te2S are predicted to have an isolated Dirac cone on their naturally cleaved surface. This finding paves the way for the realization of the topological transport regime.

A competing order scenario of two-gap behavior in hole doped cuprates

Angle-dependent studies of the gap function provide evidence for the coexistence of two distinct gaps in hole-doped cuprates, where the gap near the nodal direction scales with the superconducting transition temperature

Emergence of non-Fermi-liquid behavior due to Fermi surface reconstruction in the underdoped cuprate superconductors

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Nonmonotonic d x 2 − y 2 superconducting gap in electron-doped Pr 0.89 La Ce 0.11 Cu O 4 : Evidence of coexisting antiferromagnetism and superconductivity?

, while that in the antinodal direction scales with the pseudogap temperature. We present model calculations which show that most of the characteristic features observed in the recent angle-resolved photoemission spectroscopy (ARPES) as well as scanning tunneling microscopy (STM) two-gap studies are consistent with a scenario in which the pseudogap has a nonsuperconducting origin in a competing phase. Our analysis indicates that, near optimal doping, superconductivity can quench the competing order at low temperatures, and that some of the key differences observed between the STM and ARPES results can give insight into the superlattice symmetry of the competing order.

Strong correlation effects and optical conductivity in electron-doped cuprates

We present an intermediate coupling scenario together with a model analytic solution where the non-Fermi-liquid behavior in the underdoped cuprates emerges through the mechanism of Fermi surface (FS) reconstruction. Even though the fluctuation spectrum remains nearly isotropic, FS reconstruction driven by a density wave order breaks the lattice symmetry and induces a strong momentum dependence in the self-energy. As the doping is reduced to half-filling, we find that quasiparticle (QP) dispersion becomes essentially unrenormalized, but in sharp contrast the QP spectral weight renormalizes to nearly zero. This opposite doping evolution of the renormalization factors for QP dispersion and spectral weight conspires in such a way that the specific heat remains Fermi liquid like at all dopings in accord with experiments.

Intermediate coupling model of cuprates: Adding fluctuations to a weak coupling model of pseudogap and superconductivity competition

Recent experiments on

Failure of t-J models in describing doping evolution of spectral weight in x-ray scattering, optical, and photoemission spectra of cuprates

{mathrm{Pr}}_{0.89}mathrm{La}{mathrm{Ce}}_{0.11}mathrm{Cu}{mathrm{O}}_{4}