Sashi Satpathy

Bio: Sashi Satpathy is an academic researcher from University of Missouri. The author has contributed to research in topics: Electronic band structure & Antiferromagnetism. The author has an hindex of 39, co-authored 155 publications receiving 5755 citations. Previous affiliations of Sashi Satpathy include University of Illinois at Urbana–Champaign & Max Planck Society.

Electron states, magnetism, and the Verwey transition in magnetite

Abstract: Using density-functional calculations, we examine the electronic structure of magnetite in the spinel crystal structure in order to gain insight into the nature of the Verwey transition. The calculated cohesive and magnetic properties are in agreement with experimental results. The magnetic structure is analyzed using a Stoner model as well as from calculations within the framework of the local-spin-density approximation to the density-functional theory. The calculations show a minority-spin band at the Fermi energy consisting of ${\mathit{t}}_{2\mathit{g}}$ orbitals on the Fe(B) sublattice. These results suggest a three-band spinless model Hamiltonian for the description of the Verwey transition. The hopping integrals and the electron interaction parameters entering the model Hamiltonian are calculated using the ``constrained'' density-functional theory. The calculated parameters are consistent with the electronic origin of the Verwey transition.

Electronic structure and thermoelectric properties of bismuth telluride and bismuth selenide

Abstract: The electronic structures of the two thermoelectric materials and are studied using density-functional theory with the spin - orbit interaction included. The electron states in the gap region and the chemical bonding can be described in terms of interaction between the atomic p orbitals within the `quintuple' layer. For , we find both the valence-band maximum as well as the conduction-band minimum, each with a nearly isotropic effective mass, to occur at the zone centre in agreement with experimental results. For , we find that the six valleys for the valence-band maximum are located in the mirror planes of the Brillouin zone and they have a highly anisotropic effective mass, leading to an agreement between the de Haas - van Alphen data for the p-doped samples and the calculated Fermi surface. The calculated conduction band, however, has only two minima, instead of the six minima indicated from earlier experiments. The calculated Seebeck coefficients for both p-type and n-type materials are in agreement with the experiments.

Electromagnetic wave propagation in periodic structures: Bloch wave solution of Maxwell's equations

Abstract: We examine the propagation of electromagnetic waves in periodic dielectric structures by solving the vector Maxwell equations with the plane-wave method. Contrary to experimental reports, as well as results of scalar-wave calculations, we do not find a true gap extending throughout the Brillouin zone in the fcc structure. However, there is a depletion in the photon density of states, seemingly a remnant of the Mie resonance, giving rise to a pseudogap in the spectrum that is quite strong for dielectric-sphere packing fraction \ensuremath{\beta}\ensuremath{\sim}0.3--0.4. An effect analogous to the Borrmann effect in x-ray diffraction is predicted, where certain photon modes will propagate an anomalously long distance before getting absorbed.

Calculated effective Hamiltonian for La2CuO4 and solution in the impurity Anderson approximation.

Abstract: We report local-density-functional calculations of hybridization matrix elements and effective electron-electron interactions in ${\mathrm{La}}_{2}$Cu${\mathrm{O}}_{4}$ defining a general effective Hamiltonian that we propose as an appropriate starting point for many-body calculations in this material. The parameter values lend support to an Anderson lattice model. We find the impurity approximation to this model yields a magnetic ground state of ${x}^{2}\ensuremath{-}{y}^{2}$ symmetry, a 1-2-eV insulating gap bounded by ionization and affinity levels of the same symmetry, and a calculated $d$ spectral weight in qualitative agreement with photoemission experiments. We discuss anticipated modification of these results by lattice effects.

Origin of the two-dimensional electron gas carrier density at the LaAlO3 on SrTiO3 interface.

Abstract: Transport measurements of the two-dimensional electron gas at the LaAlO3-SrTiO3 interface have found a density of carriers much lower than expected from the "polar catastrophe" arguments From a detail density-functional study, we suggest how this discrepancy may be reconciled We find that electrons occupy multiple subbands at the interface leading to a rich array of transport properties Some electrons are confined to a single interfacial layer and susceptible to localization, while others with small masses and extended over several layers are expected to contribute to transport

I and i

Abstract: There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality. Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

Solid C60: a new form of carbon

Abstract: A new form of pure, solid carbon has been synthesized consisting of a somewhat disordered hexagonal close packing of soccer-ball-shaped C60 molecules. Infrared spectra and X-ray diffraction studies of the molecular packing confirm that the molecules have the anticipated 'fullerene' structure. Mass spectroscopy shows that the C70 molecule is present at levels of a few per cent. The solid-state and molecular properties of C60 and its possible role in interstellar space can now be studied in detail.

Two-Dimensional Nanosheets Produced by Liquid Exfoliation of Layered Materials

Abstract: If they could be easily exfoliated, layered materials would become a diverse source of two-dimensional crystals whose properties would be useful in applications ranging from electronics to energy storage. We show that layered compounds such as MoS2, WS2, MoSe2, MoTe2, TaSe2, NbSe2, NiTe2, BN, and Bi2Te3 can be efficiently dispersed in common solvents and can be deposited as individual flakes or formed into films. Electron microscopy strongly suggests that the material is exfoliated into individual layers. By blending this material with suspensions of other nanomaterials or polymer solutions, we can prepare hybrid dispersions or composites, which can be cast into films. We show that WS2 and MoS2 effectively reinforce polymers, whereas WS2/carbon nanotube hybrid films have high conductivity, leading to promising thermoelectric properties.

Topological insulators in Bi2Se3, Bi2Te3 and Sb2Te3 with a single Dirac cone on the surface

Abstract: Topological insulators are new states of quantum matter in which surface states residing in the bulk insulating gap of such systems are protected by time-reversal symmetry. The study of such states was originally inspired by the robustness to scattering of conducting edge states in quantum Hall systems. Recently, such analogies have resulted in the discovery of topologically protected states in two-dimensional and three-dimensional band insulators with large spin–orbit coupling. So far, the only known three-dimensional topological insulator is BixSb1−x, which is an alloy with complex surface states. Here, we present the results of first-principles electronic structure calculations of the layered, stoichiometric crystals Sb2Te3, Sb2Se3, Bi2Te3 and Bi2Se3. Our calculations predict that Sb2Te3, Bi2Te3 and Bi2Se3 are topological insulators, whereas Sb2Se3 is not. These topological insulators have robust and simple surface states consisting of a single Dirac cone at the Γ point. In addition, we predict that Bi2Se3 has a topologically non-trivial energy gap of 0.3 eV, which is larger than the energy scale of room temperature. We further present a simple and unified continuum model that captures the salient topological features of this class of materials. First-principles calculations predict that Bi2Se3, Bi2Te3 and Sb2Te3 are topological insulators—three-dimensional semiconductors with unusual surface states generated by spin–orbit coupling—whose surface states are described by a single gapless Dirac cone. The calculations further predict that Bi2Se3 has a non-trivial energy gap larger than the energy scale kBT at room temperature.

Topological insulators with inversion symmetry

Abstract: Topological insulators are materials with a bulk excitation gap generated by the spin-orbit interaction that are different from conventional insulators. This distinction is characterized by ${Z}_{2}$ topological invariants, which characterize the ground state. In two dimensions, there is a single ${Z}_{2}$ invariant that distinguishes the ordinary insulator from the quantum spin-Hall phase. In three dimensions, there are four ${Z}_{2}$ invariants that distinguish the ordinary insulator from ``weak'' and ``strong'' topological insulators. These phases are characterized by the presence of gapless surface (or edge) states. In the two-dimensional quantum spin-Hall phase and the three-dimensional strong topological insulator, these states are robust and are insensitive to weak disorder and interactions. In this paper, we show that the presence of inversion symmetry greatly simplifies the problem of evaluating the ${Z}_{2}$ invariants. We show that the invariants can be determined from the knowledge of the parity of the occupied Bloch wave functions at the time-reversal invariant points in the Brillouin zone. Using this approach, we predict a number of specific materials that are strong topological insulators, including the semiconducting alloy ${\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Sb}}_{x}$ as well as $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Sn}$ and HgTe under uniaxial strain. This paper also includes an expanded discussion of our formulation of the topological insulators in both two and three dimensions, as well as implications for experiments.