K. B. Chashka

Proximity-induced superconductivity in topological Bi2Te2Se and Bi2Se3films: Robust zero-energy bound state possibly due to Majorana fermions

Point contact conductance measurements on topological

Evolution of the Fermi surface of a doped topological insulator with carrier concentration

Bi_2Te_2Se

Pulsed versus dc I-V characteristics of resistive manganites

and

Electronic transport in MgB2, AlB2 and ZrB2--a comparative study

Bi_2Se_3

Tuning across the BCS-BEC crossover in the multiband superconductor Fe1+ySexTe1−x: An angle-resolved photoemission study

films reveal a signature of superconductivity below 2-3 K. In particular, critical current dips and a robust zero bias conductance peak are observed. The latter suggests the presence of zero energy bound states which could be assigned to Majorana Fermions in an unconventional topological superconductor. We attribute these novel observations to proximity induced local superconductivity in the films by small amounts of superconducting Bi inclusions or segregation to the surface, and provide supportive evidence for these effects.

Variable range hopping in A2MnReO6 (A=Ca, Sr, Ba)

In an ideal bulk topological-insulator (TI) conducting surface states protected by time reversal symmetry enfold an insulating crystal. However, the archetypical TI, Bi2Se3, is actually never insulating; it is in fact a relatively good metal. Nevertheless, it is the most studied system among all the TIs, mainly due to its simple band-structure and large spin-orbit gap. Recently it was shown that copper intercalated Bi2Se3 becomes superconducting and it was suggested as a realization of a topological superconductor (TSC). Here we use a combination of techniques that are sensitive to the shape of the Fermi surface (FS): the Shubnikov-de Haas (SdH) effect and angle resolved photoemission spectroscopy (ARPES) to study the evolution of the FS shape with carrier concentration, n. We find that as n increases, the FS becomes 2D-like. These results are of crucial importance for understanding the superconducting properties of CuxBi2Se3.

Gapless excitations in the ground state of 1T−TaS2

We report on pulsed and dc I-V characteristics of polycrystalline samples of three charge-ordered manganites, Pr2∕3Ca1∕3MnO3, Pr1∕2Ca1∕2MnO3, and Bi1∕2Sr1∕2MnO3, and of a double perovskite, Sr2MnReO6, in a temperature range where their Ohmic resistivity obeys the Efros-Shklovskii variable range hopping relation [J. Phys. C 8, L49 (1975)]. For all samples, the dc I(V) exhibits at high currents negative differential resistance and hysteresis, which mask a perfectly Ohmic or a moderately non-Ohmic conductivity obtained by pulsed measurements. This demonstrates that the widely used dc I-V measurements are usually misleading.

V3O5: Insulator-metal transition and electric-field-induced resistive-switching

Abstract We report on the temperature dependence of the resistivity ( ρ ) and the absolute thermopower ( S ) of the polycrystalline title materials and of AlB 2 single crystals. For all samples ρ ( T ) exhibits a Bloch–Gruneisen-like temperature dependence, with large characteristic temperatures θ R (≈ θ D ––the Debye temperature). At high temperatures the thermopower S ( T ) for ZrB 2 (n-type) is almost the mirror image of S ( T ) for MgB 2 (p-type) while S (AlB 2 ) is very small for all temperatures. The density of states distribution N ( E ) around E F seems to play a dominant role in determining S ( T ) of these materials. ln( T ) terms in the low-temperature ρ ( T ) and S ( T ) of ZrB 2 samples bear evidence for weak localization in 2D.

Internal pressure in superconducting Cu-intercalatedBi2Se3

A BCS-BEC crossover in a multiband superconductor has been observed experimentally using ARPES. The crossover from Bardeen-Cooper-Schrieffer (BCS) superconductivity to Bose-Einstein condensation (BEC) is difficult to realize in quantum materials because, unlike in ultracold atoms, one cannot tune the pairing interaction. We realize the BCS-BEC crossover in a nearly compensated semimetal, Fe1+ySexTe1−x, by tuning the Fermi energy εF via chemical doping, which permits us to systematically change Δ/εF from 0.16 to 0.50, where Δ is the superconducting (SC) gap. We use angle-resolved photoemission spectroscopy to measure the Fermi energy, the SC gap, and characteristic changes in the SC state electronic dispersion as the system evolves from a BCS to a BEC regime. Our results raise important questions about the crossover in multiband superconductors, which go beyond those addressed in the context of cold atoms.

Metal-insulator transition upon heating and negative-differential-resistive-switching induced by self-heating in BaCo0.9Ni0.1S1.8

We report on transport measurements on polycrystalline samples of A2MnReO6 (A=Ca, Sr, Ba), CMRO, SMRO, and BMRO, in the Ohmic and non-Ohmic regimes. The resistivity follows, over wide ranges of temperatures, the relation derived for variable range hopping of carriers localized in a parabolic gap, ρ=ρo exp(To/T)1/2. The values of To are unusually high for the Efros Shklovskii Coulomb gap interpretation. The thermopower is large and positive for SMRO and BMRO and even larger and negative for CMRO. The magnitude of the thermopower and its temperature dependence indicate that it may be unrelated to the hopping conductivity. The theoretical relations for hopping in the intermediate electric field regime provide reasonable approximations for the measured nonlinear conductivity of most samples (and for one sample also for the high-field regime). However, localization lengths derived from the non-Ohmic regime are much larger than those estimated from To. Clues to settle these inconsistencies are suggested.