In an ultrathin topological insulator (TI) film, a hybridization gap opens in the TI surface states, and the system is expected to become either a trivial insulator or a quantum spin Hall insulator when the chemical potential is within the hybridization gap. Here we show, however, that these insulating states are destroyed by the presence of a large and long-range-correlated disorder potential, which converts the expected insulator into a metal. We perform transport measurements in ultrathin, dual-gated topological insulator films as a function of temperature, gate voltage, and magnetic field, and we observe a metallic-like, non-quantized conductivity, which exhibits a weak antilocalizationlike cusp at low magnetic field and gives way to a nonsaturating linear magnetoresistance at large field. We explain these results by considering the disordered network of electron-and hole-type puddles induced by charged impurities. We argue theoretically that such disorder can produce an insulator-to-metal transition as a function of increasing disorder strength, and we derive a condition on the band gap and the impurity concentration necessary to observe the insulating state. We also explain the linear magnetoresistance in terms of strong spatial fluctuations of the local conductivity, using both numerical simulations and a theoretical scaling argument.

Nonuniform strain distributions in a graphene lattice can give rise to uniform pseudomagnetic fields and associated pseudo-Landau levels without breaking time-reversal symmetry. We demonstrate that by inducing superconductivity in a nonuniformly strained graphene sheet, the lowest pseudo-Landau levels split by a pairing gap can be inverted by changing the sign of the pairing potential. As a consequence of this inversion, we predict that a Josephson π junction deposited on top of a strained graphene sheet exhibits one-dimensional gapless modes propagating along the junction. These gapless modes mediate single electron tunneling across the junction, giving rise to the 4π-periodic fractional Josephson effect.

We report magnetotransport measurements on magnetically doped (Bi,Sb)2Te3 films grown by molecular beam epitaxy. In Hallbar devices, we observe logarithmic dependence of transport coefficients in temperature and bias voltage which can be understood to arise from electron -electron interaction corrections to the conductivity and self-heating. Submicron scale devices exhibit intriguing quantum oscillations at high magnetic fields with dependence on bias voltage. The observed quantum oscillations can be attributed to bulk and surface transport.

Recent studies predict that LaPtBi is a half-Heusler has multi-functionalities: the superconductivity 1 and topological edge states, a namely topological superconductor under certain conditions 2. It can be stimulated under substantial uniaxial strain conditions. The superconductivity in LaPtBi has been shown in bulk. However, the topological superconductive surface state can not be observed yet. In this study, we report the transmission electron microscopy work for observation of superconductivity and its relationship with a uniaxial strain in MBE grown epitaxial non-centrosymmetric LaPtBi film on MgO (001).