Date of Award

8-2018

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Physics

Committee Chair

Yong P. Chen

Committee Member 1

Gabor Csathy

Committee Member 2

Supriyo Datta

Committee Member 3

Rudro Rana Biswas

Abstract

In this thesis, I will describe our electrical transport approach in studying three-dimensional topological insulators (3D TIs). A 3D TI is a quantum material characterized by a gapped bulk and gapless surface states that are topologically protected by time reversal symmetry, thus immune to backscattering. It provides an excellent platform to pursue a plethora of exotic physics and novel device applications such as spintronics. However, in most of the prototypical 3D TIs (like Bi2Se3, Bi2Te3, and Sb2Te3), the bulk carriers always unavoidably present and overwhelm the surface conduction in transport measurements.

Here, taking advantage of material engineering, we have successfully grown an intrinsic 3D TI crystal, BiSbTeSe2 (BSTS), which has robust topological surface states (TSS) and demonstrated by our experiments to have negligible bulk conduction at low temperatures or even at room temperature in thin samples. In the presence of high perpendicular magnetic field, we have observed well-defined quantum Hall (QH) effect arising from the 2D TSS on flake devices (prepared by the so called “Scotch tape method”) with thickness even above ~100 nm. Furthermore, to have a fully control over the 3D TI samples with both top and bottom surfaces possessing a single species of spin-helical massless Dirac fermions, we fabricated dual-gated BSTS devices with the capability of gating the two surfaces independently. Such a controllable system enables us to explore minimum conductivity σmin at the double Dirac point (when both surfaces are tuned to the Dirac point or charge neutrality point) and two- species (two-component) Dirac fermion QH effect of electron+electron, electron+hole and hole+hole types, involving various combinations of top and bottom surface half-integer filling factors νt and νb.

Later we attempt to push the BSTS sample thickness into thin limit and observe more exotic phenomena. When the sample thickness is lowered to ~10 nm, we find a gap opening at the double Dirac point, presumably due to the hybridization effect between the two surfaces. The surface band diagram then is modified into a massive Dirac dispersion. Such a hybridization gap responses to perpendicular and in-plane magnetic field very differently. Upon applying a perpendicular magnetic field, where orbital effect dominates, we observe a linear splitting and growth of the zeroth Landau level (LL). The realistic band structure of BSTS has to be considered in order to understand such an effect. However an in-plane field tends to drive the system back into a semi-metallic states and gives rise to an large negative magnetoresistance (MR). This observation supports a predicted effect of an in-plane magnetic field to reduce the hybridization gap and eventually restore and split again the Dirac points in the momentum space, inducing a distinct 2D topological semimetal (TSM) phase with 2 single-fold Dirac cones of opposite spin-momentum windings.

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