Electrical transport in 3-dimensional topological insulators

Abstract

Topological insulators (TI) are a new class of materials in which gapped bulk states coexist with gapless surface-states having linear energy-momentum dispersion relation protected by time-reversal symmetry. These surface-states are the consequence of the closing of the bulk gap at an interface as a result of the change of the non-trivial topology of the Hilbert space spanned by the wave function describing the insulator to a trivial one at the boundary of the material. The strong spin-orbit coupling of the bulk material implies that Dirac fermions at the interface have helical spin polarization - the spin degeneracy of the Dirac fermions is lifted, and the spin becomes transversely locked to the crystal momentum. This leads to exciting possibilities, chief among them being the existence of Majorana modes in the proximity-induced superconducting state. This and the possible applications in spintronics has led the search for new robust TI materials. We report transport studies performed on three types of 3D topological insulators, (1) Kondo topological insulator SmB6 in bulk single-crystal form, (2) mechanically exfoliated BiSbTeSe2 films, and (c) pulse laser deposited BiSbTeSe1.6 thin films. We developed a measurement scheme, namely a local-nonlocal measurement scheme, which, as we show, is an excellent probe for deconvoluting the contribution of surface-states to transport from that of the bulk. Under a perpendicular magnetic field, we find weak antilocalization, which is indicative of spin-orbit coupling in the system. We see the surface-states for the TI systems to be robust under magnetic fields as high as 16T. We performed low-frequency 1/f noise measurements in both local, and nonlocal configurations. We find that in the surface dominant transport regime, the noise in these TIs arises from universal conductance fluctuations. Temperature-dependent resistance and resistance fluctuation studies allowed us to extract information about the band structure of both SmB6 and BiSbTeSe systems. Comparative analysis of the high-quality films of BiSbTeSe2 and disordered PLD-grown BiSbTeSe1.6 films establish PLD as a viable route to develop high-quality topological insulator materials.