Articles written in Bulletin of Materials Science

    • Exploration of the role of disorder and the behaviour of the surface state in the three-dimensional topological insulator—Bi$_2$Se$_3$


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      Here we present an overview of some of our recent experimental investigation on the high conducting topologically protected surface state properties of a 3D topological insulator (TI), Bi$_2$Se$_3$, in both bulk and single-crystals form. Selenium (Se) vacancies in Bi$_2$Se$_3$ are natural bulk charge dopants, hence these vacancies act as non-magnetic defects in these materials. We use Bi$_2$Se$_3$ material as a prototype of a 3D TI) material with the disorder, for exploring the effects of non-magnetic disorder on the topological conducting surface states. Using a sensitive non-contact mutual inductance-based measurement technique, we identify distinct signatures of surface and bulk contributions to electrical conductivity in a TI Bi$_2$Se$_3$. We show a temperature-dependent transformation from surface to bulk dominated electrical conductivity and unravel the unusual resurfacing of a surface-dominated electrical conductivity at high temperatures. We show that the surface to bulk conductivity transformation is related to a unique activation energy scale ${\Delta}$ in the range of tens of meV, which is smaller than the bulk-insulating gap in Bi$_2$Se$_3$. This gap, we believe is related to a defect state created by the charge dopant Se vacancies in Bi$_2$Se$_3$. We also see interesting effects related to disorder-related coupling of the surface states. The Se vacancies which dope the bulk of the Bi$_2$Se$_3$ crystal lead to an unusual inductive-type coupling of the high conducting 2D-like surface states. The coupling leads to a critical thickness feature in bulk TI crystal, which is distinct from the direct coupling limit in TI’s. We find that if the thickness of the Bi$_2$Se$_3$ crystal exceeds this criticalthickness, then the surface states are no longer inductively coupled. To probe the temperature-related surface to bulk transformation in conductivity of Bi$_2$Se$_3$, we use a high sensitivity magneto-optical imaging technique to directly imagethe distribution of current in single crystal and a thin film of Bi$_2$Se$_3$. At low temperatures, we observed a strong sheet current from the topological surface state. Above 80 K, we report that the emergence of a temperature-dependentin homogeneous, grainy current distribution state in Bi$_2$Se$_3$ single crystals. The grainy state has mixed regions with high and low current densities. The observation of the emergence of a temperature-dependent inhomogeneous phase in the TI suggests the possibility of a disorder-driven spontaneous phase separation scenario.

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