Transport Properties of Topological insulators

Gopal, Radha Krishna (2016) Transport Properties of Topological insulators. PhD thesis, Indian Institute of Science Education and Research Kolkata.

PDF (PhD thesis of Radha Krishna Gopal) Thesis_Radha_Krishna_Gopal.pdf - Submitted Version Restricted to Repository staff only Download (3MB)

Abstract

Topological insulators, a distinct state of quantum matter are insulating in the bulk and conducting on the surface, with unique counter propagating spin momentum locked surface Dirac states. These surface states of a topological insulator (TI) preserve time reversal symmetry. In this thesis, we studied magneto-transport studies of BST thin films, one of the recently discovered ternary tetradymite based topological insulators (M₂X₂Y where M= Bi; X and Y = Se or Te). To the best of our knowledge this is the first time BST thin films have been fabricated using Pulsed Laser Deposition (PLD). This material exhibits property very similar to its parent compounds in terms of single Dirac cone but differ in terms of chemical potential which lies in the bulk band gap in these compounds thereby making them better TI candidates. Magneto-transport data (conductance as a function of field) obtained at different temperatures for these thin films exhibits archetypal weak anti-localization (WAL) character in the low temperature regime which is a hallmark of topologically protected surface states with an odd number of Dirac cones and hence signifies a surface dominant transport of charge carriers. Pulsed laser deposition (PLD) has the advantage of maintaining stoichiometry of the target composition in the thin films since it is a non-equilibrium process and is capable of forming samples which may not otherwise form through conventional routes including single crystal growth techniques. This is especially useful since we deposited a multi-element compound such as Bi2Se2Te (BST) and for PLD grown films the stoichiometry is preserved during deposition in addition to maintaining a high deposition rate in comparison to other similar techniques like sputtering or molecular beam epitaxy. PLD grown films also have an added advantage that apart from being inexpensive it offers a unique opportunity of depositing multilayers of TI with ready integration with superconductor or magnetic materials for devices applications. For better understanding of the intricacies of the surface and bulk conduction in topological insulator thin films a nonlinear optical study could be a useful tool. Hence, we studied Nonlinear optical absorption of Bi2Se3 deposited on quartz substrate (which is transparent) using open aperture Z-scan technique. Saturable absorption (SA) is a phenomena that is observed for the given sample which has also been observed earlier in Dirac fermionic systems like Graphene. But contrary to graphene TI consists of both the bulk and surface states. Hence, which of the two is responsible for this saturable absorption is an interesting problem to address not only for fundamental understanding of TI but also for technical advancement in the field of optics. Our work further extends this study by magnetic doping of the sample and repeating the experiment which yields some interesting results that can settle this ongoing debate on the role of bulk or surface states for SA phenomena. We conclude that the bulk is responsible for SA in TI.

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