ECE Seminar Series Fall 2015
Thursday November 13th 1-2 PM, ITEB 336
The Emergence of Topological Insulators as Candidates for Optoelectronics and Spin-Based Applications
Abstract: The advent of topological ideas in condensed matter is a new paradigm where the traditional notions of Fermi-liquid theory and order parameter do not explain experimentally observed phenomena, for instance, the integer and fractional quantum Hall effect and the more recently discovered topological insulators (TI). In this presentation, I will focus on topological insulators and go over some of the key facts that typically characterize these materials. I will begin by presenting analytic results on band structure of topological insulators derived using a simple Dirac Hamiltonian and connect them to more elaborate semi-empirical tight binding and continuum k.p calculations. An important aspect of TI band structure is the helical dispersion where the spin is locked perpendicularly to momentum giving rise to 1) spin-polarized photocurrents when the surface is illuminated with circularly-polarized light and 2) spin-dependent optical transition from valence to conduction surface bands. Using the phenomenon of circular dichroism (preferential absorption of right- or left-circularly polarized light), I will emphasize on light absorption on the surface of 3D TIs such as Bi2Se3 with a single Dirac cone and contrast them with the C2v group symmetric (at X symmetry point) triple Dirac-coned topological Kondo insulator (TKI) samarium hexaboride (SmB6). I will explicitly show how the helical band structure of SmB6 at the X symmetry point of the surface Brillouin zone with Rashba- and Dresselhaus-like terms give rise to a dual-valued circular dichroism. Further, using the Berry curvature I will try to draw a parallel between the emerging field of valleytronics in transition metal dichalcogenides such as MoS2 and TIs that conform to the C2v symmetry. I will seek to emphasize that the strong polarization-dependent light absorbance on account of the Dirac fermions on surface of a TI and an easily tunable surface band gap can lead to design of optoelectronic devices with greater efficiency. Since spin and its myriad manifestations in solid state is crucial to topological insulators, I will present results on the frequency-dependent spin susceptibility in the TKI SmB6 which may prove useful to probe spin density currents that serve as the foundational block of spin-based applications. In the last part of the talk I will draw attention to the fact that while most topological insulators are strongly spin-orbit coupled driven, the multi-layered Dirac semi-metal black phosphorus which is a 2D material undergoes a giant Stark effect induced topological phase transition when doped with potassium. The topological features of BP with Dirac fermions and a linear band structure can lead to graphene-like transport properties for improved device performance. I will map the dynamic optical conductivity and spin current density changes to the transitions of BP from a trivial insulator with finite band gap to a zero gap material and then to a topological insulator with increasing dopant (K) density.
Short Bio: Parijat Sengupta received his PhD in electrical engineering from Purdue University, West Lafayette in December 2013 where he primarily worked on the electronic structure of materials and focused on topological insulators for his dissertation. Following his PhD, he joined the computational materials group at the University of Wisconsin-Madison, Madison in January 2014 as a postdoctoral research associate and was involved in modeling of defects in nuclear materials using ab-initio principles and molecular dynamics. He moved to the Photonics Center at Boston University in March of 2015 as a postdoctoral research associate and is currently working on light-matter interaction and spin transport in topological insulators and electron transport in colloidal quantum dots. Prior to joining Purdue university, he was employed with Nvidia Corp., Santa Clara as a product engineer.