Gapless chiral excitons in thin films of topological insulators

Department of Physics, Sharif University of Technology - Tehran 11155-9161, Iran, Center of Excellence for Complex Systems and Condensed Matter (CSCM), Sharif University of Technology Tehran 145888-9694, Iran and School of Physics, Institute for Research in Fundamental Sciences - Tehran 19395-5531, Iran

^(a) jafari@physics.sharif.edu

Received: 21 May 2014
Accepted: 23 June 2014

Abstract

In a nanoscopic thin film of a strong topological insulator (TI) the Coulomb interaction in the channel that exchanges the two electrons with the same chirality in two different planes of the slab takes advantage of the minus sign resulting from such “exchange” and gives rise to a bound state between the positive-energy states in one surface and the negative-energy states in the opposite surface. Therefore particle and hole pairs in the undoped Dirac cone of the TI thin film form an inter-surface spin-singlet state that lies below the continuum of free particle-hole pairs. This mode is similar to the excitons of semiconductors, albeit formed between the electron and hole pairs from two different two-dimensional surfaces. For low momenta the dispersion relation characterizing this collective mode is linear. Experimental comparison of two slabs with different thicknesses can capture the exponential dependence of the present effect on the slab thickness.