Interaction-induced criticality in Z_2 topological insulators

TL;DR

This paper predicts a new universal critical state in topological insulators caused by the interplay of topology and Coulomb interactions, which influences phase transitions and surface conductivity.

Contribution

It introduces the concept of a self-organized quantum criticality state emerging from interactions in topological insulators, a novel phenomenon in disordered systems.

Findings

Discovery of a universal critical state with fixed electrical conductivity

Prediction of a direct quantum spin Hall phase transition via this critical state

Emergence of the critical state on 3D topological insulator surfaces without adjustable parameters

Abstract

Critical phenomena and quantum phase transitions are paradigmatic concepts in modern condensed matter physics. A central example in the field of mesoscopic physics is the localization-delocalization (metal-insulator) quantum phase transition driven by disorder -- the Anderson transition. Although the notion of localization has appeared half a century ago, this field is still full of surprising new developments. The most recent arenas where novel peculiar localization phenomena have been studied are graphene and topological insulators, i.e., bulk insulators with delocalized (topologically protected) states on their surface. Besides exciting physical properties, the topological protection renders such systems promising candidates for a variety of prospective electronic and spintronic devices. It is thus of crucial importance to understand properties of boundary metallic modes in the realistic systems when both disorder and interaction are present. Here we find a novel critical state which emerges in the bulk of two-dimensional quantum spin Hall (QSH) systems and on the surface of three-dimensional topological insulators with strong spin-orbit interaction due to the interplay of nontrivial Z_2 topology and the Coulomb repulsion. At low temperatures, this state possesses a universal value of electrical conductivity. In particular, we predict that the direct QSH phase transition occurs via this novel state. Remarkably, the interaction-induced critical state emerges on the surface of a three-dimensional topological insulator without any adjustable parameters. This ``self-organized quantum criticality'' is a novel concept in the field of interacting disordered systems.