Excitonic and Nematic Instabilities on the Surface of Topological Kondo Insulators

TL;DR

This paper investigates how strong electron-electron interactions on the surface of cubic topological Kondo insulators can lead to nematic and excitonic phases, explaining recent experimental observations in SmB$_6$ and related materials.

Contribution

It provides a detailed theoretical analysis of interaction-driven surface instabilities, including phase diagrams and connections to experimental data in topological Kondo insulators.

Findings

Identification of nematic and excitonic phases on the surface

Explanation of two-fold symmetric magnetoresistance in SmB$_6$

Phase diagrams derived from Ginzburg-Landau and Ashkin-Teller models

Abstract

We study the effects of strong electron-electron interactions on the surface of cubic topological Kondo insulators (such as samarium hexaboride, SmB$_6$). Cubic topological Kondo insulators generally support three copies of massless Dirac nodes on the surface, but only two of them are energetically degenerate and exhibit an energy offset relative to the third one. With a tunable chemical potential, when the surface states host electron and hole pockets of comparable size, strong interactions may drive this system into rotational symmetry breaking nematic and translational symmetric breaking excitonic spin- or charge-density-wave phases, depending on the relative chirality of the Dirac cones. Taking a realistic surface band structure into account we analyze the associated Ginzburg-Landau theory and compute the mean field phase diagram for interacting surface states. Beyond mean field theory, this system can be described by a two-component isotropic Ashkin-Teller model at finite temperature, and we outline the phase diagram of this model. Our theory provides a possible explanation of recent measurements which detect a two-fold symmetric magnetoresistance and an upturn in surface resistivity with tunable gate voltage in SmB$_6$. Our discussion can also be germane to other cubic topological insulators, such as ytterbium hexaboride (YbB$_6$), plutonium hexaboride (PuB$_6$).