Low temperature thermal transport at the interface of a topological insulator and a d-wave superconductor

TL;DR

This paper investigates how the low-temperature thermal conductivity at the interface of a topological insulator and a d-wave superconductor varies with chemical potential, revealing a transition from a Dirac fermion state to a conventional superconductor.

Contribution

It provides analytical and numerical analysis of the universal thermal conductivity across the transition, highlighting the impact of disorder and the unique properties of topological surface states.

Findings

Thermal conductivity is half of that in a d-wave superconductor at large chemical potential.

The transition in thermal transport depends on disorder and chemical potential tuning.

Analytical expressions for thermal conductivity are derived in both large and small chemical potential limits.

Abstract

We consider the low-temperature thermal transport properties of the 2D proximity-induced superconducting state formed at the interface between a 3D strong topological insulator (TI) and a d-wave superconductor (dSC). This system is a playground for studying massless Dirac fermions, as they enter both as quasiparticles of the dSC and as surface states of the TI. For TI surface states with a single Dirac point, the four nodes in the interface-state quasiparticle excitation spectrum coalesce into a single node as the chemical potential, $\mu$, is tuned from above the impurity scattering rate ($|\mu| \gg \Gamma_0$) to below ($|\mu| \ll \Gamma_0$). We calculate, via Kubo formula, the universal limit ($T \rightarrow 0$) thermal conductivity, $\kappa_0$, as a function of $\mu$, as it is tuned through this transition. In the large and small $|\mu|$ limits, we obtain disorder-independent, closed-form expressions for $\kappa_0/T$. The large-$|\mu|$ expression is exactly half the value expected for a d-wave superconductor, a demonstration of the sense in which the TI surface topological metal is half of an ordinary 2D electron gas. Our numerical results for intermediate $|\mu|$ illustrate the nature of the transition between these limits, which is shown to depend on disorder in a well-defined manner.