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

Abstract

We consider the low-temperature thermal transport properties of the two-dimensional (2D) proximity-induced superconducting state formed at the interface between a three-dimensional strong topological insulator (TI) and a d-wave superconductor (dSC). This system is a playground for studying massless Dirac fermions, because 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 μ is tuned from above the impurity scattering rate (|μ|≫Γ0) to below (|μ|≪Γ0). We calculate, via Kubo formula, the universal-limit (T→0) thermal conductivity κ0 as a function of μ as it is tuned through this transition. In the large- and small-|μ| limits, we obtain disorder-independent, closed-form expressions for κ0/T. The large-|μ| 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 |μ| illustrate the nature of the transition between these limits, which is shown to depend on disorder in a well-defined manner.

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