Heat transport as a probe of superconducting gap structure

TL;DR

This paper discusses how low-temperature thermal conductivity measurements can reveal the presence and nature of nodes in the superconducting gap, providing insights into the pairing symmetry and mechanisms.

Contribution

It introduces a method to identify and distinguish different gap structures in superconductors using thermal conductivity data, especially at very low temperatures.

Findings

Finite residual linear term indicates symmetry-imposed nodes.

Impurity scattering dependence distinguishes line versus point nodes.

Magnetic field dependence probes low-energy quasiparticles.

Abstract

The structure of the superconducting gap provides important clues on the symmetry of the order parameter and the pairing mechanism. The presence of nodes in the gap function imposed by symmetry implies an unconventional order parameter, other than s-wave. Here we show how measurements of the thermal conductivity at very low temperature can be used to determine whether such nodes are present in a particular superconductor, and shed light on their nature and location. We focus on the residual linear term at T goes to 0. A finite value in zero magnetic field is strong evidence for symmetry-imposed nodes, and the dependence on impurity scattering can distinguish between a line of nodes or point nodes. Application of a magnetic field probes the low-energy quasiparticle excitations, whether associated with nodes or with a small value of the gap on some part of the Fermi surface, as in a multi-band superconductor. We frame our discussion around archetypal materials: Nb for s-wave, Tl2Ba2CuO(6+\delta) for d-wave, Sr2RuO4 for p-wave, and NbSe2 for multi-band superconductivity. In that framework, we discuss three heavy-fermion superconductors: CeIrIn5, CeCoIn5 and UPt3.