Experimental consequences of Bogoliubov Fermi surfaces

TL;DR

This paper explores the experimental signatures of Bogoliubov Fermi surfaces in multiband superconductors with broken time-reversal symmetry, providing theoretical predictions for various measurable low-temperature properties.

Contribution

It derives the low-energy density of states and calculates key observables to aid in experimental detection of Bogoliubov Fermi surfaces in superconductors.

Findings

Derived the low-energy density of states for various nodal structures.

Calculated temperature-dependent behaviors of tunneling, specific heat, and other observables.

Found no topologically protected surface states associated with the Fermi surfaces.

Abstract

Superconductors involving electrons with internal degrees of freedom beyond spin can have internally anisotropic pairing states that are impossible in single-band superconductors. As a case in point, in even-parity multiband superconductors that break time-reversal symmetry, nodes of the superconducting gap are generically inflated into two-dimensional Bogoliubov Fermi surfaces. The detection and characterization of these quasiparticle Fermi surfaces requires the understanding of their experimental consequences. In this paper, we derive the low-energy density of states for a broad range of possible nodal structures. Based on this, we calculate the low-temperature form of observables that are commonly employed for the characterization of nodal superconductors, i.e., the single-particle tunneling rate, the electronic specific heat and Sommerfeld coefficient, the thermal conductivity, the magnetic penetration depth, and the NMR spin-lattice relaxation rate, in the clean limit. We also address the question whether the topological invariant of the Bogoliubov Fermi surfaces is associated with topologically protected surface states, with negative results. This work is meant to serve as a guide for experimental searches for Bogoliubov Fermi surfaces in time-reversal-symmetry-breaking superconductors.