Nodeless superconductivity in the topological nodal-line semimetal CaSb2

TL;DR

This study reveals that CaSb2, a topological nodal-line semimetal, exhibits nodeless, two-gap s-wave superconductivity below 1.6 K, highlighting its potential for exploring topological superconducting states.

Contribution

First direct evidence of nodeless, two-gap s-wave superconductivity in CaSb2, a topological nodal-line semimetal, using magnetic penetration depth and specific heat measurements.

Findings

CaSb2 exhibits exponential behavior in penetration depth, indicating nodeless gaps.

Superfluid density and specific heat fit a two-gap s-wave model.

Superconductivity is fully gapped with multiple Fermi surfaces.

Abstract

CaSb2 is a topological nodal-line semimetal that becomes superconducting below 1.6 K, providing an ideal platform to investigate the interplay between topologically nontrivial electronic bands and superconductivity. In this work, we investigated the superconducting order parameter of CaSb2 by measuring its magnetic penetration depth change {\Delta}{\lambda}(T) down to 0.07 K, using a tunneling diode oscillator (TDO) based technique. Well inside the superconducting state, {\Delta}{\lambda}(T) shows an exponential activated behavior, and provides direct evidence for a nodeless superconducting gap. By analyzing the temperature dependence of the superfluid density and the electronic specific heat, we find both can be consistently described by a two-gap s-wave model, in line with the presence of multiple Fermi surfaces associated with distinct Sb sites in this compound. These results demonstrate fully-gapped superconductivity in CaSb2 and constrain the allowed pairing symmetry.