S-wave Superconductivity in the Dirac Line-nodal Material CaSb2

TL;DR

This study uses nuclear quadrupole resonance to reveal that CaSb2 exhibits conventional s-wave superconductivity with a full gap, despite its line-nodal electronic structure, indicating a complex interplay between normal and superconducting states.

Contribution

It provides microscopic evidence of s-wave superconductivity in CaSb2, a material with line-nodal electronic structure, challenging previous assumptions about its pairing symmetry.

Findings

Normal state shows Fermi liquid behavior in 1/T1.

Superconducting state exhibits a coherence peak and exponential decay in 1/T1.

CaSb2 is confirmed to have s-wave full-gap superconductivity.

Abstract

We performed 121/123Sb-nuclear quadrupole resonance (NQR) measurements on the superconducting (SC) line-nodal material CaSb2 in order to investigate electronic properties in the normal and SC states from a microscopic point of view. In the normal state, the nuclear spin-lattice relaxation rate 1/T1 for the Sb(1) site, which is responsible for the line-nodal parts, is approximately proportional to temperature, indicating the conventional Fermi liquid state. From comparison with band structure calculations, it is considered that the NQR properties related to the line-nodal character are hidden because the conventional behavior originating from Fermi-surface parts away from the nodes is dominant. In the SC state, a clear coherence peak just below the transition temperature and an exponential decrease at lower temperatures were observed in 1/T1. These results strongly suggest that conventional s-wave superconductivity with a full gap is realized in CaSb2.