Possible Multiple Gap Superconductivity with Line Nodes in Heavily Hole-Doped Superconductor KFe2As2 Studied by 75As-NQR and Specific Heat

TL;DR

This study investigates the superconducting properties of KFe2As2 using 75As-NQR and specific heat measurements, revealing evidence for multiple gap superconductivity with line nodes rather than a fully gapped s_-wave state.

Contribution

It provides experimental evidence supporting the existence of multiple nodal gaps in heavily hole-doped KFe2As2, challenging the simple s_-wave model.

Findings

Gradual temperature dependence of 1/T1 without coherence peak

Small specific heat jump below Tc

Evidence of low-energy quasiparticle excitations

Abstract

We report the 75As nuclear quadrupole resonance (NQR) and specific heat measurements of the heavily hole-doped superconductor KFe2As2 (Tc = 3.5 K). The spin-lattice relaxation rate 1/T1 in the superconducting state exhibits quite gradual temperature dependence with no coherence peak below Tc. The quasi-particle specific heat C_QP/T shows small specific heat jump which is about 30% of electronic specific heat coefficient just below Tc. In addition, it suggests the existence of low-energy quasi-particle excitation at the lowest measurement temperature T = 0.4 K \simeq Tc/10. These temperature dependence of 1/T1 and C_QP/T can be explained by multiple nodal superconducting gap scenario rather than multiple fully-gapped s_\pm-wave one within simple gap analysis.