Fully Gapped Superconducting State Based on a High Normal State Quasiparticle Density of States in Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ Single Crystals

TL;DR

This study reveals a fully gapped superconducting state in Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ with a high normal state quasiparticle density of states, based on specific heat measurements indicating a dominant s-wave gap.

Contribution

The paper provides detailed specific heat data showing a fully gapped superconducting state with a high quasiparticle density of states, and identifies a dominant s-wave gap in this iron-based superconductor.

Findings

Electronic specific heat coefficient is temperature independent.

Superconducting state is fully gapped with a dominant s-wave gap.

High normal state quasiparticle density of states (~63 mJ/mol K^2).

Abstract

We report the specific heat (SH) measurements on single crystals of hole doped $FeAs$-based superconductor $Ba_{0.6}K_{0.4}Fe_2As_2$. It is found that the electronic SH coefficient $\gamma_e(T)$ is not temperature dependent and increases almost linearly with the magnetic field in low temperature region. These point to a fully gapped superconducting state. Surprisingly the sharp SH anomaly $\Delta C/T|_{T_c}$ reaches a value of 98 $mJ/mol K^2$ suggesting a very high normal state quasiparticle density of states ($\gamma_n \approx 63 mJ/mol K^2$). A detailed analysis reveals that the $\gamma_e(T)$ cannot be fitted with a single gap of s-wave symmetry due to the presence of a hump in the middle temperature region. However, our data indicate that the dominant part of the superconducting condensate is induced by an s-wave gap with the magnitude of about 6 meV.