Specific heat of Ba0.68K0.32Fe2As2: evidence for multiband strong-coupling superconductivity

TL;DR

This study reveals multiband strong-coupling superconductivity in Ba0.68K0.32Fe2As2, characterized by two superconducting gaps, high coupling strength, and consistency with an Eliashberg model involving multiple bands.

Contribution

The paper provides experimental evidence for multiband strong-coupling superconductivity in Ba0.68K0.32Fe2As2 and demonstrates the applicability of a two-band Eliashberg model.

Findings

Identification of two superconducting gaps with different magnitudes.

Evidence of strong-coupling superconductivity from specific heat measurements.

The Eliashberg model accurately reproduces critical temperature and gap values.

Abstract

The specific heat of high-purity $\rm Ba_{0.68}K_{0.32}Fe_2As_2$ single crystals with the highest reported superconducting $T_c$ = 38.5 K was studied. The electronic specific heat, $C_p$, below $T_c$ shows two gap features, with $\Delta_1 \approx 11$ meV and $\Delta_2 \approx 3.5$ meV obtained from an $\alpha$-model analysis. The reduced gap value, $2\Delta^{\rm max} / k_B T_c \approx 6.6$, the magnitude of the specific heat jump, $\Delta C_p(T_c)/T_c$, and its slope below $T_c$ exhibit strong-coupling character. We also show that an Eliashberg model with two hole and two electron bands gives the correct values of $T_c$, the superconducting gaps, and the temperature dependence of the free-energy difference.