Probing Bulk Superconducting Order Parameter in Ba(K)Fe$_2$As$_2$ by Four Complementary Techniques

TL;DR

This study uses four experimental techniques to comprehensively analyze the bulk superconducting properties of Ba(K)Fe₂As₂, revealing two nodeless gaps with consistent data indicating extended s-wave symmetry and larger-than-BCS gap ratios.

Contribution

It provides a comprehensive multi-technique analysis of the superconducting gaps and symmetry in Ba(K)Fe₂As₂, confirming the presence of two nodeless gaps with extended s-wave symmetry.

Findings

Presence of at least two nodeless superconducting gaps.

Quantitative consistency across four different measurement techniques.

Large gap ratio exceeds the BCS value, indicating strong coupling.

Abstract

Using four different experimental techniques, we performed comprehensive studies of the bulk superconductive properties of single crystals of the nearly optimally doped $Ba_{1-x}K_xFe_2As_2$ ($T_{c} \approx 36\,K$), a typical representative of the 122 family. We investigated temperature dependencies of the (i) specific heat $C_{el}(T)$, (ii) first critical magnetic field $H_{c1}(T)$, (iii) intrinsic multiple Andreev reflection effect (IMARE), and (iv) infrared reflectivity spectra. All data clearly show the presence of (at least) two superconducting nodeless gaps. The quantitative data on the superconducting spectrum obtained by four different techniques are consistent with each other: (a) the small energy gap $\Delta_S(0) \approx 1.8 - 2.5\,meV$, and the large gap energy $\Delta_L(0) \approx 9.5 - 11.3\,meV$ that demonstrates the signature of an extended s-wave symmetry ($\sim~33 \%$ in-plane anisotropy), (b) the characteristic ratio $2\Delta_L/k_BT_C$ noticeably exceeds the BCS value.