Nodeless energy gaps of single-crystalline Ba0.68K0.32Fe2As2 as seen via 75As NMR

TL;DR

This study uses $^{75}$As NMR to reveal that single-crystalline Ba$_{0.68}$K$_{0.32}$Fe$_{2}$As$_{2}$ has a nodeless energy gap, with anisotropic antiferromagnetic spin fluctuations above $T_c$ that diminish below it.

Contribution

It provides direct NMR evidence for nodeless superconducting gaps and analyzes the spin fluctuation anisotropy related to spin-orbit coupling in this material.

Findings

Exponential decrease of $1/T_1$ indicates a fully opened energy gap.

Antiferromagnetic spin fluctuation anisotropy is observed above $T_c$ and disappears at low temperatures.

Spin-orbit coupling influences the anisotropy, which vanishes when spin-singlet pairs form.

Abstract

We report $^{75}$As nuclear magnetic resonance studies on a very clean hole-doped single-crystal Ba$_{0.68}$K$_{0.32}$Fe$_{2}$As$_{2}$ ($T_{\rm {c}}=38.5$ K). The spin-lattice relaxation rate $1/T_{1}$ shows an exponential decrease below $T \simeq 0.45 T_{\rm c}$ down to $T \simeq 0.11 T_{\rm c}$, which indicates a fully opened energy gap. From the ratio $(T_{1})_{c} / (T_{1})_{a}$, where $a$ and $c$ denote the crystal directions, we find that the antiferromagnetic spin fluctuation is anisotropic in the spin space above $T_{\rm c}$. The anisotropy decreases below $T_{\rm c}$ and disappears at $T \rightarrow 0$. We argue that the anisotropy stems from spin-orbit coupling whose effect vanishes when spin-singlet electron pairs form with a nodeless gap.