Strong-coupling Spin-singlet Superconductivity with Multiple Full Gaps in Hole-doped Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ Probed by Fe-NMR

TL;DR

This study uses Fe-NMR to investigate the superconducting properties of Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$, revealing strong-coupling spin-singlet pairing with multiple full gaps and wave-number-dependent spin fluctuations.

Contribution

It provides direct NMR evidence supporting an $s^$-wave, multiple full-gap superconducting state in hole-doped Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$, highlighting the role of spin fluctuations.

Findings

Spin-singlet superconductivity confirmed by Knight shift decrease

Multiple full gaps consistent with $s^$-wave model

Wave-number-dependent spin fluctuations observed

Abstract

We present $^{57}$Fe-NMR measurements of the novel normal and superconducting-state characteristics of the iron-arsenide superconductor Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ ($T_c$ = 38 K). In the normal state, the measured Knight shift and nuclear spin-lattice relaxation rate $(1/T_1)$ demonstrate the development of wave-number ($q$)-dependent spin fluctuations, except at $q$ = 0, which may originate from the nesting across the disconnected Fermi surfaces. In the superconducting state, the spin component in the $^{57}$Fe-Knight shift decreases to almost zero at low temperatures, evidencing a spin-singlet superconducting state. The $^{57}$Fe-$1/T_1$ results are totally consistent with a $s^\pm$-wave model with multiple full gaps, regardless of doping with either electrons or holes.