Spin Fluctuations and Unconventional Superconductivity in the Fe-based Oxypnictide Superconductor LaFeAsO_0.7 probed by 57Fe-NMR

TL;DR

This study uses $^{57}$Fe-NMR to investigate the spin fluctuations and unconventional superconductivity in LaFeAsO$_{0.7}$, revealing evidence for extended s$_{ ext{±}}$-wave pairing and unique normal state spin dynamics.

Contribution

First $^{57}$Fe-NMR evidence supporting extended s$_{ ext{±}}$-wave pairing in LaFeAsO$_{0.7}$, highlighting the role of spin fluctuations in its superconductivity.

Findings

Spin-singlet pairing confirmed by Knight shift decrease.

Absence of coherence peak in relaxation rate supports unconventional pairing.

Decrease of spin fluctuations in the normal state over all q-space.

Abstract

We report $^{57}$Fe-NMR studies on the oxygen-deficient iron (Fe)-based oxypnictide superconductor LaFeAsO$_{0.7}$ ($T_{c}=$ 28 K) enriched by $^{57}$Fe isotope. In the superconducting state, the spin component of $^{57}$Fe-Knight shift $^{57}K$ decreases almost to zero at low temperatures and the nuclear spin-lattice relaxation rate $^{57}(1/T_{1})$ exhibits a $T^{3}$-like dependence without the coherence peak just below $T_{c}$, which give firm evidence of the unconventional superconducting state formed by spin-singlet Cooper pairing. All these events below $T_c$ are consistently argued in terms of the extended s$_{\pm}$-wave pairing with a sign reversal of the order parameter among Fermi surfaces. In the normal state, we found the remarkable decrease of $1/T_1T$ upon cooling for both the Fe and As sites, which originates from the decrease of low-energy spectral weight of spin fluctuations over whole ${\bm q}$ space upon cooling below room temperature. Such behavior has never been observed for other strongly correlated superconductors where an antiferromagnetic interaction plays a vital role in mediating the Cooper pairing.