77Se NMR Evidence of Strongly Coupled Superconductivity in K0.8Fe2-xSe2

TL;DR

This study uses 77Se NMR to provide bulk evidence of strongly coupled, isotropic-gap superconductivity in K0.8Fe2-xSe2 with a Tc around 30 K, showing singlet pairing and unusual low-temperature behavior.

Contribution

First NMR evidence demonstrating strongly coupled, isotropic-gap superconductivity in K0.8Fe2-xSe2, linking it to other iron-based high-temperature superconductors.

Findings

Immediate drop in Knight shift below Tc indicating singlet pairing

Rapid decrease in relaxation rate consistent with isotropic gap ~3.8 k_BTc

Absence of Hebel-Slichter coherence peak

Abstract

We report the 77Se NMR Knight shift and spin-lattice relaxation studies on the superconducting state of the ternary iron selenide K0.8Fe2-xSe2 with Tc~30 K. Just below Tc, the Knight shift 77Kn shows an immediate drop, indicating a singlet pairing. The spin-lattice relaxation rate 1/77T1 decreases rapidly in the temperature range from TC to Tc/2, which can be fit with an isotropic gap of \Delta~3.8\pm 0.5 k_BTc. The Hebel-Slichter coherence peak is not observed. These data give bulk evidence for a strongly coupled superconductivity with isotropic gaps in K0.8Fe2-xSe2, which is similar to other iron-based high temperature superconductors. Below Tc/2, the spin-part of the Knight shift levels off to a constant value (~0.09%), and the spin-lattice relaxation follows a 1/T1 T^2 behavior, which are not well understood.