Weak Spin Fluctuation with Finite Wave Vector and Superconducting Gap Symmetry in KxFe2-ySe2: 77Se Nuclear Magnetic Resonance

TL;DR

This study uses $^{77}$Se NMR to investigate the superconducting gap symmetry and weak spin fluctuations with finite wave vector in K$_x$Fe$_{2-y}$Se$_2$, revealing insights into its magnetic and electronic properties.

Contribution

It provides new evidence of weak spin fluctuations with finite wave vector and supports nodeless superconducting gap models in K$_x$Fe$_{2-y}$Se$_2$ using NMR data.

Findings

Weak spin fluctuations with finite wave vector detected.

Field-independent $1/T_1T$ suggests sign-changing nodeless gaps.

Gap magnitude consistent with ARPES measurements.

Abstract

We report $^{77}$Se-nuclear magnetic resonance (NMR) results down to sufficiently low temperatures under magnetic fields parallel to both the $ab$-plane and the c-axis in a paramagnetic/superconducting (PM/SC) phase of K$_x$Fe$_{2-y}$Se$_2$. The observation of anisotropy in the orbital part of the Knight shift results in the anisotropy of its spin part increasing on approaching the transition temperature. The anisotropy of the Korringa relation suggests the presence of the weak spin fluctuations with a finite wave vector $\bm{q}$, which induce the magnetic fluctuations along the ab-plane at the Se site. Such fluctuations do not correspond to the stripe $(\pi,0)$ correlation of the Fe moment observed in many Fe-based superconductors, and are not contradictory to weak $(\pi,\pi)$ correlations. The nuclear spin-lattice relaxation rate $1/T_1$ shows a field-independent $T_1T \sim const.$ behavior at low temperatures for $H \parallel ab$, which is attributed to the nonzero density of states at the Fermi level and can be explained by the sign-changing order parameter even for nodeless gaps. The temperature dependence of $1/T_1$ is reproduced well by nodeless models with two isotropic gaps or a single anisotropic gap. The obtained gap magnitude in the isotropic two-gap model is comparable to those obtained in the angle-resolved photoemission spectroscopy experiments.