Antiferromagnetic Spin Fluctuations and Unconventional Nodeless Superconductivity in an Iron-based New Superconductor (Ca_4Al_2O_{6-y})(Fe_2As_2):75As-NQR Study

TL;DR

This study investigates the normal and superconducting states of a new iron-based superconductor, revealing that antiferromagnetic spin fluctuations coexist with nodeless superconductivity, challenging the idea that such fluctuations are solely responsible for high Tc.

Contribution

It provides the first NQR evidence of antiferromagnetic spin fluctuations coexisting with nodeless superconductivity in (Ca_4Al_2O_{6-y})(Fe_2As_2), highlighting the role of lattice structure in Tc enhancement.

Findings

Antiferromagnetic spin fluctuations develop down to Tc.

Superconductivity is well described by an s(+-)-wave multiple gaps model.

Tc is comparable despite differing magnetic fluctuation characteristics.

Abstract

We report 75As-nuclear quadrupole resonance (NQR) studies on (Ca_4Al_2O_{6-y})(Fe_2As_2) with Tc=27K, which unravel unique normal-state properties and point to unconventional nodeless superconductivity (SC). Measurement of nuclear-spin-relaxation rate 1/T_1 has revealed a significant development of two dimensional (2D) antiferromagnetic (AFM) spin fluctuations down to Tc, in association with the fact that FeAs layers with the smallest As-Fe-As bond angle are well separated by thick perovskite-type blocking layer. Below Tc, the temperature dependence of 1/T_1 without any trace of the coherence peak is well accounted for by an s(+-)-wave multiple gaps model. From the fact that Tc=27K in this compound is comparable to Tc=28K in the optimally-doped LaFeAsO_{1-y} in which AFM spin fluctuations are not dominant, we remark that AFM spin fluctuations are not a unique factor for enhancing Tc among existing Fe-based superconductors, but a condition for optimizing SC should be addressed from the lattice structure point of view.