75As NQR/NMR Studies on Oxygen-deficient Iron-based Oxypnictide Superconductors LaFeAsO_{1-y} (y=0,0.25,0.4) and NdFeAsO_{0.6}

TL;DR

This study investigates the superconducting properties of oxygen-deficient iron-based oxypnictides using 75As-NQR/NMR, revealing an unconventional superconducting state with a line-node gap and a correlation between local atomic configuration and T_c.

Contribution

It provides new insights into how local atomic configurations influence T_c and demonstrates the unconventional nature of superconductivity in these materials.

Findings

Unconventional superconductivity with line-node gap confirmed by T^3 dependence of 1/T_1.

Strong correlation between 75As quadrupole frequency and T_c.

Optimal local Fe-As configuration can enhance T_c up to 50 K.

Abstract

We report 75As-NQR/NMR studies on the oxygen-deficient iron(Fe)-based oxypnictide superconductors LaFeAsO_{0.6} (T_c=28 K) along with the results on LaFeAsO, LaFeAsO_{0.75}(T_c=20 K) and NdFeAsO_{0.6}(T_c=53 K). Nuclear spin-lattice relaxation rate 1/T_1 of 75As NQR at zero field on LaFeAsO_{0.6} has revealed a T^3 dependence below T_c upon cooling without the coherence peak just below T_c, evidencing the unconventional superconducting state with the line-node gap. We have found an intimate relationship between the nuclear quadrupole frequencyof 75As and T_c for four samples used in this study. It implies microscopically that the local configuration of Fe and As atoms is significantly related to the T_c of the Fe-oxypnictide superconductors, namely, the T_c can be enhanced up to 50 K when the local configuration of Fe and As atoms is optimal, in which the band structure may be also optimized through the variation of hybridization between As 4p orbitals and Fe 3d orbitals.