High-Tc Nodeless s_\pm-wave Superconductivity in (Y,La)FeAsO_{1-y} with Tc=50 K: 75As-NMR Study

TL;DR

This study uses 75As-NMR to investigate the nodeless s_-wave superconductivity in a high-Tc Fe-pnictide, revealing a fully-gapped state and linking Tc enhancement to Fermi-surface nesting.

Contribution

It demonstrates that the higher Tc in (Y,La)FeAsO_{1-y} is due to improved Fermi-surface nesting, supported by NMR data and band-structure calculations.

Findings

Nodeless bulk superconductivity at Tc=50 K confirmed by NMR.

Moderate antiferromagnetic spin fluctuations in the normal state.

Enhanced Tc linked to maximized Fermi-surface multiplicity.

Abstract

We report 75As-NMR study on the Fe-pnictide high-Tc superconductor Y0.95La0.05FeAsO_{1-y} (Y0.95La0.051111) with Tc=50 K that includes no magnetic rare-earth elements. The measurement of the nuclear-spin lattice-relaxation rate 75(1/T1) has revealed that the nodeless bulk superconductivity takes place at Tc=50 K while antiferromagnetic spin fluctuations (AFSFs) develop moderately in the normal state. These features are consistently described by the multiple fully-gapped s_\pm-wave model based on the Fermi-surface (FS) nesting. Incorporating the theory based on band calculations, we propose that the reason that Tc=50 K in Y0.95La0.051111 is larger than Tc=28 K in La1111 is that the FS multiplicity is maximized, and hence the FS nesting condition is better than that in La1111.