Unusual disorder effects in superconducting LaFeAs_(1-d)O_0.9F_0.1 as revealed by NMR spectroscopy

TL;DR

This study investigates how arsenic deficiency affects the superconducting properties of LaFeAsO_(0.9)F_(0.1), revealing unusual disorder effects through NMR measurements and suggesting possible changes in the superconducting gap symmetry.

Contribution

It provides new insights into the impact of As deficiency on superconductivity in Fe-pnictides, highlighting potential disorder-induced transitions in gap symmetry.

Findings

Change from T^3 to T^5 dependence in 1/T_1 below T_c due to As deficiency

Unexpected increase in T_c and upper critical field slope with As deficiency

Evidence for disorder-driven change from s+- to s++ superconductivity

Abstract

We report 75As NMR measurements of the spin-lattice relaxation in the superconducting state of LaFeAsO_(0.9)F_(0.1) and As-deficient LaFeAs_(1-d)O_0.9F_0.1. The temperature behavior of 1/T_1 below T_c changes drastically from a T^3-dependence for LaFeAsO_(0.9)F_(0.1) to a T^5-dependence for the As-deficient sample. These results, together with the previously reported unexpected increase of T_c and the slope of the upper critical field near T_c for the As-deficient sample, are discussed in terms of non-universal SC gaps in Fe-pnictides and the effect of As deficiency as an exotic case where nonmagnetic 'smart' impurities even stabilize an s+- wave superconductor or within a scenario of a disorder-driven change to s++ superconductivity.