Unusual disorder effects in superconductingLaFeAs1−δO0.9F0.1as revealed byA75sNMR spectroscopy

We report $^{75}\text{A}\text{s}$ nuclear magnetic resonance measurements of the spin-lattice relaxation in the superconducting (SC) state of ${\text{LaFeAsO}}_{0.9}{\text{F}}_{0.1}$ and As-deficient ${\text{LaFeAs}}_{1\ensuremath{-}\ensuremath{\delta}}{\text{O}}_{0.9}{\text{F}}_{0.1}$. The temperature behavior of $1/{T}_{1}$ below ${T}_{c}$ changes drastically from a ${T}^{3}$ dependence for ${\text{LaFeAsO}}_{0.9}{\text{F}}_{0.1}$ to a ${T}^{5}$ dependence for the As-deficient sample. These results, together with the previously reported unexpected increase in ${T}_{c}$ and the slope of the upper critical field near ${T}_{c}$ for the As-deficient sample, are discussed in terms of nonuniversal SC gaps in Fe pnictides and the effect of As deficiency as an exotic case where nonmagnetic ``smart'' impurities even stabilize an ${s}_{\ifmmode\pm\else\textpm\fi{}}$-wave superconductor or within a scenario of a disorder-driven change to ${s}_{++}$ superconductivity.