Unusual disorder effects in superconducting LaFeAs1-δO0.9F0.1 as revealed by 75As NMR spectroscopy

We report As-75 nuclear magnetic resonance measurements of the spin-lattice relaxation in the superconducting (SC) state of LaFeAsO0.9F0.1 and As-deficient LaFeAs1-delta O0.9F0.1. The temperature behavior of 1/T-1 below T-c changes drastically from a T-3 dependence for LaFeAsO0.9F0.1 to a T-5 dependence for the As-deficient sample. These results, together with the previously reported unexpected increase in Tc and the slope of the upper critical field near Tc 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(+/-)-wave superconductor or within a scenario of a disorder-driven change to s(++) superconductivity.