Spin-singlet superconductivity with multiple gaps in PrFeAsO 0.89 F 0.11

We report 75 As and 19 F NMR studies on the superconducting properties of the newly discovered compound PrFeAsO 0.89 F 0.11 ( T c =45 K). The Knight shift in the direction of H ∥ ab -plane decreases below T c to almost zero, which indicates spin-singlet pairing. The temperature dependencies of both the Knight shift and the spin lattice relaxation rate indicate that there are two gaps opening below T c , with nodes in the gap function.