NMR study of the Peculiar Superconducting Mixed State in the Np Superconductor

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A^^<27> l nuclearspin-lattice relaxation rate 1/T^^<27>_1 has been measured in heavy fermion superconductor NpPd_5Al_2 for a wide range of external fields between 0.7 and 11T. We found that 1/T^^<27>_1 exhibits a strong, fields-dependence behavior at low field s below H^～ 3T, which is attributed to unlike-spin cross relaxation between N^^<237> p-A^^<27> l. An analysis based on the cross relaxation process allows us to make the first quantitative estimation of the Np nuclear relaxation rate 1/^<237>τin an itinerant 5f electron system. The estimated values of 1/^<237>τ^～ 6×10^7 s^<-1>, with a weak temperature variation, are below the resolution of NMR relaxation time, indicating that a direct observation of the N^^<237> p NMR signal would be very difficult, at least in the paramagnetic state. It is proposed that a direct observation of N^^<237> p NMR might be possible in the superconducting state at T^～ 1K.