27Al NMR evidence for the strong-coupling d-wave superconductivity in NpPd5Al2

We present 27Al NMR studies for a single crystal of the newly developed superconductor NpPd5Al2 (Tc=4.9 K). In the superconducting state, the 27Al NMR spectrum shifts to lower frequencies owing to a decrease of the spin part of the Knight shift, and exhibits broadening characteristic of the local field distribution associated with a vortex lattice. We have also found that the 27Al nuclear spin-lattice relaxation rate (1/T1) decreases just below Tc with no coherence peak and shows T3 behavior at low temperatures. The observed temperature dependences of the Knight shift and 1/T1 in general accord with those assuming a line-node superconducting gap with the gap value of 2Δ/kBTc≅6.4. Therefore, the present NMR data provides evidence for strong-coupling d-wave superconductivity in NpPd5Al2. (author)