27Al NMR studies of NpPd5Al2

We present 27Al NMR studies for a single crystal of the Np-based superconductor NpPd5Al2(Tc=4.9K). We have observed a five-line 27Al NMR spectrum with a center line and four satellite lines separated by first-order nuclear quadrupole splittings. The Knight shift clearly drops below TcTc. The temperature dependence of the 27Al nuclear spin-lattice relaxation rate shows no coherence peak below TcTc, indicating that NpPd5Al2 is an unconventional superconductor with an anisotropic gap. The analysis of the present NMR data provides evidence for strong-coupling d-wave superconductivity in NpPd5Al2.