d-Wave Superconductivity in Antiferromagnetic Heavy-Fermion Compound UPd₂Al₃ –Evidence from ²⁷Al NMR/NQR Studies–

The nuclear-spin-lattice relaxation rate, 27(1/T-1), Of the high-quality polycrystalline UPd(2)A1(3) with a record T-c = 2 K and the narrowest Al NQR linewidth to date of 12 kHz has been measured in zero magnetic field by the NQR technique. 27(1/T-1) below T-c has been found to obey the T-3-law at least down to 0.18 K, giving strong evidence that the energy gap vanishes along lines on the Fermi surface. From the analysis of Al-27 Knight shift below T-c, it is shown that the anisotropic residual Knight shift originates from the f-electron susceptibility in the antiferromagnetic state, whereas the isotropic reduction of the shift below T-c is due to a singlet pairing nature of the superconductivity. Both the results of 27(1/T-1) and Al-27 Knight shift in UPd2Al3 can be reproduced consistently by a d-wave pairing model characterized by lines of vanishing gap on the Fermi surface