Si-29 NMR study of antiferromagnetic superconductor URu2Si2

We present Si-29 NMR spectra, nuclear spin-lattice relaxation time, T-1, and nuclear spin echo decay time, T-2, measurements in URu2Si2 over the temperature range from 0.15 K to 300 K. 1/T-1 obtained with c-axis perpendicular to the external field H-ext, 1/T-1 perpendicular to, is much larger than that obtained with c-axis parallel to H-ext, 1/T-parallel to. Existence of large anisotropy of 1/T-1 together with the nearly isotropic Si-29 hyperfine field imply that the spin fluctuations exist predominantly in the direction of the tetragonal c-axis. The temperature, T, dependence of 1/T-1 perpendicular to,T-parallel to is nearly T-independent above 60 K and is proportional to T at low temperatures, which indicates the crossover from the localized state at high temperatures to the Fermi liquid state below about 60 K occurs in the electronic state of 5f electrons. 1/T-1 and 1/T-2 decrease markedly below T-N=17.5 K associated with the antiferromagnetic order. Below T-C, 1/T-1 has no coherence peak and decreases proportionally to T-3 at low temperatures, which is well described in term of an anisotropic superconducting energy gap model with Line nodes. Although the decrease of Knight shift was expected in spin-singlet superconductor mediated by antiferromagnetic electron-electron coupling mechanism, no decrease of Si-29 Knight shift was detected within experimental errors.