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-axi s perpendicular to the external field H-ext, 1/T-1 perpendicular to, i s much larger than that obtained with c-axis parallel to H-ext, 1/T-pa rallel to. Existence of large anisotropy of 1/T-1 together with the ne arly isotropic Si-29 hyperfine field imply that the spin fluctuations exist predominantly in the direction of the tetragonal c-axis. The tem perature, T, dependence of 1/T-1 perpendicular to,T-parallel to is nea rly T-independent above 60 K and is proportional to T at low temperatu res, which indicates the crossover from the localized state at high te mperatures to the Fermi liquid state below about 60 K occurs in the el ectronic state of 5f electrons. 1/T-1 and 1/T-2 decrease markedly belo w 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 superc onducting energy gap model with Line nodes. Although the decrease of K night shift was expected in spin-singlet superconductor mediated by an tiferromagnetic electron-electron coupling mechanism, no decrease of S i-29 Knight shift was detected within experimental errors.