Abstract
We present 29Si NMR spectra, nuclear spin-lattice relaxation time, T_1, and nuclear spin echo decay time, T_2, measurements in URu_2Si_2 over the temperature range from 0.15 K to 300 K. 1/T_1 obtained with c-axis perpendicular to the external field Hext, 1/T1⊥, is much larger than that obtained with c-axis parallel to Hext, 1/T1//. Existence of large anisotropy of 1/T_1 together with the nearly isotropic 29Si hyperfine field imply that the spin fluctuations exist predominantly in the direction of the tetragonal c-axis. The temperature, T, dependence of 1/T1⊥, // 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 TN=17.5 K associated with the antiferromagnetic order. Below TC, 1/T_1 has no coherence peak and decreases proportionally to T3 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 29Si Knight shift was detected within experimental errors.
