^<b>29</b>Si NMR Study of Antiferromagnetic Superconductor URu_<b>2</b>Si_<b>2</b>

Abstract

We present 29 Si NMR spectra, nuclear spin-lattice relaxation time, T 1 , and nuclear spin echo decay time, T 2 , measurements in URu 2 Si 2 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⊥ , is much larger than that obtained with c -axis parallel to H ext , 1/ T 1// . Existence of large anisotropy of 1/ T 1 together with the nearly isotropic 29 Si 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⊥,// 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 29 Si Knight shift was detected within experimental errors.