Ru NMR and NQR probes of the metamagnetic transition in CeRu2Si2

We report Ru-99 NMR and NQR studies on CeRu2Si2 in a magnetic field (H) range of 0-15.5 T across the metamagnetic transition at H-M=7.8 T. From measurements of the hyperfine field, Delta H-hf at the Ru sites induced by the 4f-spin polarization and the nuclear-spin lattice relaxation rate, 1/T-1, three heavy-fermion (HF) regimes have been clarified. (1) For 0 less than or equal to H<3 T, the HF state is formed below 8 K. (2) At H(M)7.8 T, 1/T1T continues to increase upon cooling to 1.4 K, suggesting a very low characteristic energy. (3) In the polarized state above H-M, the 1/T1T probing the quasiparticle excitation is dramatically suppressed at low T, accompanying a peak at high T. The crossover in the HF state takes place not only in the H variation but also in the T variation across the metamagnetic transition in CeRu2Si2.