Possible Mass Enhancement by Multipole Fluctuations Excited via the Singlet-Triplet Crystal Electric Field States in PrOs4Sb12: Sb-NMR Studies Using a Single Crystal

Sb-121,Sb-123 nuclear magnetic resonance (NMR) measurements of a high-quality single crystalline PrOs4Sb12 have been carried out in magnetic fields up to 98 kOe and in the temperature range of 0.4-30 K. From the analysis of the Sb-121 Knight shift (K) and magnetic susceptibility at H approximate to 90 kOe, the transferred hyperfine coupling constants (A(THF)) are obtained. The Sb isotropic hyperfine coupling constant and anisotropic hyperfine coupling tensor are determined as approximate to 27.6 Oe/mu(B) and approximate to(149.9, 42.3, -192.2)Oe/mu(B), respectively. The anisotropic part originates from the spin-dipolar field due to the local spin density at Sb 5p orbital transferred through the hybridization with 4f electrons. The results are consistent with the Sb-12 5p molecular orbital with a(u) symmetry predicted from the band structure calculation. A(THF) for H parallel to [001] is dependent on the magnetic field, suggesting the coupling of the Sb nuclear dipole moment with not only dipole moments but also octupole moments of the Pr 4f electrons with the same time reversal. The NMR relaxation rate depends both on magnetic field and temperature, and the temperature dependence of 1/T-1 is explained by the phenomenological expression 1/(T(1)TA(THF)(2)) proportional to (1/T)exp[-Delta(NMR)(H)/T] with excitation gap Delta(NMR)(H). Here, we found that Delta(NMR)(H) is proportional to the largest excitation gap between Gamma(1) and Gamma(-)(t) of the triplet Gamma((2))(4) state under the magnetic field. These results indicate that the crystal electric field (CEF) excitation and the degeneracy of the triplet state are important for understanding Sb-NMR results. It is strongly indicated that the mass enhancement in PrOs4Sb12 is caused by the scattering conduction electrons with multipole fluctuations via the singlet-triplet CEF excitation.