Quadrupole order and field-induced heavy-fermion state in the filled skutterudite PrFe4P12 via 31P NMR

We have measured 31P NMR in the filled skutterudite PrFe4P12. In the antiferro-quadrupole ordered phase, field-induced, site-dependent line splitting was observed. We propose that the order parameter is of Γ23 type and both of the quadrupoles O20 and O22 have finite magnitude. In the high-field phase, the nuclear spin-lattice relaxation rate 1/T1 exhibits remarkable anisotropy. For the field along <111>, we observed non-Fermi-liquid behavior characterized by the enhanced, almost temperature-independent 1/T1