Low-energy spin fluctuations in heavy-Fermion filled-skutterudite compounds YbFe4P12 and YbFe4Sb12 investigated by 31P-NMR and 121Sb-NQR

To microscopically elucidate the magnetic quantum criticality of Yb-based filled skutterudites YbFe4Sb12 and YbFe4P12, each of which has an intermediate valence and transforms into the heavy Fermi-liquid (HFL) state at low temperatures, we have carried out Sb-121-NQR below similar to 30K and P-31-NMR below similar to 200 K. The nuclear spin-lattice relaxation rates divided by temperature 1/T1T in both compounds are dominated by the generalized susceptibility chi(q) with a finite wave vector q far from q = 0.1= T/T of Sb-121 in YbFe4Sb12 in zero field exhibits a near T-independent behavior with the small broad maximum around 4K, indicating that the HFL state with rather small antiferromagnetic (AFM) spin fluctuations with T-N similar to 4K takes place below at least similar to 20 K. 1/T1T of 31 P in YbFe4P12 at high field H = 7T also exhibits the near T-independent behavior, indicating that the HFL state takes place below at least similar to 200 K. The most important feature is that 1/T1T of P-31 at low-H strongly depends on both temperature and field and follows the power-law relation 1/T1T AT(-alpha) over one decade with alpha similar or equal to 0: 7 at 0.2 T. This result leads to the conclusion that YbFe4Sb12 at low-H region is located very close to the non-Fermi-liquid state than YbFe4Sb12 is. (c) 2006 Published by Elsevier B.V.