Low-energy spin fluctuations in heavy-Fermion filled-skutterudite compounds [formula omitted] and [formula omitted] investigated by 31P-NMR and 121Sb-NQR

Abstract

To microscopically elucidate the magnetic quantum criticality of Yb-based filled skutterudites YbFe 4 Sb 12 and YbFe 4 P 12 , each of which has an intermediate valence and transforms into the heavy Fermi-liquid (HFL) state at low temperatures, we have carried out 121Sb-NQR below ∼ 30 K and 31P-NMR below ∼ 200 K . The nuclear spin–lattice relaxation rates divided by temperature 1 / T 1 T in both compounds are dominated by the generalized susceptibility χ ( q ) with a finite wave vector q far from q = 0 . 1 / T 1 T of 121Sb in YbFe 4 Sb 12 in zero field exhibits a near T -independent behavior with the small broad maximum around 4 K, indicating that the HFL state with rather small antiferromagnetic (AFM) spin fluctuations with T N ∼ 4 K takes place below at least ∼ 20 K . 1 / T 1 T of 31P in YbFe 4 P 12 at high field H = 7 T also exhibits the near T -independent behavior, indicating that the HFL state takes place below at least ∼ 200 K . The most important feature is that 1 / T 1 T of 31P at low- H strongly depends on both temperature and field and follows the power-law relation 1 / T 1 T = AT - α over one decade with α ≃ 0.7 at 0.2 T. This result leads to the conclusion that YbFe 4 Sb 12 at low- H region is located very close to the non-Fermi-liquid state than YbFe 4 Sb 12 is.