Low-energy spin fluctuations in filled skutterudites YbFe₄Sb₁₂ and LaFe₄Sb₁₂ investigated through ¹²¹Sb nuclear quadrupole and ¹³⁹La nuclear magnetic resonance measurements

We have elucidated low-energy spin fluctuations in the new filled skutterudites YbFe₄Sb₁₂ and LaFe₄Sb₁₂ synthesized at high pressures, through ¹²¹Sb nuclear quadrupole resonance (NQR) and ¹³⁹La nuclear magnetic resonance (NMR) measurements. The longitudinal spin–lattice relaxation rate 1/T₁ of ¹²¹Sb in YbFe₄Sb₁₂ provides evidence that upon cooling below ∼20 K, the compound transforms from the localized 4f electron state of Yb³⁺ ions to a nonmagnetic heavy Fermi liquid state, originating from the mixing of 4f electrons with conduction electrons. Whereas, the Curie–Weiss type behaviour of the ¹³⁹La Knight shift and ¹²¹Sb- 1/T₁ in LaFe₄Sb₁₂ indicate that the compound remains in the localized electron state down to 1.4 K, it in fact originates from 3d electrons of Fe in [Fe₄Sb₁₂ ] anions. In both compounds, the transversal nuclear spin–spin relaxation rate 1/T₂ exhibits a clear peak at and  K, respectively. The origin of the 1/T₂ peak is discussed in terms of the freezing of the thermal vibration of Sb cages or rare-earth ions filled in each Sb cage. By comparing the experimental results of the present study with those previously reported for the compounds synthesized at ambient pressure, it is pointed out that both the strongly correlated electron properties and the thermal vibrations are greatly modified with the increase of rare-earth atom deficiency.