Unusual Suppression of Low Energy Spin Excitations near Fermi Level in AFe₄Sb₁₂ (A=Ca, Sr, and Ba) Probed by Sb-NQR

Abstract

Sb nuclear quadrupole resonances (NQR) have been performed on the alkaline-earth filled skutterudite AFe₄Sb₁₂ (A=Ca, Sr, and Ba) being close to ferromagnetic instabiity. No hyperfine splitting in NQR spectrum even at the lowest temperature (1.5 K) confirms an absence of uniform ferromagnetic order in these compounds. Above about T^*∼70 K, the nuclear spin–lattice relaxation rate divided by temperature, 1⁄T₁T, follows an empirical relationship 1⁄T₁T=a+bχ_spin with respect to the Curie Weiss (CW) like T-dependence of the static spin susceptibility χ_spin. The correlation between 1⁄T₁T and χ_spin is consistent with the self-consistent renormalization (SCR) theory for the three-dimensional (3D) nearly ferromagnet despite the relatively large contribution of the first term not correlating to χ_spin. However, below about T^*, 1⁄T₁T exhibits a gap-like suppression with a gap magnitude of about 65 K. This unusual suppression of the low energy spin excitations is contrasting to the case in LaFe₄Sb₁₂, and a characteristic feature in the filled skutterudites with divalent filler ion. The pseudo-gap structure near the Fermi level is proposed for the hybridized Fe3d–Sb5p band (mainly partial p state) for the unusual suppression.