Field-insensitive heavy fermion features and phase transition in the caged-structure quasi-skutterudite Sm${}_3$Ru${}_4$Ge${}_{13}$

Abstract

The robust field-insensitive heavy fermion features in Sm${}_3$Ru${}_4$Ge${}_{13}$ and the magnetic phase transition at $T_N\approx$ 5~K are studied using magnetization $M(T)$, specific heat $C_p(T)$, resistivity $\rho (T)$ and thermal conductivity ${\kappa }_T(T)$. The average crystal structure of Sm${}_3$Ru${}_4$Ge${}_{13}$ conforms to the cubic space group $Pm\bar{3}n$ however, signatures of subtle structural distortions are obtained from the x ray data. The magnetic susceptibility, $\chi (T)$, follows a modified Curie-Weiss law indicating the presence of crystal fields of Sm${}^{3+}$ and the significance of van Vleck terms. No sign of ferromagnetism is observed in $M(H)$ of Sm${}_3$Ru${}_4$Ge${}_{13}$ which yields only 0.025~${\mu }_{\mathrm{B}}$/f.u.-Sm at 2~K, 7~T. The Sommerfeld coefficient, $\gamma \approx$ 220~mJ/mol-Sm K${}^2$, estimated from the analysis of low temperature specific heat suggests the formation of heavy quasi particles at low temperature. Though a ln$T$ dependence of $\rho (T)$ is observed till 60~K, the resistivity behavior is accounted for by assuming a two-band model for activated behavior of charge carriers. The field scans of resistivity, $\rho (H)$, below $T_N$ display significant nonlinearity while those above the $T_N$ are more metal-like. Low values of thermal conductivity, ${\kappa }_T(T)$, are observed in Sm${}_3$Ru${}_4$Ge${}_{13}$ however, displaying an anomaly at $T_N$ which signifies magnetoelastic coupling. A fairly high value of Seebeck coefficient, $S\approx$ 40~$\mu$V/K is observed at 300~K. We identify Sm${}_3$Ru${}_4$Ge${}_{13}$ as a low charge carrier density system with unusual field-insensitive heavy fermion features very similar to the filled skutterudites.

18 pages, 7 figures, Accepted to J. Alloys and Comp., 2016