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

TL;DR

This study reveals that Sm$_3$Ru$_4$Ge$_{13}$ exhibits field-insensitive heavy fermion behavior and undergoes a magnetic phase transition at 5 K, with detailed analysis of its structural, magnetic, and transport properties indicating complex electron correlations.

Contribution

It provides the first detailed investigation of heavy fermion features and phase transition in the caged-structure quasi-skutterudite Sm$_3$Ru$_4$Ge$_{13}$, highlighting its unusual field-insensitive properties.

Findings

Heavy fermion behavior with a Sommerfeld coefficient of ~220 mJ/mol-Sm K$^2$

Magnetic transition at T_N ≈ 5 K with no ferromagnetism observed

Significant magnetoelastic coupling evidenced by thermal conductivity anomaly

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.