Field-insensitive heavy fermion features and phase transition in the caged-structure quasi-skutterudite Sm₃ Ru₄ Ge₁₃

The robust field-insensitive heavy fermion features in Sm₃ Ru₄ Ge₁₃ and the magnetic phase transition at T_N ≈ 5 K are studied using magnetization M(T), specific heat C_p(T), resistivity ρ(T) and thermal conductivity κ_T(T). The average crystal structure of Sm₃ Ru₄ Ge₁₃ conforms to the cubic space group Pm3¯n however, signatures of subtle structural distortions are obtained from the x ray data. The magnetic susceptibility, χ(T), follows a modified Curie–Weiss law indicating the presence of crystal fields of Sm³⁺ and the significance of van Vleck terms. No sign of ferromagnetism is observed in M(H) of Sm₃ Ru₄ Ge₁₃ which yields only 0.025 μ_B/f.u.-Sm at 2 K, 7 T. The Sommerfeld coefficient, γ≈ 220 mJ/mol-Sm K², estimated from the analysis of low temperature specific heat suggests the formation of heavy quasi particles at low temperature. Though a lnT dependence of ρ(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, ρ(H), below T_N display significant nonlinearity while those above the T_N are more metal-like. Low values of thermal conductivity, κ_T(T), are observed in Sm₃ Ru₄ Ge₁₃ however, displaying an anomaly at T_N which signifies magnetoelastic coupling. A fairly high value of Seebeck coefficient, S ≈ 40 μV/K is observed at 300 K. We identify Sm₃ Ru₄ Ge₁₃ as a low charge carrier density system with unusual field-insensitive heavy fermion features very similar to the filled skutterudites.