Transport properties in the filled-skutterudite compounds RERu₄Sb₁₂ (RE—La, Ce, Pr and Nd); an exotic heavy fermion semimetal CeRu₄Sb₁₂

We report transport measurements on high quality RERu₄Sb₁₂ single crystals under magnetic fields up to 14 T and temperatures down to 0.3 K. CeRu₄Sb₁₂ exhibits anomalous features in all the transport properties, compared to those for RE = La, Pr and Nd. The thermoelectric power (TEP) shows a huge positive peak over 100 μ V K⁻¹ near 90 K. The Hall coefficient, ∼2.0 × 10⁻⁹m³ C⁻¹ at room temperature, exhibits a drastic increase below ∼100 K to R_H ≈ 5.2× 10⁻⁸m³ C⁻¹ at 0.3 K. At low temperatures below ∼3 K, the TEP exhibits an apparent anomaly correlated with the non-Fermi-liquid behaviour reported in the resistivity and specific heat. We have succeeded in observing Shubnikov–de Haas oscillations both in the magnetoresistance and Hall effect. This result along with the Hall effect suggests small semimetallic Fermi surfaces with highly correlated electrons at low temperatures in contrast with the large FS for LaRu₄Sb₁₂. NdRu₄Sb₁₂ exhibits anomalies suggesting a ferromagnetic ground state at around 1.4 K above which the resistivity exhibits a negative temperature coefficient up to 6 K.