Possible Partially Disordered Sm State Coexisting with Heavy Quasiparticles in SmAu3Al7

Abstract

Single crystals of SmAu3Al7 with an anisotropic cage structure have been investigated by single-crystal structural analysis and measurements of magnetization, electrical resistivity, specific heat, and X-ray absorption spectroscopy. The average valence of Sm ions is +2.96, which is close to magnetic trivalent, with no significant temperature dependence. The crystalline-electric-field ground state is a |±3/2〉 doublet with the strong Ising nature with the easy axis along the [0001] direction. In the temperature dependence of specific heat, two clear λ-type peaks appear at 0.9 K (= T*) and 2.8 K (= TN), indicating the existence of multiple ordered phases: phase I (T* < T < TN) and phase II (T < T*) in zero field. In the resistivity for the current along the [0001] direction, a sharp increase appears just below TN, suggesting the development of a superzone gap opening on the Fermi surface perpendicular to the [0001] direction. The temperature dependence of magnetic susceptibility along the magnetic easy [0001] direction exhibits a small cusp at TN, confirming the antiferromagnetic nature of phase I. Below TN, however, a pronounced Curie-type behavior appears, indicating that ∼40% of Sm magnetic moments remain paramagnetic in phase I. The Sommerfeld coefficient is estimated to be over 1 J/mol K2 deep inside the ordered state. These results can be interpreted that disordered Sm ions form a Kondo sublattice through hybridization with conduction electrons (partial Kondo screening state) along with quasiparticle mass enhancement.