Electronic and magnetic properties of the topological semimetal SmMg2Bi2

Dirac semimetals show nontrivial physical properties and can host exotic quantum states like Weyl semimetals and topological insulators under suitable external conditions. Here, by combining angle-resolved photoemission spectroscopy measurements (ARPES) and first-principle calculations, we demonstrate that the Zintl-phase compound SmMg2Bi2 is in close proximity to a topological Dirac semimetallic state. ARPES results show a Dirac-like band crossing at the zone center near the Fermi level (EF), which is further confirmed by first-principle calculations. Theoretical studies also reveal that SmMg2Bi2 belongs to a Z2 topological class and hosts spin-polarized states around the EF. Zintl's theory predicts that the valence state of Sm in this material should be Sm2+, however, we detect many Sm-4f multiplet states (flat-bands) whose energy positions and relative intensities suggest the presence of both dominant Sm2+ and minor Sm3+. The small concentration (2.5%) of Sm3+ in the bulk of a crystal is inferred to arise from Sm vacancies in the crystal. It is also evident that these flat bands and other dispersive states are strongly hybridized when they cross each other. Due to the presence of Sm3+ ions, the temperature dependence of the magnetic susceptibility χ(T) shows a Curie-Weiss-like contribution in the low-temperature region, in addition to the Van Vleck-like behavior expected for the Sm2+ ions. The present study will help to better understand the electronic structure, magnetism, and transport properties of related materials.