Electronic and magnetic properties of the topological semimetal SmMg$_2$Bi$_2$

TL;DR

This study combines ARPES measurements and first-principle calculations to reveal that SmMg$_2$Bi$_2$ is a topological Dirac semimetal with complex mixed valence states and spin-polarized surface states, advancing understanding of its electronic and magnetic properties.

Contribution

It demonstrates that SmMg$_2$Bi$_2$ is a topological Dirac semimetal with mixed Sm valence states, confirmed by ARPES and theoretical calculations, providing new insights into its electronic structure.

Findings

Presence of Dirac-like band crossing near Fermi level

Detection of mixed Sm$^{2+}$ and Sm$^{3+}$ valence states

Observation of spin-polarized states and magnetic susceptibility behavior

Abstract

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 Zintl-phase compound SmMg$_2$Bi$_2$ belongs to the close proximity to a topological Dirac semimetallic state. ARPES results show a Dirac-like band crossing at the zone-center near the Fermi level ($E_\mathrm {F}$) which is further confirmed by first-principle calculations. Theoretical studies also reveal that SmMg$_2$Bi$_2$ belongs to a $Z_2$ topological class and hosts spin-polarized states around the $E_\mathrm {F}$. Zintl's theory predicts that the valence state of Sm in this material should be Sm$^{2+}$, however we detect many Sm-4$f$ multiplet states (flat-bands) whose energy positions suggest the presence of both Sm$^{2+}$ and Sm$^{3+}$. 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 Sm$^{3+}$ ions, the temperature dependence of magnetic susceptibility $\chi(T)$ shows Curie-Weiss-like contribution in the low temperature region, in addition to the Van Vleck-like behaviour expected for the Sm$^{2+}$ ions. The present study will help in better understanding of the electronic structure, magnetism and transport properties of related materials.