Optical study of the topological materials LnSbTe (Ln = La, Ce, Sm, Gd)

The interaction among nontrivial topology, symmetry breaking orders, and strong electron correlation could generate many exotic physical phenomena. Recently, charge-density wave (CDW) orders and the Kondo effect are reported to occur in some of the LnSbTe compounds, which are suggested to be either topological insulators or topological semimetals. Here we perform optical spectroscopic measurements on four of the LnSbTe (Ln = La, Ce, Sm, Gd) compounds in order to investigate their exact electronic ground states. Metallic responses are commonly observed in all of the four compounds, despite their different transport behaviors. Furthermore, We observe some temperature-dependent excitations in the midinfrared region which are most likely to stem from CDW gaps. There is one such gap of about 452 meV in LaSbTe, one about 471 meV in CeSbTe, and two around 374 and 480 meV in GdSbTe, but none in SmSbTe, which is inconsistent with their highly similar electronic structures. Additionally, the presence of a Kondo hybridization gap is a bit too weak to be clearly revealed in our measured temperature range. Our results provide more detailed information on the LnSbTe family, which helps to understand the underlying physics of these systems and inspires more related explorations.