Observation of multiple nodal lines in SmSbTe

Having been a ground for various topological fermionic phases, the family of ZrSiS-type 111 materials has been under experimental and theoretical investigations. Within this family of materials, the subfamily $\mathit{Ln}\mathrm{SbTe}$ $(Ln=\text{lanthanide}\phantom{\rule{4.pt}{0ex}}\text{elements})$ is gaining interest in recent times as the strong correlation effects and magnetism arising from the $4f$ electrons of the lanthanides can provide an important platform to study the link between topology, magnetism, and correlation. In this Letter, we report the systematic study of the electronic structure of $\mathrm{SmSbTe}$---a member of the $\mathit{Ln}\mathrm{SbTe}$ subfamily---by utilizing angle-resolved photoemission spectroscopy in conjunction with first-principles calculations, transport, and magnetic measurements. Our experimental results identify multiple Dirac nodes forming the nodal lines along the $\mathrm{\ensuremath{\Gamma}}\text{\ensuremath{-}}X$ and $Z\text{\ensuremath{-}}R$ directions in the bulk Brillouin zone (BZ) as predicted by our theoretical calculations. A surface Dirac-like state is also observed at the $\overline{X}$ point of the surface BZ. Our study highlights $\mathrm{SmSbTe}$ as a promising candidate to understand the topological electronic structure of $\mathit{Ln}\mathrm{SbTe}$ materials.