Metallic layered materials with magnetic frustration: An ARPES view of SmAuAl 4 Ge 2 and TbAuAl 4 Ge 2

Compounds of the materials class $LnT{\mathrm{Al}}₄{X}₂$ ($Ln$ = lanthanide, $X$ = tetrel, $T$ = transition metal) host exotic magnetic phenomena due to geometric frustration induced by their triangular lattice. Complex spin arrangements, magnetic fluctuations, and double magnetic transitions have been well observed by means of magnetotransport. Nevertheless, the experimental electronic structure of this family of materials has been poorly studied. We have investigated the experimental electronic structure of two members of this class of materials: ${\mathrm{SmAuAl}}₄{\mathrm{Ge}}₂$ and ${\mathrm{TbAuAl}}₄{\mathrm{Ge}}₂$. By means of angle-resolved photoemission spectroscopy (ARPES) accompanied by density functional theory calculations, we reveal common trends and features, the important effect of localized spin moments on the electronic structure, the presence of surface-localized electronic states, and the nature of the surface termination layer. Low-dimensionality, exchange interaction, and spin-orbit coupling are all important ingredients of the electronic structure.