Electron pocket and (topological) surface states of the Yb(0001) surface

Among the lanthanides, Yb seems simple since it is non-magnetic with a filled 4f shell, but the electronic structure still holds some surprises: In the limit of vanishing spin-orbit coupling, fcc Yb was recently identified as nodal line semimetal. We present a combined experimental and theoretical study of Yb on Mo(110) that shows that Yb grows in hcp stacking and its (0001) surface hosts two surface state where one of them is of topological origin, located in a region between two nodal lines. Angular resolved photoemission spectroscopy clearly shows an electron pocket at the zone center that can be reproduced by density functional theory calculations including Hubbard U corrections on the (unoccupied) d-orbitals. We discuss the impact of spin-orbit coupling effects and similarities to topological properties of the lighter earth-alkaline elements like Be(0001) and Ca(111).