Unoccupied spin texture of ordered Bi films on vicinal Ge substrates

Abstract

Vicinal surfaces offer a compelling testing ground in surface science: breaking the high symmetry of a singular surface introduces periodic step arrays that can modify surface states on the terraces and influence epitaxial ordering; the interplay between these effects governs morphology and electronic structure, especially in heavy, spin--orbit-coupled materials. We combine low-energy electron diffraction (LEED) and spin-polarized inverse photoemission (SP-IPES) with ab-initio calculations for the Bi$(1\,1\,1)$ reference, for the first time on a true high-index vicinal bismuth surface. On the ordered Bi$(9\,9\,11)$ vicinal film selected by Ge$(2\,2\,3)$, the measured unoccupied-state spin-sign sequence matches the calculated initial-state spectral function of Bi$(1\,1\,1)$ terraces. Conversely, the pseudo-cubic Bi domains retained on Ge$(4\,4\,3)$ and Ge$(3\,3\,1)$ exhibit a characteristic spin-polarized surface-state response referenced to the macroscopic vicinal surface. These results show that the substrate cut selects not only the morphology of the Bi film, but also the crystallographic reference frame of its unoccupied spin texture.