Surface and bulk electronic structure of the unconventional superconductor Sr ₂RuO ₄: Unusual splitting of the β band

We present an angle-resolved photoemission study of the surface and bulk electronic structure of the single layer ruthenate Sr ₂RuO ₄. As the early studies by photoemission and scanning tunneling microscopy were confronted with a problem of surface reconstruction, surface ageing was previously proposed as a possible remedy to access the bulk states. Here, we suggest an alternative way by demonstrating that, in the case of Sr ₂RuO ₄, circularly polarized light can be used to disentangle the signals from the bulk and surface layers, thus opening the possibility to investigate many-body interactions both in bulk and surface bands. The proposed procedure results in improved momentum resolution, which enabled us to detect an unexpected splitting of the surface β band. We discuss the origin of the splitting of the β band and the possible connection with the Rashba effect at the surface.