Fermi surface sheet-dependent band splitting in Sr2RuO4 revealed by high-resolution angle-resolved photoemission spectroscopy

High-resolution angle-resolved photoemission measurements have been carried out on Sr2RuO4. We observe clearly two sets of Fermi surface sheets near the (pi,0)-(0,pi) line, which are most likely attributed to the surface and bulk Fermi surface splitting of the beta band. This is in strong contrast to the nearly null surface and bulk Fermi surface splitting of the alpha band, although both have identical orbital components. Extensive band structure calculations are performed by considering various scenarios, including structural distortion, spin-orbit coupling, and surface ferromagnetism. However, none of them can explain such a qualitative difference of the surface and bulk Fermi surface splitting between the alpha and beta sheets. This unusual behavior points to an unknown order on the surface of Sr2RuO4 that remains to be uncovered. Its revelation will be important for studying and utilizing novel quantum phenomena associated with the surface of Sr2RuO4 as a result of its being a possible p-wave chiral superconductor and a topological superconductor.