Observation of two-dimensional bulk electronic states in the superconducting topological insulator heterostructure Cuₓ(PbSe)₅(Bi₂Se₃)₆: Implications for unconventional superconductivity

We have performed angle-resolved photoemission spectroscopy (ARPES) on Cuₓ(PbSe)₅(Bi₂Se₃)₆ [(CPSBS); x=1.47], a superconductor derived from a topological insulator heterostructure, to elucidate the electronic states relevant to the occurrence of possible unconventional superconductivity. Upon Cu intercalation into the parent compound (PbSe)₅(Bi₂Se₃)₆, we observed a distinct energy shift of the bulk conduction band due to electron doping. Photon-energy-dependent ARPES measurements of CPSBS revealed that the observed bulk band forms a cylindrical electronlike Fermi surface at the Brillouin-zone center. The two-dimensional nature of the bulk electronic states puts strong constraints on the possible topological character of the superconducting state in CPSBS.