Standing-wave photoemission study of the high-Tc superconductor Bi2Sr2CaCu2O8$+$d

It is believed that the key element of superconductivity in the high-Tc cuprates is the electron- or hole- doping of the CuO2 planes within their layered structures with large c-axis lattice parameters. An important challenge remaining is the unambiguous differentiation of the electronic structure of these CuO2 layers and those of the intermediate layers. Conventional angle-resolved photoemission spectroscopy (ARPES) with energies of 6 to 150 eV has provided much information, but collects photoelectrons from only the topmost surface layers rather than the full unit cell for the typical cuprate Bi2Sr2CaCu2O8$+$d (Bi-2212) Here we present a soft x-ray standing-wave photoemission study of Bi-2212, providing depth resolution of the different atomic planes (CuO2, Ca, SrO, and BiO). Rocking curves of core-level and valence spectra were used to derive layer-resolved densities of states (DOSs) within Bi-2212. DFT calculations incorporating the Bi-2212 supermodulation structures are compared to the layer-specific DOSs. Our work thus supplies new insights into the electronic structure of the cuprates.