Unoccupied electronic states of the high-temperature superconductor Bi2Sr2CaCu2O8 investigated by two-photon photoemission spectroscopy -: art. no. 134517

Two-photon photoemission spectroscopy (2PPES) has been applied to an optimally doped high-temperature superconductor (HTSC) Bi2Sr2CaCu2O8(Bi2212). 2PPES with photon energy between 4.0 and 4.9 eV revealed four structures, labeled as A to D, arising from unoccupied states at <0.5, 1.8, 2.7, and 3.7 eV above the Fermi level (E-F), respectively. Structures A and B show no strong polarization dependence, whereas structures C and D appear in p-polarization configuration and completely disappear in s-polarization. The polarization dependence indicates that the unoccupied states for structures A and B are composed of orbitals parallel to the CuO2 plane. The unoccupied states for structures C and D are composed of atomic orbitals perpendicular to the CuO2 plane. Unoccupied states for structures A and B are attributed to the unoccupied band of carrier and the upper Hubbard band, respectively.