Two-photon photoemission of high-temperature superconductor Bi2212

The electronic structure of high-Tc superconductor Bi2Sr2CaCu2O8 (Bi2212) has been studied by two-photon photoemission (2PPE) spectroscopy. The 2PPE spectra measured with different photon energies in a range between 3.96 and 4.89 eV exhibited two sharp structures, A and B. These structures have been found to originate from unoccupied states at 2.7 and 3.7 eV, respectively, above the Fermi level. The characteristics of these structures are as follows: (i) the width of structure A is as narrow as 0.3 eV in full width at half maximum and (ii) both structures appear in p-polarization configuration, and completely disappear in s-polarization. Based on these characteristics, we have attributed structure A to the unoccupied state having Cu 3dz2 character. Structure B has been attributed to the satellite of structure A.