Surface electronic structure of the 83 K superconductor $Bi_2Sr_2CaCu_2O_8$

We have investigated the occupied and unoccupied electronic structure of $Bi_2Sr_2CaCu_2O_8$ single crystal surfaces by high resolution angle-resolved photoemission, applying He I and synchrotron radiation, and angle-resolved inverse photoemission. The geometric surface structure was determined by temperature-dependent LEED experiments. The electronic states far from EF are found to be almost dispersionless due to correlation effects. Near the Fermi level a band reveals weak but distinct k-dispersion, intersecting EF at about 12⋉X. Starting with a well-developed Fermi edge for T >Tc we observe the opening of a superconducting energy gap in the quasiparticle density of states of 30 meV for T < Tc. This indicates that $Bi_2Sr_2CaCu_2O_8$ is a strong-coupling superconductor