SURFACE EFFECTS ON YBA2CU3O7-DELTA BY ANGLE-RESOLVED PHOTOEMISSION

High-resolution angle-resolved photoemission performed on cleaved surfaces of twinned YBa2Cu3O7-delta single crystals at 20 K revealed an intrinsic surface-derived electronic state for this high-temperature superconductor. In the spectra it is very pronounced around the X(Y) point of the Brillouin zone where its binding energy is 0.92 eV. It shows slight k parallel-to-, no k perpendicular-to -dispersion, and a strong sensitivity to surface contamination. The existence of this state points to a rearrangement of electron charge in the surface layers and consequences for the superconducting behaviour on the surface of 1:2:3 superconductors will be discussed.