Realistic Surface Scattering and Surface Bound State Formation in the   High T_c Superconductor YBa_2Cu_3O_{6+x}
  Surface Umklapp scattering of quasiparticles, and surface roughness are shown to play essential roles in the formation of the surface bound states in realistic models for YBa_2Cu_3O_{6+x}. The results account for the shape, the impurity dependence of the height, and for a proposed universal width of the zero bias conductance peak. 
M.B. Walker and P. Pairor