Direct, Experimental Evidence of the Fermi Surface in YBa2Cu3O7-x

We report new measurements of the electron-positron momentum spectra of YBa2Cu3O7-x performed with ultra-high statistical precision. These data differ from previous results in two significant respects: They show the D2 symmetry appropriate for untwinned crystals and, more importantly, they show unmistakable, statistically significant, discontinuities that are evidence of a major Fermi surface section.