WHY IS PHOTOEMISSION BETTER THAN INVERSE PHOTOEMISSION FOR STUDYING HIGH-TC OXIDES

Within the model of hole superconductivity a fundamental asymmetry between electrons and holes exists. Mathematically this translates into a dependence of the superconducting energy gap function DELTA-k on the band energy epsilon-k. We discuss here the consequences of this dependence for high resolution photoemission and inverse photoemission experiments. In particular we find: (1) the angle-integrated spectrum in the superconducting state should be sharper in photoemission than in inverse photoemission, and there should be a net gain (loss) in spectral weight in going from the normal to the superconducting state in the former (latter) technique. (2) High resolution angle-resolved spectra should exhibit small qualitative differences with usual BCS behavior. In particular, in a range of k values the peak in the photoemission spectrum in the superconducting state should move closer to the Fermi energy with k while the corresponding peak in the normal state is moving away from it. In the photoemission spectrum the peaks should stay close to the Fermi energy for a larger range of k values than in the inverse photoemission case.