Why is photoemission better than inverse photoemission for studying high Tc oxides?

Physica C 182, 277 (1991).

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 Δk on the band energy εk. We discuss herethe 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.