MEASURING THE RELATIVE PHASE OF THE ENERGY-GAP IN A HIGH-TEMPERATURE SUPERCONDUCTOR WITH EELS

A method of measuring the relative phase of the energy gap in a high-t emperature superconductor is suggested for electron energy loss spectr oscopy. Energy-resolved measurements of off-specular scattering should show a feature similar to the specular feature associated with the ga p. Unlike the specular feature, which reflects an average of the gap o ver the (normal) Fermi surface, the energy loss of the off-specular fe ature depends on the superconducting energy gap at only two locations on the Fermi surface. The onset of the feature reflects the relative p hase between these two points. This result is independent of surface c haracteristics. Such characteristics affect the magnitude of the off-s pecular feature, not its location or onset. The size of the feature is estimated for a simple surface model. Implications of specific measur ements on Bi2Sr2CaCu2O8 are discussed.