X-ray photoemission spectroscopy as a probe of charge-gap opening in many-electron systems

Core-hole (x-ray) photoemission (XPS) provides a powerful indirect probe of the low-energy charge excitations of a many-electron system. We have previ ously argued that XPS can be used to study the way in which a (pseudo)gap o pens for charge excitations across, for example, a metal-superconductor tra nsition, independently of any change in the spin excitation spectrum, and s o provide information about spin-charge separation. Here we consider how th e loss of low-energy excitations modifies XPS spectra in the context of sev eral simple models, considering particularly the case of gap opening for bo th s- and d-wave superconductors, and find that XPS, like the nuclear magne tic resonance technique, is in principle sensitive to nodes in the supercon ducting gap function.