Theoretical and experimental analysis of the superconducting transition effects on the Fermi-edge photoemission spectra

We analyze the line shape differences between ${\mathrm{Bi}}_4$ ${\mathrm{Ca}}_3$ ${\mathrm{Sr}}_3$ ${\mathrm{Cu}}_4$ ${\mathrm{O}}_{16+\mathit{x}}$ Fermi-edge photoemission spectra taken at room temperature and at temperatures below $T_c$. A quasiparticle theoretical approach indicates that these changes can be interpreted, in first approximation, in terms of a displacement of the leading edge, reflecting the creation of a superconductivity gap of a few tens of millivolts.