Resonant photoemission study of Nd2-xCexCuO4-y: Nature of electronic states near the Fermi level

The electronic structures of single-crystal and ceramic samples of the electron-doped superconductor Nd2-xCexCuO4-y with x=0 and 0.15 have been studied using resonant photoemission spectro- scopy. Comparing these spectra with previous results for La2-xSrxCuO4 shows that for both electron and hole doping the Fermi level lies at nearly the same energy, in states that fill in the x=0 insulator gap. We point out that this would occur if the states at the Fermi level obey a Luttinger-type sum rule, i.e., according to standard theory, if the metallic state is a normal Fermi liquid