Photoemission broadening of Fermi-liquid systems, and its relevance to high-temperature superconductors

We measured at high energy and angular resolution the spectral width of photoemission features near the Fermi energy for Ag(100) and for the nearly-two-dimensional metal 2H-${\mathrm{NbSe}}_2$. The results do not agree with the quadratic width-energy relationship predicted for the lifetime broadening of a Fermi liquid: We found a nearly inverse linear relationship for the sp band of Ag(100), and a similar behavior in the case of ${\mathrm{NbSe}}_2$. These results indicate that the broadening is primarily affected by extrinsic factors rather than by the lifetime—and also that the ‘‘anomalous’’ broadening of the photoemission spectra found for the normal state of the high-temperature superconductors cannot be automatically interpreted as evidence of non-Fermi-liquid behavior.