Observation of phonon structure in electron density of states of normal metal

Within standard treatments of the interacting electron--phonon system the electron density of states (EDOS) shows no sign of the phonons when the system is in the normal state. On the other hand, emergence of a fine energy scale in the superconducting state makes the phonons readily observable in the EDOS and other spectroscopic quantities. Here we wish to re-examine the conditions under which phonon structure might be present in the normal state. Accounting for a finite, i.e. non-infinite, electronic bandwidth results in easily distinguishable phonon structure in the EDOS. We argue that for narrow band metals, the resolution of photoemission spectroscopy is sufficient for resolving these structures. Manifestations of finite band effects in $K_3C_{60}$ are discussed.