Phonon Spectrum of La

Abstract

Observed phonon structure in the tunneling density of states of superconducting La films (${T}_{c}={4.9}^{\ensuremath{\circ}}\mathrm{K}$, $\frac{2\ensuremath{\Delta}}{k{T}_{c}}=3.8$) contains resolvable detail not reported previously, with maximum deviations from the BCS prediction amounting to 1.7%. The McMillan-Rowell method of analysis produces a phonon spectrum ${\ensuremath{\alpha}}^{2}F(\ensuremath{\omega})$ characterized by parameters which predict ${T}_{c}=5.0$ \ifmmode^\circ\else\textdegree\fi{}K via the McMillan equation. Implications of this agreement are discussed.