Observation of Normal-Metal Phonons with Proximity-Effect Tunneling

Abstract

We have measured the tunneling characteristics ($\frac{\mathrm{dV}}{\mathrm{dI}}$ and $\frac{d^{2}V}{d{I}^2}$ versus $V$) of thin film junctions of the form $\mathrm{Al}-I-M-\mathrm{Pb}$, where $M$ is Ag, Cu, or Al. These tunneling characteristics have dips at voltages corresponding to peaks in the phonon density of states of $M$ and peaks at voltages corresponding to the peaks in the phonon density of states of Pb. From the amplitude of the dips due to the phonons of $M$ we can crudely estimate the electron-phonon coupling constant ${\lambda }_M$ for Al and Cu.