  We have examined the role of the substrate on electron-phonon coupling in
normal metal films of Mn-doped Al at temperatures below 1 K. Normal
metal-insulator-superconductor junctions were used to measure the electron
temperature in the films as a function of Joule heating power and phonon
temperature. Theory suggests that the distribution of phonons available for
interaction with electrons in metal films may depend on the acoustic properties
of the substrate, namely, that the electron-phonon coupling constant Sigma
would be larger on the substrate with smaller sound speed. In contrast, our
results indicate that within experimental error (typically +/- 10%), Sigma is
unchanged among the two acoustically distinct substrates used in our
investigation.
