Heterocontact effects in point-contact electron-phonon spectroscopy of the alkali metals

For a small contact between two different materials (a heterocontact), we derive the free-electron expression for the electron-phonon spectral function determined from the measured I-V characteristic. The heterocontact spectral function differs strikingly from the homocontact spectral function in that it excludes scattering through angles less than a minimum angle in the larger-bandwidth material. We calculate realistic heterocontact spectra for pairs of alkali metals. If in a given pair of alkalis the smaller-bandwidth material is replaced by one with a still smaller bandwidth, the size of the signal from the larger-bandwidth material decreases (sometimes dramatically), and the part of the spectrum due to scattering by 2kF phonons is relatively enhanced.