Point-contact noise spectroscopy of phonons in metals

Abstract

The spectral density of low-frequency electrical fluctuations (noise) in pure metal point contacts is found to depend nonmonotonically on the potential difference at low temperatures. For contacts satisfying the conditions for the ballistic regime of electron current flow the noise is made up of a monotonic part proportional to the square of the dc voltage (1/f noise) and a “spectral part” with extrema at biases corresponding to the characteristic phonon energies. Comparison of the noise spectrum part for sodium with the calculated contribution introduced by Umklapp processes in the point-contact electron-phonon interaction function shows that these characteristics are similar. Analysis of noise spectra and dispersion curves of phonons in the main crystallographic directions for three metals (sodium, copper, and tin) makes it possible to establish that the positions of maxima in the noise spectra correspond to the energies at which Umklapp processes are allowed. At the same time the positions of minima correlate well with the energies of phonons possessing maximum wave vectors and minimum group velocities. The latter regularity is attributed to stimulated emission of coherent phonons due to normal processes. This emission reduces the noise due to the correlation between separate electron-phonon scattering events. Thus, the spectral density of low-frequency fluctuations as a function of dc potential difference (the “noise spectrum”) is found to contain more detailed information about phonons, enabling one to determine their energies at a number of characteristic points on the dispersion curves.