Relaxation kinetics of nonequilibrium phonons in copper and gold point contacts

The response spectra of copper–copper and gold–gold metal point contacts in the ballistic and diffusion regimes are measured in the frequency range from 3 · 103 to 8 · 1010 Hz by different techniques: video detection, recording of the second-harmonic amplitude, and recording of an intermediate frequency in the mixing of two rf radiations. The kinetics of transient phenomena in the contacts is analyzed on the basis of the experimental data. It is shown that the saturation of the frequency dependence of the signal and its smooth dependence on the bias voltage beyond the limits of the phonon spectrum can be attributed to (respectively) the bolometric effect (inertial heating of the contact by external radiation) and the existence of an intermediate thermalization stage of the phonons during their relaxation in interaction with electrons. The relaxation frequency of high-energy phonons is determined and is found to be equal to 5 · 109 for copper and 0.8 · 109 Hz for gold. Nonuniform relaxation is observed, and the elastic mean free path of nonequilibrium phonons is estimated (∼ 1.1 nm). The existence of latent rf anomalies (spectral features) in the point-contact spectra in the diffusion regime is established experimentally.

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