Microcontact spectroscopy of electron–phonon interaction in zinc and cadmium

The nonlinear current–voltage characteristics of microscopic (d≃10–100 Å) zinc and cadmium point contacts are measured at 1.5°K. It is shown that the second derivatives of the characteristics are proportional to the density of phonon states and permit the maxima in the phonon spectrum to be located more precisely. The microcontact spectra display seven features: five maxima at eV = 7.6 ± 0.2, 10.1 ± 0.7, 15.2 ± 0.7, 19.5 ± 1.0 and 23.3 ± 0.5 MeV for zinc and 4.8 ± 0.3, 6.6 ± 0.1, 10.5 ± 0.3, 13.3 ± 0.8 and 17.0 ± 0.5 MeV for cadmium as well as two additional maxima, observed more rarely, at eV = 12.5 and 26.5 MeV for zinc and 8.5 and 20 MeV for cadmium. All of these values are fairly consistent with the position of the singular points ∂ω/∂q = 0 on the phonon dispersion curves. Slight energy shifts and redistribution of line strength observed in the spectra of different samples are interpreted as being due to the different orientation of crystallites in the region of the contact. The effect of temperature on the spectral linewidth is studied and an empirical method is proposed for taking account of the background observed in the spectra of most contacts.