Anisotropy and phonon-nonequilibrium effects in point-contact spectra of copper
Abstract
This is the first combined study of point-contact and noise spectra of single-crystalline point contacts. The high anisotropy and high sensitivity of noise spectra to the presence of impurities in the region of the point contact are used to select point contacts with a given crystallographic direction and given conditions of motion of electrons through the point contact. Copper point contacts were studied in the symmetric directions [100], [111], and [110] for the ballistic (li≫d)and intermediate (li∼ d) states of electron motion through the contact. It turned out that when the smooth background is subtracted out the point-contact (PC) spectra differ from one another for these two states by ∼10% for the [100] and [110] directions and almost coincide for the [111] direction. This difference is in agreement with the spectral correction to the smooth background in the ballistic state, appearing because of the nonequilibrium state of the phonons in the region of the point contact, proposed in the theory. The correlation between the positions of the minima on the energy scale in the noise spectra and the position of the maxima of this correction was observed experimentally. In the intermediate state and for the [111] direction, distinct minima are not observed in the noise spectra in the ballistic state, and the spectral correction to the background is relatively small. For this reason, the experimental PC spectra for symmetric directions, obtained in the intermediate state, are in better agreement with the calculations of anisotropic PC functions in which the nonequilibrium phonons were neglected.