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The experimentally observed nonlinearities of the current-voltagecharacteristics (CVCs) of tantalum-based point homo- and hetero- contacts inboth normal and superconducting states related to electron-phonon interaction(EPI) were analyzed. It was taken into account that additional nonlinearity ofCVCs arising upon contact transition to the superconducting state(superconducting spectral component) is formed not only near the constrictionin the region roughly equal to the contact diameter (as is the case for thenormal state, and as predicted theoretically for the superconducting state),but also in a markedly larger region that is about the size of the coherencelength. In this case, a considerable role in the formation of thissuperconducting component is played by nonequilibrium phonons with low groupvelocity, which account for the experimentally observed sharpening of thephonon peaks in the EPI spectra (the second derivatives of the CVCs) during thesuperconducting transition of the contacts, instead of the theoreticallyexpected peak broadening (spreading), and for the increase in thesuperconducting contribution to the point contact spectrum in the low andmedium energy regions. The high-energy part of the EPI spectrum changes muchless significantly during the superconducting transition, which is attributableto suppression of the excess contact current by nonequilibrium quasi-particles.A detailed procedure was proposed for the recovery of the EPI spectral functionfrom the point contact spectrum contribution (the second derivative of the CVC)that arises during the superconducting transition of one or both contactingmetals.

Recovery of the electron-phonon interaction function in superconducting tantalum ballistic contacts