Phonon self-energy effects in the superconducting energy gap of MgB2 point-contact spectra

In strong-coupling superconductors with a short electron mean free path the self-energy effects in the superconducting order parameter play a major role in the phonon manifestation of the point-contact spectra at the above-gap energies. We derive asymptotic expressions for the nonlinear conductivity of tunnel, ballistic, and diffusive point contacts in the case eV>>Delta, and show that these expressions not only qualitatively, but also semiquantitatively correspond to the measurements of the phonon structure in the point-contact spectra for the pi band of MgB2 thin films.