Phonon self-energy effects in the superconducting energy gap ofMgB2point-contact spectra

Abstract

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 eVӷ⌬, 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 band of MgB 2 thin films.

Andreev-spectroscopy study of unconventional superconductivity in MgB2:(La,Sr)MnO3 nanocomposite

“…The phonon spectrum and the electron-phonon coupling in MgB 2 are well known at present (see Refs. 5,16,[26][27][28]. Using these data, we first calculate the dI/dV-V characteristics of a PC for the π-and σ-bands of MgB 2 .…”

“…The elastic term is pro-Unconventional superconductivity of MgB:LSMO nanocomposite portional to the energy-dependent part of the excess current I exc (eV) and appears because of frequency dependence of the energy gap function Δ(ε = eV). At T = 0 the (normalized) difference conductance related to the excess current reads for the S-c-N contact [13][14][15][16]…”

“…Nonetheless, we hope that some features observed in PC's dI exc /dV characteristics provide evidence that the frequency dependence of the composite's energy gap Δ tr (ε) differs from those for MgB 2 . Note, that the differential conductance dI exc /dV characteristics of any point contact (tunnel, ballistic, diffusive) are roughly similar in the region of ε = eV, where the contribution of ImΔ(ε) is negligible (see, e.g., [16]). For MgB 2 this region is below 80 meV [5,6].…”

Phonon Structure in Point-Contact Spectra of MGB2

“…In this section we review what has recently become known about the phonon structure in the point-contact spectra 12 . In order to do this, we should refer to the works where the excess current in point contact is considered in terms of the strong-coupling theory.…”

“…For diffusive point contact with the restriction: l i ≪ d ≪ √ l i ξ, the same quantity takes the form 12 :…”