Cancellation of quasiparticle mass enhancement in the conductance of point contacts.

Abstract

The high fraction of Andreev reflections that has been observed in noble-metal point contacts with the high-${\mathit{T}}_{\mathit{c}}$ oxides ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ and ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_{4}$ as well as with the heavy-fermion superconductors ${\mathrm{UPt}}_{3}$ and ${\mathrm{URu}}_{2}$${\mathrm{Si}}_{2}$ suggests that the boundary condition at the interface involves Fermi velocities without mass-enhancement factors. We show theoretically that this is indeed the case for a self-energy \ensuremath{\Sigma}(\ensuremath{\omega},k) only weakly dependent on k. A comparison between the velocities derived from point-contact experiments with known values of the fully dressed quasiparticle velocities allows a determination of the mass-enhancement factor.