Comment on “Josephson Current as a Boundary Condition for Gor'kov Equations”

In a recent paper, Ovchinnikov in J Supercond Nov Magn 35, 663 (2022) claimed that the Josephson current remains purely sinusoidal in the second order of the perturbation theory with respect to the barrier (interface) transparency. In this comment, we explain that this wrong claim arises due to neglecting inhomogeneous suppression of the order parameter in the vicinity of the barrier, which is inevitably accompanied by nonlinear spatial dependence of the superconducting phase. With both effects taken into account, the second order of the perturbation theory produces nonzero second Josephson harmonic.