Title: Quasiparticle interference in multiband superconductors with strong coupling,
Abstract: We develop a theory of the quasiparticle interference (QPI) in multiband
superconductors based on strong-coupling Eliashberg approach within the Born
approximation. In the framework of this theory, we study dependencies of the
QPI response function in the multiband superconductors with nodeless s-wave
superconductive order parameter. We pay a special attention to the difference
of the quasiparticle scattering between the bands having the same and opposite
signs of the order parameter. We show that, at the momentum values close to the
momentum transfer between two bands, the energy dependence of the quasiparticle
interference response function has three singularities. Two of these correspond
to the values of the gap functions and the third one depends on both the gaps
and the transfer momentum. We argue that only the singularity near the smallest
band gap may be used as an universal tool to distinguish between $s_{++}$ and
$s_{\pm}$ order parameters. The robustness of the sign of the response function
peak near the smaller gap value, irrespective of the change in parameters, in
both the symmetry cases is a promising feature that can be harnessed
experimentally.