Strong electron-phonon interaction in multiband superconductors

We discuss the effects of anisotropy on superconducting critical temperature and order parameter in a strongly coupled regime. The multiband representation is used as a model for anisotropy. We show that strong coupling effects in multiband superconductors lead to pair-breaking due to interband coupling because soft phonon modes play the same role as usual impurities. This effect makes the order parameters in different bands equal to each other and limits the upper bound on critical temperature.