Upper Limit for the Superconducting Transition Temperature in Eliashberg–McMillan Theory

We present simple qualitative estimates for the maximal superconducting transition temperature, which may be achieved due to electron–phonon coupling in Eliashberg–McMillan theory. It is shown that in the limit of very strong coupling the upper limit for transition temperature is determined in fact by a combination of atomic constants and density of conduction electrons.