A possibility of increase of a superconducting gap in two-dimensional   BCS superconductor
  According to the BCS model the probability of filling of pair electron states vk2 as a function of kinetic energy is smeared i.e. is not a step function. This means that part of energy of connection through virtual phonons is consumed to increase the kinetic energy in an electron system at the transition to the superconducting state. In this work a modified mechanism of vk2 formation is proposed which takes into account the dynamics of fluctuations near the phase transition. The estimations of superconducting gap and isotopic effect for a matrial with parameters of high-temperature superconductors give values comparable with these observed 
I. N. Zhilyaev
cond-mat.supr-con