Phonon squeezing via correlations in the superconducting electron-phonon interaction

Superconductivity in the conventional BCS model with correlated squeezed phonons is discussed. It is shown that the energy gap and the critical temperature are maximally enhanced in an optimum and finite range of squeezed coupling. For finite-squeezed coupling the ratio 2Δ/Tc becomes coupling-constant dependent and increases beyond the BCS value of 3.53. Ion-mass dependence of the squeezed coupling constant can yield variations of the isotope exponent from its conventional BCS value of 0.5. © 1995 The American Physical Society.