Polaron Coherence as Origin of the Pseudogap Phase in High Temperature   Superconducting Cuprates
  Within a two component approach to high Tc copper oxides including polaronic couplings, we identify the pseudogap phase as the onset of polaron ordering. This ordering persists in the superconducting phase. A huge isotope effect on the pseudogap onset temperature is predicted and in agreement with experimental data. The anomalous temperature dependence of the mean square copper oxygen ion displacement observed above, at and below Tc stems from an s-wave superconducting component of the order parameter, whereas a pure d-wave order parameter alone can be excluded. 
A. Bussmann-Holder, H. Keller, A. R. Bishop, A. Simon, and K. A.   Mueller