Three-orbital study on the orbital distillation effect in the high Tc   cuprates
  Our recent study has revealed that the mixture of the dz2 orbital component into the Fermi surface suppresses Tc in the cuprates such as La2CuO4. We have also shown that applying hydrostatic pressure enhances Tc due to smaller mixing of the Cu4s component. We call these the "orbital distillation" effect. In our previous study, the 4s orbital was taken into account through the hoppings in the dx2-y2 sector, but here we consider a model in which of the dx2-y2, dz2 and 4s orbitals are all considered explicitly. The present study reinforces our conclusion that smaller 4s hybridization further enhances Tc. 
H. Sakakibara, K. Suzuki, H. Usui, K. Kuroki, R. Arita, D.J. Scalapino   and H. Aoki
cond-mat.supr-con cond-mat.mtrl-sci cond-mat.str-el