Correlation between mixing of Cu d orbitals and Tc determined by polarized CuL3XAS

Experimental evidence for the increasing probability of the Cu 3dz2 holes with increasing Tc has been found in BiCaSrCuO superconductors by high resolution Cu L3 XAS. The weight of the Cu 3dz2 holes increases by about 5% going from the normal phase at 300K to the superconducting phase at 10K. The energy splitting Δz between the dz2 and d-y2 hole states decreased by increasing Tc by doping within each class of high Tc superconductors. The range of values of Δz increases going from one class of high Tc superconductor to another class with higher Tc. These results give a positive experimental test for pairing mechanisms involving Cu 3dz2 holes and in particular for pairing mediated by virtual d-d excitations