Orbital structure of copper in high temperature superconductors

The orbital structure of copper ions in high- T c cuprates is discussed in a localized picture. Experimental indications that the d z 2 -orbital becomes relevant, especially at high doping levels, are summarized. The possible mechanism of this effect, notably the crossing of d x 2 − y 2 and d z 2 -levels with doping, is identified and corresponding numerical calculations are carried out. It is shown that e.g. for La 2− x Sr x CuO 4 the crossing of these levels may occur at x ∼0.4. The consequences of this phenomenon for the stability of the system and for its superconductivity are briefly discussed.