Role of apical holes for superconductivity in p-type cuprates: NEXAFS results

From the early days of high-temperature superconductivity on, the question of a possible role of apical sites in p-type superconductors has been contr oversely discussed in the community. Several recent observations made by us , using polarization-dependent O1s and Cu2p near-edge X-ray absorption spec troscopy on single crystals of infinite-layer compounds, of Ca-doped T ' ph ase compounds, and of Y1-xCaxBa2Cu3Oy, are all consistent with the conjectu re that there is significant influence of the apical site on high-T-c super conductivity. Most notably, the Ca-dependent hole counts for the latter fam ily show that superconductivity is absent if there are no holes on the apic al site. This is independent of the hole count in the CuO2 planes.