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 controversely discussed in the community. Several recent observations made by us, using polarization-dependent O1 s and Cu2 p near-edge X-ray absorption spectroscopy on single crystals of infinite-layer compounds, of Ca-doped T′ phase compounds, and of Y 1− x Ca x Ba 2 Cu 3 O y , are all consistent with the conjecture that there is significant influence of the apical site on high- T c superconductivity. Most notably, the Ca-dependent hole counts for the latter family show that superconductivity is absent if there are no holes on the apical site. This is independent of the hole count in the CuO 2 planes.