Hole redistribution across interfaces in superconducting cuprate superlattices

With the aim of disentangling the role of the constituent blocks in the high-T-c cuprate superconductors we have measured x-ray absorption spectra at the Cu L-3 edge on (Cu1-delta C delta)Ba-2 CuOx (BCO), CaCuO2 (CCO), and BCO/CCO superconducting superlattices obtained by stacking in a sequence the two blocks. By observing the polarization dependence we could discriminate between the in-plane and out-of-plane hole densities. The results strongly indicate that charge is redistributed across the interfaces from the BCO charge reservoir block to the CCO infinite layer block. In the superconducting samples a relatively high density of out-of-plane holes is still present, whereas when the CCO layer thickness is excessive the holes appear to have preferential in-plane orientation and superconductivity is suppressed.