Superconducting-insulator transition driven by out-of-plane carrier localization in Nd1.2Ba1.8Cu3O7+x ultrathin films

Polarized x-ray absorption spectroscopy (XAS) at the copper L-2,L-3 and oxygen K edges has been used to study the superconducting-insulating (S-I) transition in Nd1.2Ba1.8Cu3O7+x epitaxial films, realized by reducing the thickness from 9 to 6 unit cells. XAS shows that the main differences between the superconducting and the insulating films cannot be ascribed to changes of Cu density of states within the conducting CuO2 planes, but rather to the increase of the 3d(3z)(2)-r(2) states of copper and to the effective reduction of the charge transfer of holes to the oxygen 2p(xy) orbitals in the CuO2 planes. The experimental results show that the S-I transition is related to the hole localization at the Cu sites of the charge reservoir layer, i.e., outside the CuO2 planes, while the electronic states, lying within the conducting planes, appear to be similar in the insulating and superconducting states.