Superconducting to insulating transition in Nd1.2Ba1.8Cu3O7+x thin films studied by polarized X-ray absorption spectroscopy

We report a study of the superconducting-insulator (S-I) transition in epitaxial untwinned Nd1.2Ba1.8Cu3O7+x films using Cu-L and O-K edge polarized X-ray absorption spectroscopy (XAS). The S-I transition in realized by reducing the films thickness from 9 to 6 u.c. Due to strain, thin NdBCO films are characterized by structurally identical CuO2 planes. The major differences in the XAS spectra are not associated to states within the CuO2 units in the ab-plane, but to the unoccupied 3d(3z(2)-r(2)) states of copper at the Cu(1) and Cu(2) sites hybridized with the apical 2p(z) O(4) state, and to the unoccupied 2p(z) O(4) states themselves. The experimental results show that the S-I transition induced by change of thickness in Nd1.2Ba1.8Cu3O7+x films is correlated to holes localization at the copper sites of the Cu(1)O charge reservoir layer. The results open new perspectives for the interpretation of the superconducting to insulating transition in the HTS compounds. (c) 2007 Elsevier B.V. All rights reserved.