Polarized x-ray absorption spectroscopy (XAS) at the copper $L_{2,3}$ and oxygen $K$ edges has been used to study the superconducting-insulating (S-I) transition in ${\mathrm{Nd}}_{1.2}{\mathrm{Ba}}_{1.8}{\mathrm{Cu}}_3{\mathrm{O}}_{7+x}$ epitaxial films, realized by reducing the thickness from $9\text{to}6\text{unit}\text{cells}$. 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 $\mathrm{Cu}{\mathrm{O}}_2$ 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 $\mathrm{Cu}{\mathrm{O}}_2$ 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 $\mathrm{Cu}{\mathrm{O}}_2$ planes, while the electronic states, lying within the conducting planes, appear to be similar in the insulating and superconducting states.

• Received 22 December 2006

DOI:https://doi.org/10.1103/PhysRevB.75.054519