Thickness Induced Superconducting Insulating Transition in ${\rm Nd}_{1.2}{\rm Ba}_{1.8}{\rm Cu}_{3}{\rm O}_{\rm z}$ Ultra-Thin Films

A study of the thickness induced superconducting to insulator transition in Nd
_1.2
Ba
_1.8
Cu
₃
O
_z
(NdBCO) epitaxial films is presented. The samples have been deposited by high oxygen pressure diode sputtering on SrTiO
₃
(100) substrates. Accurate structural characterizations have been carried out in function of the film thickness by X-ray diffraction using conventional and synchrotron radiation sources. The superconductor to insulator transition is correlated to a structural transformation from a pseudotetragonal phase, characterized by the presence of short Cu(1)O chains in both a and b directions, to a tetragonal structure where Cu(1)O chains are completely destroyed. We demonstrate that the ordering of oxygen ions is essential for the appearance of the superconductivity, while oxygen disorder in the Cu(1)O planes, leads to a two-dimensional MOTT insulating behavior of the resistivity associated to a localization of the carriers within lengths lower than 3 nm.