Localization and oxygen concentration in epitaxial YBa₂Cu₃O_x thin films on (100) SrTiO₃ substrates deposited by laser ablation

Abstract

Thin films of YBa2Cu3Ox were deposited by laser ablation on (100) SrTiO3 at 730°C substrate surface temperature and 0.2 Torr O2 ambient using the third-harmonic Nd:YAG laser pulses. This was followed by a relatively rapid cooling down to 200°C (10-15 min) in various oxygen pressures. X-ray diffraction in the normal Bragg mode and in grazing incidence show that all films were grown epitaxially with their c-axis normal to the interface. Films which were cooled down in 1, 10 and 700 Torr O2 were found to be orthorhombic-superconducting with Tc (R=0) pf 60, 88 and 90 K, and had x values of 6.64, 6.78 and 7.00, respectively. Films which were cooled down in 50 and 200 mTorr O2 were tetragonal with x values of 5.95 and 6.08 and showed localization (a metal to insulator transition) with minimum resistivities at 140 and 50 K, respectively. It is concluded that the films are deposited in the tetragonal-localized phase, which transforms into the orthorhombic-superconducting phase upon cooling only if a sufficient pressure of oxygen ambient is present. © 1989.