Phase transformation in ultrathin Y–Ba–Cu–O films by high-pressure oxygen annealing

To examine the thermodynamic phase stability of Y–Ba–Cu–O superconductors in ultrathin film form, we study the effects of high-pressure oxygen annealing and Cu enrichment on YBa2Cu3O[Formula: see text] films ranging from 8 nm to 15 nm in thickness. The films are grown epitaxially on SrTiO3 substrates by pulsed laser-ablated deposition, and post-annealed in 475 atm of oxygen at 800[Formula: see text]C while subjected to varying concentrations of excess CuO. X-ray diffraction (XRD) on the annealed films shows evidence for suppression of YBa2Cu3O[Formula: see text] and formation of Y2Ba4Cu8O[Formula: see text]. This structural transformation towards a higher-oxidation phase is expectedly correlated with film thinness and Cu enrichment.