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

Abstract

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 YBa 2 Cu 3 O 7−δ films ranging from 8 nm to 15 nm in thickness. The films are grown epitaxially on SrTiO 3 substrates by pulsed laser-ablated deposition, and postannealed in 475 atm of oxygen at 800 • C while subjected to varying concentrations of excess CuO. X-ray diffraction (XRD) on the annealed films shows evidence for suppression of YBa 2 Cu 3 O 7−δ and formation of Y 2 Ba 4 Cu 8 O 16 . This structural transformation towards a higher-oxidation phase is expectedly correlated with film thinness and Cu enrichment.