Novel Superoxygenated Phases in Superconducting Cuprate Thin Films*

The superconducting critical temperature ($T_c$) of hole-doped cuprates tends to increase with their lattice complexity, which is generally correlated with higher states of oxidation. For $\mathrm{YBa_2Cu_3O_{7-\delta}}$ (YBCO-123), it is known that solid-state reaction in high-pressure oxygen can induce the formation of more complex and oxidized phases such as $\mathrm{Y_2Ba_4Cu_7O_{15-\delta}}$ (YBCO-247) and $\mathrm{Y_2Ba_4Cu_8O_{16}}$ (YBCO-248). In this work, we apply this superoxygenation concept of material synthesis to nanoscale thin films which, owing to their large surface-to-volume ratio, are more thermodynamically reactive than bulk samples. Epitaxial thin films of YBCO-123 were grown by pulsed laser deposition on $\mathrm{(La,Sr)(Al,Ta)O_3}$ substrates, and post-annealed in up to 500 atm of oxygen at 800C. Our post-annealed films show robust superconducting transitions with $T_c$ ranging from 80 to 93K. Transmission electron microscopy and X-ray absorption spectroscopy were used to probe the lattice structure and oxygen stoichiometry. Our measurements show clear evidence of conversion to YBCO-247 and YBCO-248 in the superoxygenated films, as well as YBCO-125, a novel YBCO phase that has three CuO chains per unit cell and potentially higher $T_c$.