Interaction of ${\mathrm{H}}_2$O with a high-temperature superconductor

The effect of water on the high-temperature superconductor ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ has been studied. Photoemission measurements show that water modifies valence-band states, reduces the number of inequivalent Ba sites and induces O 1s corelevel components at 534 and 531 eV. The 534-eV core-level component is coincident with that of cryosorbed ice and the enhanced 531-eV component is due to a new water-induced bonding environment in the superconductor; this is most probably a hydroxyl species. Changes in the Ba core-level appearance suggest that water may be preferentially modifying ${\mathrm{YBa}}_2$${\mathrm{CuO}}_7$ in a way that affects the Ba atomic environment. The effect of water on superconductivity and its possible implications for bulk superconducting transport in ceramics is discussed. Previous reports suggesting oxygen dimerization below $T_c$, as well as other reported changes in the spectrum, are improbable in view of these present results.