Sequential electron beam evaporated films of Tl2CaBa2Cu2Oy with zero resistance at 97 K

Thin unoriented polycrystalline films in the Tl-Ca-Ba-Cu-O system were prepared by sequential electron beam evaporation of multiple Tl, Ca, Ba, and Cu layers and a two-stage anneal under controlled Tl and oxygen overpressures. These films show zero resistance as high as 97 K and transport critical current densities at 76 K up to 110 000 A/cm² with typical values of 50 000 A/cm². The critical currents exhibit a modest magnetic field dependence. Meissner effect data show an onset at 110 K and a superconducting fraction of 75%. Compositional and structural studies indicate that the films, although polyphase, are predominantly Tl₂Ca₁Ba₂Cu₂O_y. There is no evidence of other superconducting phases.