T-c enhancement by partial substitution of Ca in superconducting YbBa2Cu4O8 epitaxial films prepared by coating-pyrolysis process

Ca-substituted epitaxial YbBa2Cu4O8 (Yb124) films were prepared by a coatin g-pyrolysis process to enhance the superconducting transition temperature. The Ca-doped Yb124 films showed higher T-c values, e.g., 81K, by AC-suscept ibility measurement for the film with a nominal composition of Yb :Ca: Ba: Cu = 0.95 : 0.05: 2: 4. These films contained a small proportion of Ca-dope d YbBa2Cu3Oy [Yb(Ca)123] as an impurity phase. However, these high T, value s should be attributed not to Yb(Ca)123 but to (Yb1-xCax)Ba2Cu4O8, because both the slow- and fast-cooled films showed almost the same T-c values, tha t are higher than those of the Ca-free films. (C) 2001 Elsevier Science B.V . All rights reserved.