Unconventional Variation of T_c in the Multilayered Cuprate Superconductor (Cu,C)Ba₂Ca₄Cu₅O_y

Abstract

We report the anomalous T_c behavior in a five-layered compound of the cuprate superconductor (Cu,C)Ba₂Ca₄Cu₅O_y. The T_c decreases once from 94.7 K (as-synthesized) to 93.3 K (annealed at 460 °C under N₂ flow), and then increases to 99.0 K (annealed at 530 °C under N₂ flow). We argue that this T_c change reflects the CuO₂ planes governing the bulk T_c, shifting from the three inner CuO₂ planes to the two outer CuO₂ planes. A model to explain this behavior is proposed, drawing two bell-shaped curves against the oxygen content (doping state), and the T_c values change along the curves from that of the inner CuO₂ plane to that of outer one.