Anomalous behaviour of irreversibility lines in multi-layered superconductor (Cu, C)Ba2Ca3Cu4Oy

The physical properties of a multi-layered cuprate superconductor (Cu0.6C0.4)Ba2Ca3Cu4Oy are investigated using high-quality samples of varying oxygen content (y approximate to 11.18-10.34). Samples are synthesized using a newly developed method capable of fine control of the starting oxygen and carbon compositions, followed by annealing under N-2 flow at various temperatures to modify the oxygen content. While T-c, the lattice constants and T-o change monotonically with y, the irreversibility lines exhibit a discontinuous drop at y less than or equal to 10.45 amongst a more gradual decrease. This discontinuity has not been observed before for single- or double-layered CuO2 superconductors. This anomaly is attributed to the presence of two different order parameters in this material, arising from the large difference in carrier concentrations of the outer five-coordinated CuO2 planes and inner four-coordinated CuO2 planes. The superconducting gap in the outer planes develops with decreasing y (approaching the optimal doping state), suppressing the decrease in the irreversibility line, while the carrier insensitivity of the inner planes limits the change in T-c Two different kinks are also observed in the irreversibility lines, tentatively attributed to the development of the superconducting gap and 2D-3D crossover in a vortex system. Control of the doping state of both the inner and outer planes is demonstrated to be important in the design of J(c) and B-irr, even if the change in T-c is small. This result is also expected to be applicable to other multi-layered superconductors.