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 (–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 N2 flow at various temperatures to modify the oxygen content. While Tc, the lattice constants and T0 change monotonically with y, the irreversibility lines exhibit a discontinuous drop at 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 Tc. 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 Jc and Birr, even if the change in Tc is small. This result is also expected to be applicable to other multi-layered superconductors.