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

Abstract

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 controlof the starting oxygen and carbon compositions, followed by annealing underN2 flow at various temperatures to modify the oxygen content. WhileTc, the lattice constantsand T0 changemonotonically 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-layeredCuO2 superconductors. This anomaly is attributed to the presence of two different order parameters inthis material, arising from the large difference in carrier concentrations of the outer five-coordinatedCuO2 planes and innerfour-coordinated CuO2 planes. The superconducting gap in the outer planes develops with decreasingy (approaching the optimal doping state), suppressing the decrease in the irreversibilityline, while the carrier insensitivity of the inner planes limits the change inTc. Two different kinks are also observed in the irreversibility lines, tentatively attributed to thedevelopment of the superconducting gap and 2D–3D crossover in a vortex system. Control of thedoping state of both the inner and outer planes is demonstrated to be important in the design ofJc andBirr, even if thechange in Tc is small. This result is also expected to be applicable to other multi-layered superconductors.