Vortex melting line and anisotropy of aBa2Ca3Cu4O8(O1−yFy)2 multilayered superconductor

Abstract

We have measured the vortex melting lines of high-pressure-synthesizedBa2Ca3Cu4O8(O1−yFy)2 (nominalcomposition, 2y = 1.3, 1.6and 2.0) F(2y)-0234multilayered high-Tc superconductor using fundamental and third harmonic susceptibility responsescarried out on preferentially oriented crystallites with very low ac field amplitude(5 µT) and in applied dc fields up to 6 T. The vortex melting lines of all three F-substitutedsamples show interesting doping dependence and are very well described by the commonlyaccepted theory of melting lines. Interestingly, we discovered that the models describing thetemperature dependence of the in-plane London penetration depth also dependon the doping level: the 3D XY model for the nearly-optimum-doped sample[F(1.3)], the mean-field theory for the under-doped sample [F(1.6)] and the two-fluidmodel for the heavily-under-doped sample [F(2.0)]. We found that the vortexmelting lines in F-0234 are determined by the interplay in the coupling of pancakevortices through the charge reservoir layer (CRL), and through the block of innerCuO2 planes (IPs), respectively. The anisotropy values (47 for the near-optimally-doped sample)derived from experimental vortex melting lines are consistent with the values obtained byfirst-principles electronic-band-structure calculations.