Vortex melting line and dimensional crossover in Ba2Can−1CunO2n(O1−y,Fy)2 cuprate superconductors

Ba2Can−1CunO2n(O1−y,Fy)2[F(2y)-02(n − 1)n], homologous series with F-substitution at apical oxygen site possess a thinner, 7.4 Å, charge reservoir layer (CRL) and a strong interlayer vortex coupling was expected in these systems. We measured the vortex melting lines of high pressure synthesized Ba2Ca3Cu4O8(O1−yFy)2 (2y = 2.0) multilayered system with 2 inner planes (IP) of CuO2 layers and Ba2Ca1Cu2O4(O1−yFy)2 (2y = 1.4, 1.6 and 2.0) bilayer system with no IP; using fundamental and third harmonic susceptibility response carried out onto preferentially oriented crystallites at very low ac field amplitude and applied dc fields up to 12 T. These experimental melting lines are described by the commonly accepted theory of two-fluid model. We observed 3D to 2D crossover in heavily underdoped F(2.0)-0212 sample, whereas such a vortex lattice crossover is not evident in F(2.0)-0234 sample; and explained on the basis of heavily underdoped and self doped states.