Long-range-order correlations in isotropic systems: HT phase diagram of polycrystalline high T_c superconductors

We report on the effect of thermal fluctuations in the superconducting state of polycrystalline La_1.8Sr_0.2CuO_7-δ in the presence of an applied magnetic field. The intrinsic isotropy of the system allowed us to study the EJ characteristics with J either parallel or perpendicular to H. The scaling behavior of the EJ curves are consistent with predictions of critical phenomena theories. However, the long-range-order correlation loss takes place in two stages in the high field region. When increasing temperature, we first observe the long-range-order loss in the direction transversal to the field. At higher temperatures the correlation parallel to the field also disappears. In the weak field region thermal fluctuations cause the transition to occur in a single line.