Topological phase transition of decoupling quasi-two-dimensional vortex pairs in [La.sub.1-y][Sm.sub.y]Mn[O.sub.3 + [delta]]

Characteristic signs of the universal Nelson-Kosterlitz jump of the superconducting liquid density in the temperature dependences of the magnetization of [La.sub.1-y][Sm.sub.y]Mn[O.sub.3 + [delta]] samples with samarium concentrations y = 0.85 and 1.0, which are measured in magnetic fields 100 Oe [less than or equal to] H [less than or equal to] 3.5 kOe, are detected. As the temperature increases, the sample with y = 0.85 exhibits a crescent-shaped singularity in the dc magnetization curve near the critical temperature of decoupling vortex-antivortex pairs ([T.sub.KT] = [T.sub.c] [congruent to] 43 K), which is independent of measuring magnetic field H and is characteristic of the dissociation of 2D vortex pairs. A similar singularity is also detected in the sample with a samarium concentration y = 1.0 at a significantly lower temperature ([T.sub.KT] [congruent to] 12 K). The obtained experimental results are explained in terms of the topological Kosterlitz-Thouless phase transition of dissociation of 2D vortex pairs in a quasi-two-dimensional weak Josephson coupling network.