Topological phase transition of decoupling quasi-two-dimensional vortex pairs in La1–ySmyMnO3 + δ (y = 0.85, 1.0)

Abstract

Characteristic signs of the universal Nelson–Kosterlitz jump of the superconducting liquid density in the temperature dependences of the magnetization of La_1–ySm_yMnO_{3 + δ} samples with samarium concentrations y = 0.85 and 1.0, which are measured in magnetic fields 100 Oe ≤ H ≤ 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_KT ≡ T_c ≈ 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_KT ≈ 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.