Josephson vortex lattice melting in Bi-2212 probed by commensurate oscillations of Josephson flux-flow
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We studied the commensurate semifluxon oscillations of Josephson flux-flow in Bi-2212 stacked structures near Tc as a probe of melting of a Josephson vortex lattice. We found that oscillations exist above 0.5 T. The amplitude of the oscillations is found to decrease gradually with the temperature and to turn to zero without any jump at T = T0 (3.5 K below the resistive transition temperature Tc), thus, indicating a phase transition of the second order. This characteristic temperature T0 is identified as the Berezinskii-Kosterlitz-Thouless (BKT) transition temperature, TBKT, in the elementary superconducting layers of Bi-2212 at zero magnetic field. On the basis of these facts, we infer that melting of a triangular Josephson vortex lattice occurs via the BKT phase with formation of characteristic flux loops containing pancake vortices and antivortices. The B-T phase diagram of the BKT phase found from our experiment is consistent with theoretical predictions.
PACS numbers74.25.Qt 74.25.Dw 74.50.+r 74.72.Hs
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