Josephson vortex lattice melting in Bi-2212 probed by commensurate oscillations of Josephson flux-flow

We studied the commensurate semi-fluxon oscillations of Josephson flux-flow in a Bi-2212 stacked structures near $T_c$ as a probe of melting of Josephson vortex lattice. We found that oscillations exist above 0.5 T. The amplitude of oscillations is found to decrease gradually with temperature and to turn to zero without any jump at $T=T_0$, 3.5 K below the resistive transition temperature $T_c$ indicating the phase transition of the second order. This characteristic temperature $T_0$ is identified as the Berezinskii-Kosterlitz-Thouless (BKT) transition temperature, $T_\mathrm{BKT}$, in elementary superconducting layers of Bi-2212 at zero magnetic field. On the base of these facts we infer that melting of triangular Josephson vortex lattice occurs via the BKT phase with formation of characteristic flux loops containing pancake vortices and anti-vortices. The $B-T$ phase diagram of the BKT phase found out from our experiment is consistent with theoretical predictions.