  We studied the commensurate semi-fluxon oscillations of Josephson flux-flow
(JFF) in a Bi-2212 stacked structures near $T_c$ as a probe of melting of
Josephson vortex lattice (JVL). We found that oscillations exist above 0.5T.
The amplitude of oscillations is found to decrease gradually with temperature
and to turn to zero without any jump at $T=T_0$, 3.5K 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, $T_{BKT}$, 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.

Superconductivity