Josephson vortex lattice melting in Bi-2212 probed by commensurate oscillations of Josephson flux-flow
Yu. I. Latyshev, V. N. Pavlenko, A. P. Orlov, X. Hu*
Institute of Radio-Engineering and Electronics RAS, 125009 Moscow, Russia
*National Institute for Materials Science, 305-0047 Ibaraki, Japan
PACS: 74.25.Qt, 74.25.Dw, 74.50.+r, 74.72.Hs
Abstract
We studied the commensurate semi-fluxon oscillations of
Josephson flux-flow in a Bi-2212 stacked structures near Tc 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=T0, 3.5 K below the resistive transition
temperature Tc indicating the phase transition of the second
order. This characteristic temperature T0 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.