VORTEX DYNAMICS AND VORTEX MELTING IN YBCO THIN-FILMS STUDIED BY SQUID

A very sensitive de SQUID flux detection method is used to study the v ortex dynamics in epitaxially grown c-axis oriented YBCO thin films. T he samples are in a magnetic field parallel to the c-axis ranging from zero to 0.4 T. The flux fluctuation near the surface of the thin film samples is measured as a function of temperature without any external disturbance. There is a well-defined noise peak in the flux fluctuati on versus temperature curves. The boundary constructed from the peak p osition is in excellent agreement with the equation derived from the L indemann melting criterion. We suggest that the peak corresponds to a vortex melting transition.