Magneto-optical movies of the melting process
Here we present several 'movies' which demonstrate the nucleation and propagation of the vortex-lattice melting transition in BSCCO crystals as observed using the differential MO technique. The differential images in the movies are obtained by subtracting the image at field Ha from the image at Ha+dHa. The fading transition effect between the images is introduced for a more clear visualization.
Movie #1
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This is an example of the melting process in a large BSCCO crystal, which has three visible
crystallographic defects seen as three parallel inclined lines.
Initially the entire sample is in the vortex solid phase, artificially colored in
brown for clarity.
As the field is increased, a liquid droplet (blue) is nucleated at the lower-left corner, and it expands upwards with a very irregular shape. In parallel, independent liquid regions are being formed along the defects, resulting in the formation of complicated patterns. The effects of disorder are clearly visible in this movie since the solid-liquid interface (bright) has well defined preferential directions both parallel and perpendicular to the defects. At elevated field the entire central part of the sample is in the liquid phase, while the vortex solid remains along the edges due to the dome-shaped field profile. The small solid islands, which remain within in the central liquid region at intermediate fields, display hysteretic behavior, and a supercooling of the vortex liquid is observed at these sites upon decreasing the field. |
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| The melting propagation in this crystal shows that the quenched disorder modifies the first-order transition on a mesoscopic scale (the linear size of the liquid droplets is often only a dozen of vortices, which corresponds to observation of melting in atomic solid on a scale of a few hundred angstroms). Although at each location the melting is a sharp first-order transition, the global transition is completely rounded as expected from the general Imry-Wortis arguments. | ||
Movie #2
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This movie shows the melting process in a rather small BSCCO crystal of about 300 microns wide,
seen in dark. The temperature is 60 K, the modulation field is 1 Oe, and the applied
field is 158 Oe, which is slightly below the melting field, so the entire sample is
initially in the vortex solid phase. The gray scale from black to white spans a field
range of 0.2 G.
In this simplest example, as the magnetic field is increased, a bright spot appears in the upper left corner, which is the nucleation of the vortex-liquid phase. The bright spot expands with increasing the field and transforms into a bright ring. This ring is the position of the solid - liquid interface separating the liquid droplet from the surrounding vortex solid. Contrary to the expected behavior in absence of disorder, the liquid droplet is not in the center of the sample and it is not round, and also the solid-liquid interface is very irregular both in its shape and width. At higher fields the entire left part of the crystal is in the liquid phase while a narrow strip of vortex solid remains on the right side. |
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| In all the measurements the images are acquired on increasing and decreasing field. The processes of melting and freezing are mostly reversible, except special regions where hysteretic behavior is found. This movie is the simplest example of the melting process that we have observed, and usually the behavior is much more complicated as shown in the rest of the movies. | ||
Movie #3
| This is another crystal that has a defect across the center of the sample with a small step in thickness. The upper part of the crystal is slightly thicker than the lower part. The entire sample is initially in the vortex solid phase (artificially painted in brown for clarity). The vortex liquid (blue) nucleates at few places along the defect and propagates downward in a finger-like structure. The lower boundary of the liquid expands with the field, however, the upper solid-liquid interface is fully pinned at the defect. In the upper part of the sample the vortex liquid nucleates independently and displays a complicated pattern of numerous aligned solid and liquid domains. | 
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