Differential magneto-optical imaging
Movie #1
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In this movie the field is fixed at B = 30 G and
the temperature is varied up and down across the melting transition. The
images were obtained by differential magneto-optical imaging, with
temperature modulation of 0.3 K. For clarity, the vortex solid has been
colored in two shades; the orange-brown color corresponds to the vortex solid
in the pristine regions of the sample and a darker brown color for the
irradiated circular regions. Similarly, the dark blue corresponds to the
vortex liquid in the pristine regions and the lighter blue corresponds to the
vortex liquid in the irradiated apertures.
In the movie it is clearly seen that the pristine unirradiated regions of the sample melt well before the irradiated apertures, in which the melting temperature is shifted by about 1 K with respect to the neighboring unirradiated regions. In this temperature interval the vortex solid inside the apertures (dark brown) is surrounded by vortex liquid phase in the pristine regions (dark blue). This is the first observation of an upward shift of the melting line while preserving its first-order nature. In the irradiated regions one can also see that the melting transition is broadened to about 0.3 K. |
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Movie #2
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This movie is similar to movie #1, but at a higher field
of B = 50 G. What is clear from this movie is that the delay in
melting inside the irradiated apertures is significantly larger than in movie
#1 at B = 30 G. Also the width of the melting transition in the
irradiated regions has increased. At B = 50 G, the irradiated
apertures melt over a temperature range of about 1 K.
Another interesting feature is the difference in the shape of the melting patterns in the irradiated regions in the two movies. We believe that the variations in shape as well as the width of the transition are another manifestation of the porous state of the vortex matter as described below. |
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Figure 3 shows the H - T phase diagram and the upward shift of the melting line in BSCCO crystals for various densities of columnar defects. We have also found that the shifted first-order melting lines (solid lines) terminate at critical points, beyond which the melting transition becomes continuous (dotted lines). Another important observation is the sharp kink in the melting line marked by the arrows, which becomes more pronounced with increasing the density of columnar defects. |
| We propose that when vortices outnumber columnar defects a
'porous' vortex matter is formed as shown by the Bitter decoration images in
Figure 4(b).
Figure 4(c) shows the location of six fold coordinated vortices (open dots) and non - six - fold coordinated vortices (solid dots). This is a heterogeneous state which consists of two populations of vortices. The vortices localized on the columnar defects form a rigid matrix of pinned vortices. In the pores of this matrix softer vortex crystallites are formed by the interstitial vortices, which significantly outnumber the columnar defects. The confining walls of the rigid matrix cause an upward shift of the melting transition of the vortex crystallites relative to pristine melting (region 1 with respect to the Bf = 50 G data in Fig. 3). The crystallites melt at Bmpor while the matrix remains solid, forming an interstitial liquid within the pores of the matrix (region 2). At higher temperatures the matrix melts at Bmmtx and a homogeneous liquid is obtained in region 3. |
Click for enlargement |
