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Vortex Matter and its Phase Transitions
The mixed state of type II superconductors has magnetic flux penetrating the
sample in the form of vortices, with each vortex carrying an identical quantum
of flux. These vortices generally form a triangular lattice under weak mutually
repulsive forces; the lattice spacing can be easily varied over many orders of
magnitude by varying the external magnetic field. The elastic moduli of this
lattice are small and this soft vortex matter can undergo phase transitions
like normal matter, but with thermal fluctuations and underlying defects
playing an important role. We discuss experimental studies on vortex matter
phase transitions, with some emphasis on DC magnetisation measurements
investigating the nature of the phase transition.Comment: 11 pages and 4 figure
Calculation of minor hysteresis loops under metastable to stable transformations in vortex matter
We present a model in which metastable supercooled phase and stable
equilibrium phase of vortex matter coexist in different regions of a sample.
Minor hysteresis loops are calculated with the simple assumption of the two
phases of vortex matter having field-independent critical current densities. We
use our earlier published ideas that the free energy barrier separating the
metastable and stable phases reduces as the magnetic induction moves farther
from the first order phase transition line, and that metastable to stable
transformations occur in local regions of the sample when the local energy
dissipation exceeds a critical value. Previously reported anomalous features in
minor hysteresis loops are reproduced, and calculated field profiles are
presented.Comment: 9pages, 7 figure
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