1,421 research outputs found
Extruding the vortex lattice: two reacting populations of dislocations
A controllable soft solid is realised in vortex matter in a type II
superconductor. The two-dimensional unit cell area can be varied by a factor of
in the solid phase, without a change of crystal symmetry offering easy
exploration of extreme regimes compared to ordinary materials. The capacity to
confine two-dimensional vortex matter to mesoscopic regions provides an arena
for the largely unexplored metallurgy of plastic deformation at large density
gradients. Our simulations reveal a novel plastic flow mechanism in this driven
non-equilibrium system, utilising two distinct, but strongly interacting,
populations of dislocations. One population facilitates the relaxation of
density; a second aids the relaxation of shear stresses concentrated at the
boundaries. The disparity of the bulk and shear moduli in vortex matter ensures
the dislocation motion follows the overall continuum flow reflecting density
variation
First order phase transition from the vortex liquid to an amorphous solid
We present a systematic study of the topology of the vortex solid phase in
superconducting BiSrCaCuO samples with low doses of
columnar defects. A new state of vortex matter imposed by the presence of
geometrical contours associated with the random distribution of columns is
found. The results show that the first order liquid-solid transition in this
vortex matter does not require a structural symmetry change.Comment: 4 pages, 5 figure
Hysteretic behavior of the vortex lattice at the onset of the second peak for HgBaCuO superconductor
By means of local Hall probe ac and dc permeability measurements we
investigated the phase diagram of vortex matter for the HgBaCuO superconductor in the regime near the critical temperature. The second peak
line, , in contrast to what is usually assumed, doesn't terminate
at the critical temperature. Our local ac permeability measurements revealed
pronounced hysteretic behavior and thermomagnetic history effects near the
onset of the second peak, giving evidence for a phase transition of vortex
matter from an ordered qausilattice state to a disordered glass
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
Langevin Dynamics of the vortex matter two-stage melting transition in Bi_2Sr_2CaCu_2O in the presence of straight and of tilted columnar defects
In this paper we use London Langevin molecular dynamics simulations to
investigate the vortex matter melting transition in the highly anisotropic
high-temperature superconductor material Bi_2Sr_2CaCu_2O in the
presence of low concentration of columnar defects (CDs). We reproduce with
further details our previous results obtained by using Multilevel Monte Carlo
simulations that showed that the melting of the nanocrystalline vortex matter
occurs in two stages: a first stage melting into nanoliquid vortex matter and a
second stage delocalization transition into a homogeneous liquid. Furthermore,
we report on new dynamical measurements in the presence of a current that
identifies clearly the irreversibility line and the second stage delocalization
transition. In addition to CDs aligned along the c-axis we also simulate the
case of tilted CDs which are aligned at an angle with respect to the applied
magnetic field. Results for CDs tilted by with respect to c-axis
show that the locations of the melting and delocalization transitions are not
affected by the tilt when the ratio of flux lines to CDs remains constant. On
the other hand we argue that some dynamical properties and in particular the
position of the irreversibility line should be affected.Comment: 13 pages, 11 figure
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