444 research outputs found
Saddle point states and energy barriers for vortex entrance and exit in superconducting disks and rings
The transitions between the different vortex states of thin mesoscopic
superconducting disks and rings are studied using the non-linear
Ginzburg-Landau functional. They are saddle points of the free energy
representing the energy barrier which has to be overcome for transition between
the different vortex states. In small superconducting disks and rings the
saddle point state between two giant vortex states, and in larger systems the
saddle point state between a multivortex state and a giant vortex state and
between two multivortex states is obtained. The shape and the height of the
nucleation barrier is investigated for different disk and ring configurations.Comment: 10 pages, 18 figure
Induced order and reentrant melting in classical two-dimensional binary clusters
A binary system of classical charged particles interacting through a dipole
repulsive potential and confined in a two-dimensional hardwall trap is studied
by Brownian dynamics simulations. We found that the presence of small particles
\emph{stabilizes} the angular order of the system as a consequence of radial
fluctuations of the small particles. There is an optimum in the increased
rigidity of the cluster as function of the number of small particles. The small
(i.e. defect) particles melt at a lower temperature compared to the big
particles and exhibit a \emph{reentrant} behavior in its radial order that is
induced by the intershell rotation of the big particles.Comment: 7 pages, 3 figure
On the Brauer groups of symmetries of abelian Dijkgraaf-Witten theories
Symmetries of three-dimensional topological field theories are naturally
defined in terms of invertible topological surface defects. Symmetry groups are
thus Brauer-Picard groups. We present a gauge theoretic realization of all
symmetries of abelian Dijkgraaf-Witten theories. The symmetry group for a
Dijkgraaf-Witten theory with gauge group a finite abelian group , and with
vanishing 3-cocycle, is generated by group automorphisms of , by
automorphisms of the trivial Chern-Simons 2-gerbe on the stack of -bundles,
and by partial e-m dualities.
We show that transmission functors naturally extracted from extended
topological field theories with surface defects give a physical realization of
the bijection between invertible bimodule categories of a fusion category and
braided auto-equivalences of its Drinfeld center. The latter provides the
labels for bulk Wilson lines; it follows that a symmetry is completely
characterized by its action on bulk Wilson lines.Comment: 21 pages, 9 figures. v2: Minor changes, typos corrected and
references added. v3: Typos correcte
From non-semisimple Hopf algebras to correlation functions for logarithmic CFT
We use factorizable finite tensor categories, and specifically the
representation categories of factorizable ribbon Hopf algebras H, as a
laboratory for exploring bulk correlation functions in local logarithmic
conformal field theories. For any ribbon Hopf algebra automorphism omega of H
we present a candidate for the space of bulk fields and endow it with a natural
structure of a commutative symmetric Frobenius algebra. We derive an expression
for the corresponding bulk partition functions as bilinear combinations of
irreducible characters; as a crucial ingredient this involves the Cartan matrix
of the category. We also show how for any candidate bulk state space of the
type we consider, correlation functions of bulk fields for closed oriented
world sheets of any genus can be constructed that are invariant under the
natural action of the relevant mapping class group.Comment: 41 pages, several figures. version 2: typos corrected, bibliography
updated, introduction extended, a few minor clarifications adde
Vortex states in superconducting rings
The superconducting state of a thin superconducting disk with a hole is
studied within the non-linear Ginzburg-Landau theory in which the
demagnetization effect is accurately taken into account. We find that the flux
through the hole is not quantized, the superconducting state is stabilized with
increasing size of the hole for fixed radius of the disk, and a transition to a
multi-vortex state is found if the disk is sufficiently large. Breaking the
circular summetry through a non central location of the hole in the disk
enhances the multi-vortex state.Comment: 11 pages, 23 figures (postscript). To appear in Physical Review B,
Vol. 61 (2000
Dependence of the vortex configuration on the geometry of mesoscopic flat samples
The influence of the geometry of a thin superconducting sample on the
penetration of the magnetic field lines and the arrangement of vortices are
investigated theoretically. We compare superconducting disks, squares and
triangles with the same surface area having nonzero thickness. The coupled
nonlinear Ginzburg-Landau equations are solved self-consistently and the
important demagnetization effects are taken into account. We calculate and
compare quantities like the free energy, the magnetization, the Cooper-pair
density, the magnetic field distribution and the superconducting current
density for the three geometries. For given vorticity the vortex lattice is
different for the three geometries, i.e. it tries to adapt to the geometry of
the sample. This also influences the stability range of the different vortex
states. For certain magnetic field ranges we found a coexistence of a giant
vortex placed in the center and single vortices toward the corners of the
sample. Also the H-T phase diagram is obtained.Comment: 9 pages, 17 figures (submitted to Phys. Rev. B
Superconducting properties of mesoscopic cylinders with enhanced surface superconductivity
The superconducting state of an infinitely long superconducting cylinder
surrounded by a medium which enhances its superconductivity near the boundary
is studied within the nonlinear Ginzburg-Landau theory. This enhancement can be
due to the proximity of another superconductor or due to surface treatment.
Quantities like the free energy, the magnetization and the Cooper-pair density
are calculated. Phase diagrams are obtained to investigate how the critical
field and the critical temperature depend on this surface enhancement for
different values of the Ginzburg-Landau parameter \kappa. Increasing the
superconductivity near the surface leads to higher critical fields and critical
temperatures. For small cylinder diameters only giant vortex states nucleate,
while for larger cylinders multivortices can nucleate. The stability of these
multivortex states also depends on the surface enhancement. For type-I
superconductors we found the remarkable result that for a range of values of
the surface extrapolation length the superconductor can transit from the
Meissner state into superconducting states with vorticity L > 1. Such a
behaviour is not found for the case of large \kappa, i.e. type-II
superconductivity.Comment: submitted to Phys. Rev.
Classical double-layer atoms: artificial molecules
The groundstate configuration and the eigenmodes of two parallel
two-dimensional classical atoms are obtained as function of the inter-atomic
distance (d). The classical particles are confined by identical harmonic wells
and repel each other through a Coulomb potential. As function of d we find
several structural transitions which are of first or second order. For first
(second) order transitions the first (second) derivative of the energy with
respect to d is discontinuous, the radial position of the particles changes
discontinuously (continuously) and the frequency of the eigenmodes exhibit a
jump (one mode becomes soft, i.e. its frequency becomes zero).Comment: 4 pages, RevTex, 5 ps figures, to appear in Phys.Rev.Let
Vortex structure of thin mesoscopic disks in the presence of an inhomogeneous magnetic field
The vortex states in a thin mesoscopic disk are investigated within the
phenomenological Ginzburg-Landau theory in the presence of different ''model''
magnetic field profiles with zero average field which may result from a
ferromagnetic disk or circulating currents in a loop near the superconductor.
We calculated the dependences of both the ground and metastable states on the
magnitude and shape of the magnetic field profile for different values of the
order parameter angular moment, i.e. the vorticity. The regions of existence of
the multi-vortex state and the giant vortex state are found. We analysed the
phase transitions between these states and studied the contribution from
ring-shaped vortices. A new transition between different multi-vortex
configurations as the ground state is found. Furthermore, we found a vortex
state consisting of a central giant vortex surrounded by a collection of
anti-vortices which are located in a ring around this giant vortex. The limit
to a disk with an infinite radius, i.e. a film, will also be discussed. We also
extended our results to ''real'' magnetic field profiles and to the case in
which an external homogeneous magnetic field is present.Comment: 17 pages, 23 figures. Submitted to PR
- …