552 research outputs found
Square vortex lattice at anomalously low magnetic fields in electron-doped NdCeCuO
We report here on the first direct observations of the vortex lattice in the
bulk of electron-doped NdCeCuO single crystals. Using
small angle neutron scattering, we have observed a square vortex lattice with
the nearest-neighbors oriented at 45 from the Cu-O bond direction,
which is consistent with theories based on the d-wave superconducting gap.
However, the square symmetry persists down to unusually low magnetic fields.
Moreover, the diffracted intensity from the vortex lattice is found to decrease
rapidly with increasing magnetic field.Comment: 4 pages, 4 Figures, accepted for publication in Phys. Rev. Let
Transport and Entanglement Generation in the Bose-Hubbard Model
We study entanglement generation via particle transport across a
one-dimensional system described by the Bose-Hubbard Hamiltonian. We analyze
how the competition between interactions and tunneling affects transport
properties and the creation of entanglement in the occupation number basis.
Alternatively, we propose to use spatially delocalized quantum bits, where a
quantum bit is defined by the presence of a particle either in a site or in the
adjacent one. Our results can serve as a guidance for future experiments to
characterize entanglement of ultracold gases in one-dimensional optical
lattices.Comment: 14 pages, 6 figure
Structure of Flux Line Lattices with Weak Disorder at Large Length Scales
Dislocation-free decoration images containing up to 80,000 vortices have been
obtained on high quality BiSrCaCuO superconducting
single crystals. The observed flux line lattices are in the random manifold
regime with a roughening exponent of 0.44 for length scales up to 80-100
lattice constants. At larger length scales, the data exhibit nonequilibrium
features that persist for different cooling rates and field histories.Comment: 4 pages, 3 gif images, to appear in PRB rapid communicatio
Life remade: critical animation in the digital age
Introduction to Special Issue of animation: an interdisciplinary journal. Animation and contemporary life are enmeshed like never before. A growing number of the media images we consume are in animated form (from fully animated features to CGI laden blockbusters and advertisements); recourse to common animation software and aesthetic approaches significantly blur the lines between previously distinct artistic and design practices (from video games, to special effects, to architecture and contemporary art); and through techniques of computational modelling and visualization, animation is increasingly fundamental to processes of knowledge production and the creation of various modes or elements of life. This appears therefore to be a particularly 'critical' moment to ponder animation's expanded cultural and political role. This special issue also provides an opportunity to consider animation's own powers of critique – the ways in which the digital animated image is increasingly being deployed explicitly as a means of intervening in social and political arenas ranging from human rights advocacy to ecological activism. And finally, we hope this collection of essays serves to further the already rich examination of the politics of more traditional forms of animation in the current digital age. This special issue thus builds upon recent scholarship that has already begun to contend with animation's expanded presence and its inherent political and critical significance..
Plasma resonance at low magnetic fields as a probe of vortex line meandering in layered superconductors
We consider the magnetic field dependence of the plasma resonance frequency
in pristine and in irradiated BiSrCaCuO crystals near . At
low magnetic fields we relate linear in field corrections to the plasma
frequency to the average distance between the pancake vortices in the
neighboring layers (wandering length). We calculate the wandering length in the
case of thermal wiggling of vortex lines, taking into account both Josephson
and magnetic interlayer coupling of pancakes. Analyzing experimental data, we
found that (i) the wandering length becomes comparable with the London
penetration depth near T and (ii) at small melting fields ( G) the
wandering length does not change much at the melting transition. This shows
existence of the line liquid phase in this field range. We also found that
pinning by columnar defects affects weakly the field dependence of the plasma
resonance frequency near .Comment: RevTex, 4 pages, 2 PS figures, Submitted to Phys. Rev.
Monte-Carlo calculation of longitudinal and transverse resistivities in a model Type-II superconductor
We study the effect of a transport current on the vortex-line lattice in
isotropic type-II superconductors in the presence of strong thermal
fluctuations by means of 'driven-diffusion' Monte Carlo simulations of a
discretized London theory with finite magnetic penetration depth. We calculate
the current-voltage (I-V) characteristics for various temperatures, for
transverse as well as longitudinal currents I. From these characteristics, we
estimate the linear resistivities R_xx=R_yy and R_zz and compare these with
equilibrium results for the vortex-lattice structure factor and the helicity
moduli. From this comparison a consistent picture arises, in which the melting
of the flux-line lattice occurs in two stages for the system size considered.
In the first stage of the melting, at a temperature T_m, the structure factor
drops to zero and R_xx becomes finite. For a higher temperature T_z, the second
stage takes place, in which the longitudinal superconducting coherence is lost,
and R_zz becomes finite as well. We compare our results with related recent
numerical work and experiments on cuprate superconductors.Comment: 4 pages, with eps figure
Freezing transition of the vortex liquid in anisotropic superconductors
We study the solid-liquid transition of a model of pancake vortices in
laminar superconductors using a density functional theory of freezing. The
physical properties of the system along the melting line are discussed in
detail. We show that there is a very good agreement with experimental data in
the shape and position of the first order transition in the phase diagram and
in the magnitude and temperature dependence of the magnetic induction jump at
the transition. We analyze the validity of the Lindemann melting criterion and
the Hansen-Verlet freezing criterion. Both criteria are shown to be good to
predict the phase diagram in the region where a first order phase transition is
experimentally observed.Comment: 9 pages, 10 figure
Optimal Control for Generating Quantum Gates in Open Dissipative Systems
Optimal control methods for implementing quantum modules with least amount of
relaxative loss are devised to give best approximations to unitary gates under
relaxation. The potential gain by optimal control using relaxation parameters
against time-optimal control is explored and exemplified in numerical and in
algebraic terms: it is the method of choice to govern quantum systems within
subspaces of weak relaxation whenever the drift Hamiltonian would otherwise
drive the system through fast decaying modes. In a standard model system
generalising decoherence-free subspaces to more realistic scenarios,
openGRAPE-derived controls realise a CNOT with fidelities beyond 95% instead of
at most 15% for a standard Trotter expansion. As additional benefit it requires
control fields orders of magnitude lower than the bang-bang decouplings in the
latter.Comment: largely expanded version, superseedes v1: 10 pages, 5 figure
Disorder Driven Melting of the Vortex Line Lattice
We use Monte Carlo simulations of the 3D uniformly frustrated XY model, with
uncorrelated quenched randomness in the in-plane couplings, to model the effect
of random point pins on the vortex line phases of a type II superconductor. We
map out the phase diagram as a function of temperature T and randomness
strength p for fixed applied magnetic field. We find that, as p increases to a
critical value p_c, the first order vortex lattice melting line turns parallel
to the T axis, and continues smoothly down to low temperature, rather than
ending at a critical point. The entropy jump across this line at p_c vanishes,
but the transition remains first order. Above this disorder driven transition
line, we find that the helicity modulus parallel to the applied field vanishes,
and so no true phase coherent vortex glass exists.Comment: 4 pages, 6 eps figure
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