1,049 research outputs found
Energy cost associated with vortex crossing in superconductors
Starting from the Ginzburg-Landau free energy of a type II superconductor in
a magnetic field we estimate the energy associated with two vortices crossing.
The calculations are performed by assuming that we are in a part of the phase
diagram where the lowest Landau level approximation is valid. We consider only
two vortices but with two markedly different sets of boundary conditions: on a
sphere and on a plane with quasi-periodic boundary conditions. We find that the
answers are very similar suggesting that the energy is localised to the
crossing point. The crossing energy is found to be field and temperature
dependent -- with a value at the experimentally measured melting line of
, where is the Lindemann
melting criterion parameter. The crossing energy is then used with an extension
of the Marchetti, Nelson and Cates hydrodynamic theory to suggest an
explanation of the recent transport experiments of Safar {{\em et al.}\ }.Comment: 15 pages, RevTex v3.0, followed by 5 postscript figure
Free expansion of lowest Landau level states of trapped atoms: a wavefunction microscope
We show that for any lowest-Landau-level state of a trapped, rotating,
interacting Bose gas, the particle distribution in coordinate space in a free
expansion (time of flight) experiment is related to that in the trap at the
time it is turned off by a simple rescaling and rotation. When the
lowest-Landau-level approximation is valid, interactions can be neglected
during the expansion, even when they play an essential role in the ground state
when the trap is present. The correlations in the density in a single snapshot
can be used to obtain information about the fluid, such as whether a transition
to a quantum Hall state has occurred.Comment: 5 pages, no figures. v2: discussion of neglect of interactions during
expansion improved, refs adde
Morse theory of the moment map for representations of quivers
The results of this paper concern the Morse theory of the norm-square of the
moment map on the space of representations of a quiver. We show that the
gradient flow of this function converges, and that the Morse stratification
induced by the gradient flow co-incides with the Harder-Narasimhan
stratification from algebraic geometry. Moreover, the limit of the gradient
flow is isomorphic to the graded object of the
Harder-Narasimhan-Jordan-H\"older filtration associated to the initial
conditions for the flow. With a view towards applications to Nakajima quiver
varieties we construct explicit local co-ordinates around the Morse strata and
(under a technical hypothesis on the stability parameter) describe the negative
normal space to the critical sets. Finally, we observe that the usual Kirwan
surjectivity theorems in rational cohomology and integral K-theory carry over
to this non-compact setting, and that these theorems generalize to certain
equivariant contexts.Comment: 48 pages, small revisions from previous version based on referee's
comments. To appear in Geometriae Dedicat
Scaling in high-temperature superconductors
A Hartree approximation is used to study the interplay of two kinds of
scaling which arise in high-temperature superconductors, namely critical-point
scaling and that due to the confinement of electron pairs to their lowest
Landau level in the presence of an applied magnetic field. In the neighbourhood
of the zero-field critical point, thermodynamic functions scale with the
scaling variable , which differs from the variable
suggested by the gaussian approximation.
Lowest-Landau-level (LLL) scaling occurs in a region of high field surrounding
the upper critical field line but not in the vicinity of the zero-field
transition. For YBaCuO in particular, a field of at least 10 T is needed to
observe LLL scaling. These results are consistent with a range of recent
experimental measurements of the magnetization, transport properties and,
especially, the specific heat of high- materials.Comment: 22 pages + 1 figure appended as postscript fil
Rotating spin-1 bosons in the lowest Landau level
We present results for the ground states of a system of spin-1 bosons in a
rotating trap. We focus on the dilute, weakly interacting regime, and restrict
the bosons to the quantum states in the lowest Landau level (LLL) in the plane
(disc), sphere or torus geometries. We map out parts of the zero temperature
phase diagram, using both exact quantum ground states and LLL mean field
configurations. For the case of a spin-independent interaction we present exact
quantum ground states at angular momentum . For general values of the
interaction parameters, we present mean field studies of general ground states
at slow rotation and of lattices of vortices and skyrmions at higher rotation
rates. Finally, we discuss quantum Hall liquid states at ultra-high rotation.Comment: 24 pages, 14 figures, RevTe
Near threshold eta meson production in the d+d->alpha+eta reaction
The d+d->alpha+eta reaction has been investigated near threshold using the
ANKE facility at COSY-Juelich. Both total and differential cross sections have
been measured at two excess energies, Q=2.6 MeV and 7.7 MeV, with a
subthreshold measurement being undertaken at Q=-2.6 MeV to study the physical
background. While consistent with isotropy at the lower energy, the angular
distribution reveals a pronounced anisotropy at the higher one, indicating the
presence of higher partial waves. Options for the decomposition into partial
amplitudes and their consequences for determination of the s-wave eta-alpha
scattering length are discussed.Comment: 8pp, fig.3 added, normalisation in eq.4.1 correcte
Low-Energy \Lambda-\p Scattering Parameters from the Reaction
Constraints on the spin-averaged scattering length and effective
range have been obtained from measurements of the reaction
close to the production threshold by comparing model phase-space Dalitz plot
occupations with experimental ones. The data fix well the position of the
virtual bound state in the system. Combining this with information
from elastic scattering measurements at slightly higher energies,
together with the fact that the hyperdeuteron is not bound, leads to a new
determination of the low energy scattering parameters.Comment: 18 pages, 7 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
Modelling potential range expansion of an underutilised food security crop in Sub-Saharan Africa
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