12,272 research outputs found
Fractionalization and confinement in the U(1) and gauge theories of strongly correlated systems
Recently, we have elucidated the physics of electron fractionalization in
strongly interacting electron systems using a gauge theory formulation.
Here we discuss the connection with the earlier U(1) gauge theory approaches
based on the slave boson mean field theory. In particular, we identify the
relationship between the holons and Spinons of the slave-boson theory and the
true physical excitations of the fractionalized phases that are readily
described in the approach.Comment: 4 page
The tidal stripping of satellites
We present an improved analytic calculation for the tidal radius of
satellites and test our results against N-body simulations.
The tidal radius in general depends upon four factors: the potential of the
host galaxy, the potential of the satellite, the orbit of the satellite and
{\it the orbit of the star within the satellite}. We demonstrate that this last
point is critical and suggest using {\it three tidal radii} to cover the range
of orbits of stars within the satellite. In this way we show explicitly that
prograde star orbits will be more easily stripped than radial orbits; while
radial orbits are more easily stripped than retrograde ones. This result has
previously been established by several authors numerically, but can now be
understood analytically. For point mass, power-law (which includes the
isothermal sphere), and a restricted class of split power law potentials our
solution is fully analytic. For more general potentials, we provide an equation
which may be rapidly solved numerically. Over short times (\simlt 1-2 Gyrs
satellite orbit), we find excellent agreement between our analytic and
numerical models. Over longer times, star orbits within the satellite are
transformed by the tidal field of the host galaxy. In a Hubble time, this
causes a convergence of the three limiting tidal radii towards the prograde
stripping radius. Beyond the prograde stripping radius, the velocity dispersion
will be tangentially anisotropic.Comment: 10 pages, 5 figures. Final version accepted for publication in MNRAS.
Some new fully analytic tidal radii have been added for power law density
profiles (including the isothermal sphere) and some split power law
Thermal metal in network models of a disordered two-dimensional superconductor
We study the universality class for localization which arises from models of
non-interacting quasiparticles in disordered superconductors that have neither
time-reversal nor spin-rotation symmetries. Two-dimensional systems in this
category, which is known as class D, can display phases with three different
types of quasiparticle dynamics: metallic, localized, or with a quantized
(thermal) Hall conductance. Correspondingly, they can show a variety of
delocalization transitions. We illustrate this behavior by investigating
numerically the phase diagrams of network models with the appropriate symmetry,
and for the first time show the appearance of the metallic phase.Comment: 5 pages, 3 figure
Escape path complexity and its context dependency in Pacific blue-eyes (Pseudomugil signifer)
The escape trajectories animals take following a predatory attack appear to
show high degrees of apparent 'randomness' - a property that has been described
as 'protean behaviour'. Here we present a method of quantifying the escape
trajectories of individual animals using a path complexity approach. When fish
(Pseudomugil signifer) were attacked either on their own or in groups, we find
that an individual's path rapidly increases in entropy (our measure of
complexity) following the attack. For individuals on their own, this entropy
remains elevated (indicating a more random path) for a sustained period (10
seconds) after the attack, whilst it falls more quickly for individuals in
groups. The entropy of the path is context dependent. When attacks towards
single fish come from greater distances, a fish's path shows less complexity
compared to attacks that come from short range. This context dependency effect
did not exist, however, when individuals were in groups. Nor did the path
complexity of individuals in groups depend on a fish's local density of
neighbours. We separate out the components of speed and direction changes to
determine which of these components contributes to the overall increase in path
complexity following an attack. We found that both speed and direction measures
contribute similarly to an individual's path's complexity in absolute terms.
Our work highlights the adaptive behavioural tactics that animals use to avoid
predators and also provides a novel method for quantifying the escape
trajectories of animals.Comment: 9 page
QUAGMIRE v1.3: a quasi-geostrophic model for investigating rotating fluids experiments
QUAGMIRE is a quasi-geostrophic numerical model for performing fast, high-resolution simulations of multi-layer rotating annulus laboratory experiments on a desktop personal computer. The model uses a hybrid finite-difference/spectral approach to numerically integrate the coupled nonlinear partial differential equations of motion in cylindrical geometry in each layer. Version 1.3 implements the special case of two fluid layers of equal resting depths. The flow is forced either by a differentially rotating lid, or by relaxation to specified streamfunction or potential vorticity fields, or both. Dissipation is achieved through Ekman layer pumping and suction at the horizontal boundaries, including the internal interface. The effects of weak interfacial tension are included, as well as the linear topographic beta-effect and the quadratic centripetal beta-effect. Stochastic forcing may optionally be activated, to represent approximately the effects of random unresolved features. A leapfrog time stepping scheme is used, with a Robert filter. Flows simulated by the model agree well with those observed in the corresponding laboratory experiments
Correlation among Cirrus Ice Content, Water Vapor and Temperature in the TTL as Observed by CALIPSO and Aura-MLS
Water vapor in the tropical tropopause layer (TTL) has a local radiative cooling effect. As a source for ice in cirrus clouds, however, it can also indirectly produce infrared heating. Using NASA A-Train satellite measurements of CALIPSO and Aura/MLS we calculated the correlation of water vapor, ice water content and temperature in the TTL. We find that temperature strongly controls water vapor (correlation r =0.94) and cirrus clouds at 100 hPa (r = 0.91). Moreover we observe that the cirrus seasonal cycle is highly (r =0.9) anticorrelated with the water vapor variation in the TTL, showing higher cloud occurrence during December-January-February. We further investigate the anticorrelation on a regional scale and find that the strong anticorrelation occurs generally in the ITCZ (Intertropical Convergence Zone). The seasonal cycle of the cirrus ice water content is also highly anticorrelated to water vapor (r = 0.91) and our results support the hypothesis that the total water at 100 hPa is roughly constant. Temperature acts as a main regulator for balancing the partition between water vapor and cirrus clouds. Thus, to a large extent, the depleting water vapor in the TTL during DJF is a manifestation of cirrus formation
Spin-Peierls states of quantum antiferromagnets on the lattice
We discuss the quantum paramagnetic phases of Heisenberg antiferromagnets on
the 1/5-depleted square lattice found in . The possible phases of
the quantum dimer model on this lattice are obtained by a mapping to a
quantum-mechanical height model. In addition to the ``decoupled'' phases found
earlier, we find a possible intermediate spin-Peierls phase with
spontaneously-broken lattice symmetry. Experimental signatures of the different
quantum paramagnetic phases are discussed.Comment: 9 pages; 2 eps figure
Mesoscale theory of grains and cells: crystal plasticity and coarsening
Solids with spatial variations in the crystalline axes naturally evolve into
cells or grains separated by sharp walls. Such variations are mathematically
described using the Nye dislocation density tensor. At high temperatures,
polycrystalline grains form from the melt and coarsen with time: the
dislocations can both climb and glide. At low temperatures under shear the
dislocations (which allow only glide) form into cell structures. While both the
microscopic laws of dislocation motion and the macroscopic laws of coarsening
and plastic deformation are well studied, we hitherto have had no simple,
continuum explanation for the evolution of dislocations into sharp walls. We
present here a mesoscale theory of dislocation motion. It provides a
quantitative description of deformation and rotation, grounded in a microscopic
order parameter field exhibiting the topologically conserved quantities. The
topological current of the Nye dislocation density tensor is derived from a
microscopic theory of glide driven by Peach-Koehler forces between dislocations
using a simple closure approximation. The resulting theory is shown to form
sharp dislocation walls in finite time, both with and without dislocation
climb.Comment: 5 pages, 3 figure
Scaling and Crossover Functions for the Conductance in the Directed Network Model of Edge States
We consider the directed network (DN) of edge states on the surface of a
cylinder of length L and circumference C. By mapping it to a ferromagnetic
superspin chain, and using a scaling analysis, we show its equivalence to a
one-dimensional supersymmetric nonlinear sigma model in the scaling limit, for
any value of the ratio L/C, except for short systems where L is less than of
order C^{1/2}. For the sigma model, the universal crossover functions for the
conductance and its variance have been determined previously. We also show that
the DN model can be mapped directly onto the random matrix (Fokker-Planck)
approach to disordered quasi-one-dimensional wires, which implies that the
entire distribution of the conductance is the same as in the latter system, for
any value of L/C in the same scaling limit. The results of Chalker and Dohmen
are explained quantitatively.Comment: 10 pages, REVTeX, 2 eps figure
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