752 research outputs found
Classical Topological Order in Kagome Ice
We examine the onset of classical topological order in a nearest-neighbor
kagome ice model. Using Monte Carlo simulations, we characterize the
topological sectors of the groundstate using a non-local cut measure which
circumscribes the toroidal geometry of the simulation cell. We demonstrate that
simulations which employ global loop updates that are allowed to wind around
the periodic boundaries cause the topological sector to fluctuate, while
restricted local loop updates freeze the simulation into one topological
sector. The freezing into one topological sector can also be observed in the
susceptibility of the real magnetic spin vectors projected onto the kagome
plane. The ability of the susceptibility to distinguish between fluctuating and
non-fluctuating topological sectors should motivate its use as a local probe of
topological order in a variety of related model and experimental systems.Comment: 17 pages, 9 figure
Origin of the approximate universality of distributions in equilibrium correlated systems
We propose an interpretation of previous experimental and numerical
experiments, showing that for a large class of systems, distributions of global
quantities are similar to a distribution originally obtained for the
magnetization in the 2D-XY model . This approach, developed for the Ising
model, is based on previous numerical observations. We obtain an effective
action using a perturbative method, which successfully describes the order
parameter fluctuations near the phase transition. This leads to a direct link
between the D-dimensional Ising model and the XY model in the same dimension,
which appears to be a generic feature of many equilibrium critical systems and
which is at the heart of the above observations.Comment: To appear in Europhysics Letter
Topological Sector Fluctuations and Curie Law Crossover in Spin Ice
At low temperatures, a spin ice enters a Coulomb phase - a state with
algebraic correlations and topologically constrained spin configurations. In
Ho2Ti2O7, we have observed experimentally that this process is accompanied by a
non-standard temperature evolution of the wave vector dependent magnetic
susceptibility, as measured by neutron scattering. Analytical and numerical
approaches reveal signatures of a crossover between two Curie laws, one
characterizing the high temperature paramagnetic regime, and the other the low
temperature topologically constrained regime, which we call the spin liquid
Curie law. The theory is shown to be in excellent agreement with neutron
scattering experiments. On a more general footing, i) the existence of two
Curie laws appears to be a general property of the emergent gauge field for a
classical spin liquid, and ii) sheds light on the experimental difficulty of
measuring a precise Curie-Weiss temperature in frustrated materials; iii) the
mapping between gauge and spin degrees of freedom means that the susceptibility
at finite wave vector can be used as a local probe of fluctuations among
topological sectors.Comment: 10 pages, 5 figure
Magnetic Monopole Dynamics in Spin Ice
One of the most remarkable examples of emergent quasi-particles, is that of
the "fractionalization" of magnetic dipoles in the low energy configurations of
materials known as "spin ice", into free and unconfined magnetic monopoles
interacting via Coulomb's 1/r law [Castelnovo et. al., Nature, 451, 42-45
(2008)]. Recent experiments have shown that a Coulomb gas of magnetic charges
really does exist at low temperature in these materials and this discovery
provides a new perspective on otherwise largely inaccessible phenomenology. In
this paper, after a review of the different spin ice models, we present
detailed results describing the diffusive dynamics of monopole particles
starting both from the dipolar spin ice model and directly from a Coulomb gas
within the grand canonical ensemble. The diffusive quasi-particle dynamics of
real spin ice materials within "quantum tunneling" regime is modeled with
Metropolis dynamics, with the particles constrained to move along an underlying
network of oriented paths, which are classical analogues of the Dirac strings
connecting pairs of Dirac monopoles.Comment: 26 pages, 12 figure
Dense colloidal suspensions under time-dependent shear
We consider the nonlinear rheology of dense colloidal suspensions under a
time-dependent simple shear flow. Starting from the Smoluchowski equation for
interacting Brownian particles advected by shearing (ignoring fluctuations in
fluid velocity) we develop a formalism which enables the calculation of
time-dependent, far-from-equilibrium averages. Taking shear-stress as an
example we derive exactly a generalized Green-Kubo relation, and an equation of
motion for the transient density correlator, involving a three-time memory
function. Mode coupling approximations give a closed constitutive equation
yielding the time-dependent stress for arbitrary shear rate history. We solve
this equation numerically for the special case of a hard sphere glass subject
to step-strain.Comment: 4 page
Universal Fluctuations of the Danube Water Level: a Link with Turbulence, Criticality and Company Growth
A global quantity, regardless of its precise nature, will often fluctuate
according to a Gaussian limit distribution. However, in highly correlated
systems, other limit distributions are possible. We have previously calculated
one such distribution and have argued that this function should apply
specifically, and in many instances, to global quantities that define a steady
state. Here we demonstrate, for the first time, the relevance of this
prediction to natural phenomena. The river level fluctuations of the Danube are
observed to obey our prediction, which immediately establishes a generic
statistical connection between turbulence, criticality and company growth
statistics.Comment: 5 pages, 1 figur
All-sky interferometric meteor radar meteoroid speed estimation using the Fresnel transform
Fresnel transform meteor speed estimation is investigated. A spectral based technique is developed allowing the transform to be applied at low temporal sampling rates. Simulations are used to compare meteoroid speeds determined using the Fresnel transform and alternative techniques, confirming that the Fresnel transform produces the most accurate meteoroid speed estimates for high effective pulse repetition frequencies (PRFs). The Fresnel transform is applied to high effective PRF data collected during Leonid meteor showers, producing speed estimates in good agreement with the theoretical pre-atmospheric speed of the 71 kms−1. Further simulations for the standard low effective PRF sampling parameters used for Buckland Park meteor radar (BPMR) observations suggests that the Fresnel transform can successfully estimate meteor speeds up to 80 kms−1. Fresnel transform speed estimation is applied using the BPMR, producing speed distributions similar to those obtained in previous studies. The technique is also applied to data collected using the BPMR sampling parameters during Southern delta-Aquarid and Geminid meteor showers, producing speeds in very good agreement with the theoretical pre-atmospheric speeds of these showers (41 kms−1 and 35 kms−1, respectively). However, application of the Fresnel transform to high speed showers suggests that the practical upper limit for accurate speed estimation using the BPMR sampling parameters is around 50 kms−1. This limit allows speed accurate estimates to be made for about 70% of known meteor showers, and around 70% of sporadic echoes.D. A. Holdsworth, W. G. Elford, R. A. Vincent, I. M. Reid, D. J. Murphy, and W. Singe
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