2,101 research outputs found
Nature of finite-temperature transition in anisotropic pyrochlore Er2Ti2O7
We study the finite-temperature transition in a model XY antiferromagnet on a
pyrochlore lattice, which describes the pyrochlore material Er2Ti2O7. The
ordered magnetic structure selected by thermal fluctuations is six-fold
degenerate. Nevertheless, our classical Monte Carlo simulations show that the
critical behavior corresponds to the three-dimensional XY universality class.
We determine an additional critical exponent nu_6=0.75>nu characteristic of a
dangerously irrelevant scaling variable. Persistent thermal fluctuations in the
ordered phase are revealed in Monte Carlo simulations by the peculiar
coexistence of Bragg peaks and diffuse magnetic scattering, the feature also
observed in neutron diffraction experiments.Comment: 5+5 pages (including supplemental material
Onsager's Wien Effect on a Lattice
The Second Wien Effect describes the non-linear, non-equilibrium response of
a weak electrolyte in moderate to high electric fields. Onsager's 1934
electrodiffusion theory along with various extensions has been invoked for
systems and phenomena as diverse as solar cells, surfactant solutions, water
splitting reactions, dielectric liquids, electrohydrodynamic flow, water and
ice physics, electrical double layers, non-Ohmic conduction in semiconductors
and oxide glasses, biochemical nerve response and magnetic monopoles in spin
ice. In view of this technological importance and the experimental ubiquity of
such phenomena, it is surprising that Onsager's Wien effect has never been
studied by numerical simulation. Here we present simulations of a lattice
Coulomb gas, treating the widely applicable case of a double equilibrium for
free charge generation. We obtain detailed characterisation of the Wien effect
and confirm the accuracy of the analytical theories as regards the field
evolution of the free charge density and correlations. We also demonstrate that
simulations can uncover further corrections, such as how the field-dependent
conductivity may be influenced by details of microscopic dynamics. We conclude
that lattice simulation offers a powerful means by which to investigate
system-specific corrections to the Onsager theory, and thus constitutes a
valuable tool for detailed theoretical studies of the numerous practical
applications of the Second Wien Effect.Comment: Main: 12 pages, 4 figures. Supplementary Information: 7 page
Spin ice under pressure: symmetry enhancement and infinite order multicriticality
We study the low-temperature behaviour of spin ice when uniaxial pressure
induces a tetragonal distortion. There is a phase transition between a Coulomb
liquid and a fully magnetised phase. Unusually, it combines features of
discontinuous and continuous transitions: the order parameter exhibits a jump,
but this is accompanied by a divergent susceptibility and vanishing domain wall
tension. All these aspects can be understood as a consequence of an emergent
SU(2) symmetry at the critical point. We map out a possible experimental
realisation
A Three Dimensional Kasteleyn Transition: Spin Ice in a [100] Field
We examine the statistical mechanics of spin-ice materials with a [100]
magnetic field. We show that the approach to saturated magnetisation is, in the
low-temperature limit, an example of a 3D Kasteleyn transition, which is
topological in the sense that magnetisation is changed only by excitations that
span the entire system. We study the transition analytically and using a Monte
Carlo cluster algorithm, and compare our results with recent data from
experiments on Dy2Ti2O7.Comment: 4 pages, 5 figure
Mesospheric turbulent velocity estimation using the Buckland Park MF radar
Copyright © 2001 European Geosciences UnionThis paper investigates turbulent velocity estimation using the full correlation analysis (FCA) of spaced antenna (SA) data, and its application to the routine FCA observations of the Buckland Park MF (BPMF) radar. The effects of transmitter beamwidths are investigated, confirming the suggestions of previous authors that wide transmit beam widths lead to an overestimation of the turbulent velocity. The annual variation of the turbulent velocity is investigated, revealing an increase in turbulent velocity with height, and equinoctal minima and solstice maxima observed below 80 km. Investigations of the turbulent velocities about the March diurnal tide maximum reveals a diurnal variation in phase with the zonal velocity. Harmonic analysis reveals this relationship exists between February and September. Descending power layers are also observed during this period. A number of mechanisms are proposed to describe these observations.D. A. Holdsworth, R. A. Vincent, and I. M. Rei
Towards an Adaptive OS Noise Mitigation Technique for Microbenchmarking on Apple Ipad Devices
This study investigates levels of Operating System (OS) noise on Apple iPad mobile devices. OS noise causes variations in application performance that interfere with microbenchmark results. OS noise manifests in collected data through extreme outliers and variations in skewness. Using our collected data, we develop an iterative, semi-automated outlier removal process for Apple iPad OS noise profiles. The profiles generated by outlier removal represent the first step toward an adaptive noise mitigation technique, which presents opportunities for use in microbenchmarking across other mobile platforms
Quantum order by disorder and accidental soft mode Er2Ti2O7
Motivated by recent neutron scattering experiments, we derive and study an
effective "pseudo-dipolar" spin-1/2 model for the XY pyrochlore antiferromagnet
Er2Ti2O7. While a bond-dependent in-plane exchange anisotropy removes any
continuous symmetry, it does lead to a one-parameter `accidental' classical
degeneracy. This degeneracy is lifted by quantum fluctuations in favor of the
non-coplanar spin structure observed experimentally -- a rare experimental
instance of quantum order by disorder. A non-Goldstone low-energy mode is
present in the excitation spectrum in accordance with inelastic neutron
scattering data. Our theory also resolves the puzzle of the experimentally
observed continuous ordering transition, absent from previous models.Comment: 5 pages, 4 figures, final versio
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
In-situ monitoring for CVD processes
Aiming towards process control of industrial high yield/high volume CVD reactors, the potential of optical sensors as a monitoring tool has been explored. The sensors selected are based on both Fourier transform infrared spectroscopy (FTIR) and tunable diode laser spectroscopy (NIR-DLS). The former has the advantage of wide spectral capability, and well established databases. NIR-DLS spectroscopy has potentially high sensitivity, laser spatial resolution, and the benefits of comparatively easier integration capabilities-including optical fibre compatibility. The proposed technical approach for process control is characterised by a 'chemistry based' feedback system with in-situ optical data as input information. The selected optical sensors continuously analyze the gas phase near the surface of the growing layer. The spectroscopic data has been correlated with process performance and layer properties which, in turn establish data basis for process control. The new process control approach is currently being verified on different industrialised CVD coaters. One of the selected applications deals with the deposition of SnO2 layers on glass based on the oxidation of (CH3)2SnCl2, which is used in high volume production for low-E glazing
Raised immunoglobulin E and idiopathic bronchiectasis
SummaryWe describe a series of 4 subjects with markedly raised immunoglobulin E levels in association with idiopathic bronchiectasis. This has not been described previously and appears to be a distinct entity from other conditions such as allergic bronchopulmonary aspergillosis
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