6,780 research outputs found
An electric-field representation of the harmonic XY model
The two-dimensional harmonic XY (HXY) model is a spin model in which the
classical spins interact via a piecewise parabolic potential. We argue that the
HXY model should be regarded as the canonical classical lattice spin model of
phase fluctuations in two-dimensional condensates, as it is the simplest model
that guarantees the modular symmetry of the experimental systems. Here we
formulate a lattice electric-field representation of the HXY model and contrast
this with an analogous representation of the Villain model and the
two-dimensional Coulomb gas with a purely rotational auxiliary field. We find
that the HXY model is a spin-model analogue of a lattice electric-field model
of the Coulomb gas with an auxiliary field, but with a temperature-dependent
vacuum (electric) permittivity that encodes the coupling of the spin vortices
to their background spin-wave medium. The spin vortices map to the Coulomb
charges, while the spin-wave fluctuations correspond to auxiliary-field
fluctuations. The coupling explains the striking differences in the
high-temperature asymptotes of the specific heats of the HXY model and the
Coulomb gas with an auxiliary field. Our results elucidate the propagation of
effective long-range interactions throughout the HXY model (whose interactions
are purely local) by the lattice electric fields. They also imply that global
spin-twist excitations (topological-sector fluctuations) generated by local
spin dynamics are ergodically excluded in the low-temperature phase. We discuss
the relevance of these results to condensate physics.Comment: 13 pages, 10 figure
Topological-sector fluctuations and ergodicity breaking at the Berezinskii-Kosterlitz-Thouless transition
The Berezinskii-Kosterlitz-Thouless (BKT) phase transition drives the
unbinding of topological defects in many two-dimensional systems. In the
two-dimensional Coulomb gas, it corresponds to an insulator-conductor
transition driven by charge deconfinement. We investigate the global
topological properties of this transition, both analytically and by numerical
simulation, using a lattice-field description of the two-dimensional Coulomb
gas on a torus. The BKT transition is shown to be an ergodicity breaking
between the topological sectors of the electric field, which implies a
definition of topological order in terms of broken ergodicity. The breakdown of
local topological order at the BKT transition leads to the excitation of global
topological defects in the electric field, corresponding to different
topological sectors. The quantized nature of these classical excitations, and
their strict suppression by ergodicity breaking in the low-temperature phase,
afford striking global signatures of topological-sector fluctuations at the BKT
transition. We discuss how these signatures could be detected in experiments
on, for example, magnetic films and cold-atom systems.Comment: 11 pages, 6 figure
Phase order in superfluid helium films
Classic experimental data on helium films are transformed to estimate a
finite-size phase order parameter that measures the thermal degradation of the
condensate fraction in the two-dimensional superfluid. The order parameter is
found to evolve thermally with the exponent , a
characteristic, in analogous magnetic systems, of the
Berezinskii-Kosterlitz-Thouless (BKT) phase transition. Universal scaling near
the BKT fixed point generates a collapse of experimental data on helium and
ferromagnetic films, and implies new experiments and theoretical protocols to
explore the phase order. These results give a striking example of experimental
finite-size scaling in a critical system that is broadly relevant to
two-dimensional Bose fluids.Comment: 6 pages, 2 figure
Banishing AdS ghosts with a UV cutoff
A recent attempt to make sense of scalars in AdS with "Neumann boundary
conditions" outside of the usual BF-window
led to pathologies including (depending on the precise context) either IR
divergences or the appearance of ghosts. Here we argue that such ghosts may be
banished by imposing a UV cutoff. It is also possible to achieve this goal in
certain UV completions. An example is the above AdS theory with a radial cutoff
supplemented by particular boundary conditions on the cutoff surface. In this
case we explicitly identify a region of parameter space for which the theory is
ghost free. At low energies, this theory may be interpreted as the standard
dual CFT (defined with "Dirichlet" boundary conditions) interacting with an
extra scalar via an irrelevant interaction. We also discuss the relationship to
recent works on holographic fermi surfaces and quantum criticality.Comment: 20 pages, 9 figure
Symmetry Breaking Phase Transitions in ABJM Theory with a Finite U(1) Chemical Potential
We consider the U(1) charged sector of ABJM theory at finite temperature,
which corresponds to the Reissner-Nordstrom AdS black hole in the dual type IIA
supergravity description. Including back-reaction to the bulk geometry, we show
that phase transitions occur to a broken phase where SU(4) R-symmetry of the
field theory is broken spontaneously by the condensation of dimension one or
two operators. We show both numerically and analytically that the relevant
critical exponents for the dimension one operator agree precisely with those of
mean field theory in the strongly coupled regime of the large N planar limit.Comment: 22 pages, 6 figures, typos corrected, references added, improved
figures, minor changes, accepted for publication in Phys. Rev.
A Retrospective Examination of Paleoparasitology and its Establishment in the Journal of Parasitology
Volume 95 (2009) of the Journal of Parasitology represented a significant benchmark in the history of paleoparasitology when it received on the cover formal recognition as a topical area for publication. This retrospective examination chronicles the emergence of paleoparasitology, from its origins as an adjunct contribution to the study of prehistoric human populations to its modern expression as a sub-disciplinary interest. The aim of paleoparasitology is to elucidate the temporal and spatial dimensions of parasitism from the fossil record of human and non-human host populations
Motor control for a brushless DC motor
This invention relates to a motor control system for a brushless DC motor having an inverter responsively coupled to the motor control system and in power transmitting relationship to the motor. The motor control system includes a motor rotor speed detecting unit that provides a pulsed waveform signal proportional to rotor speed. This pulsed waveform signal is delivered to the inverter to thereby cause an inverter fundamental current waveform output to the motor to be switched at a rate proportional to said rotor speed. In addition, the fundamental current waveform is also pulse width modulated at a rate proportional to the rotor speed. A fundamental current waveform phase advance circuit is controllingly coupled to the inverter. The phase advance circuit is coupled to receive the pulsed waveform signal from the motor rotor speed detecting unit and phase advance the pulsed waveform signal as a predetermined function of motor speed to thereby cause the fundamental current waveform to be advanced and thereby compensate for fundamental current waveform lag due to motor winding reactance which allows the motor to operate at higher speeds than the motor is rated while providing optimal torque and therefore increased efficiency
New stability results for Einstein scalar gravity
We consider asymptotically anti de Sitter gravity coupled to a scalar field
with mass slightly above the Breitenlohner-Freedman bound. This theory admits a
large class of consistent boundary conditions characterized by an arbitrary
function . An important open question is to determine which admit stable
ground states. It has previously been shown that the total energy is bounded
from below if is bounded from below and the bulk scalar potential
admits a suitable superpotential. We extend this result and show that the
energy remains bounded even in some cases where can become arbitrarily
negative. As one application, this leads to the possibility that in
gauge/gravity duality, one can add a double trace operator with negative
coefficient to the dual field theory and still have a stable vacuum
Stellar spectroscopy: Fermions and holographic Lifshitz criticality
Electron stars are fluids of charged fermions in Anti-de Sitter spacetime.
They are candidate holographic duals for gauge theories at finite charge
density and exhibit emergent Lifshitz scaling at low energies. This paper
computes in detail the field theory Green's function G^R(w,k) of the
gauge-invariant fermionic operators making up the star. The Green's function
contains a large number of closely spaced Fermi surfaces, the volumes of which
add up to the total charge density in accordance with the Luttinger count.
Excitations of the Fermi surfaces are long lived for w <~ k^z. Beyond w ~ k^z
the fermionic quasiparticles dissipate strongly into the critical Lifshitz
sector. Fermions near this critical dispersion relation give interesting
contributions to the optical conductivity.Comment: 38 pages + appendices. 9 figure
Analyzing community resilience as an emergent property of dynamic social-ecological systems
This is the final version of the article. Available from the publisher via the DOI in this record.Community resilience is widely promoted so that communities can respond positively to a range of risks, including shocks, extreme events, and other changes. Although much research has identified characteristics or capacities that confer resilience, resilience is more than simply the sum of these. Resilience is an emergent property—the capacities are linked and act together. We present an empirical analysis of five different capacities and assess how interactions between them confer resilience in two coastal communities in Cornwall, UK. These capacities are place attachment, leadership, community cohesion and efficacy, community networks, and knowledge and learning. Based on a survey and focus group discussions, our results show that residents draw on these capacities in different combinations, enabling resilience in diverse ways. This provides a dynamic and socially nuanced perspective on community resilience as process, potentially informing theory and practice of conservation, disaster risk reduction, climate change adaptation, and community development.This work was supported by the Natural
Environment Research Council (NERC) through Belmont Forum
project, Multi-scale Adaptations to Global Change in Coastlines
(MAGIC) project no: NE/L008807/1, and by the Economic and
Social Research Council (ESRC) through the UK South West
Doctoral Training Centre Studentship Award 2013 (Environment,
Energy and Resilience)
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