367 research outputs found
Half moons are pinch points with dispersion
"Pinch points," singular features observed in (quasi-)elastic neutron
scattering, are a widely discussed hallmark of spin liquids with an emergent
gauge symmetry. Much less attention has been paid to "half moons," distinctive
crescent patterns at finite energy, which have been observed in experiments on
a number of pyrochlore magnets, and in a wide range of model calculations. Here
we unify these two phenomena within a single framework, paying particular
attention to the case of ordered, or field-saturated states, where pinch points
and half moons can be found in bands of excitations above a gap. We find that
half moons are nothing other than pinch points inscribed on a dispersing band.
Molecular dynamics simulations of the kagome lattice antiferromagnet are used
to explore how these bands evolve into the ground state and excitations of a
classical spin liquid. We explicitly demonstrate that this theory can reproduce
the pinch points and half moons observed in NdZrO.Comment: 6 pages, 4 figures. Supplementary material: 10 pages, 3 figure
On Polynomial Multiplication in Chebyshev Basis
In a recent paper Lima, Panario and Wang have provided a new method to
multiply polynomials in Chebyshev basis which aims at reducing the total number
of multiplication when polynomials have small degree. Their idea is to use
Karatsuba's multiplication scheme to improve upon the naive method but without
being able to get rid of its quadratic complexity. In this paper, we extend
their result by providing a reduction scheme which allows to multiply
polynomial in Chebyshev basis by using algorithms from the monomial basis case
and therefore get the same asymptotic complexity estimate. Our reduction allows
to use any of these algorithms without converting polynomials input to monomial
basis which therefore provide a more direct reduction scheme then the one using
conversions. We also demonstrate that our reduction is efficient in practice,
and even outperform the performance of the best known algorithm for Chebyshev
basis when polynomials have large degree. Finally, we demonstrate a linear time
equivalence between the polynomial multiplication problem under monomial basis
and under Chebyshev basis
The CMB in a Causal Set Universe
We discuss Cosmic Microwave Background constraints on the causal set theory
of quantum gravity, which has made testable predictions about the nature of
dark energy. We flesh out previously discussed heuristic constraints by showing
how the power spectrum of causal set dark energy fluctuations can be found from
the overlap volumes of past light cones of points in the universe. Using a
modified Boltzmann code we put constraints on the single parameter of the
theory that are somewhat stronger than previous ones. We conclude that causal
set theory cannot explain late-time acceleration without radical alterations to
General Relativity.Comment: 5 pages, 2 figure
Fast Computation of Voigt Functions via Fourier Transforms
This work presents a method of computing Voigt functions and their
derivatives, to high accuracy, on a uniform grid. It is based on an adaptation
of Fourier-transform based convolution. The relative error of the result
decreases as the fourth power of the computational effort. Because of its use
of highly vectorizable operations for its core, it can be implemented very
efficiently in scripting language environments which provide fast vector
libraries. The availability of the derivatives makes it suitable as a function
generator for non-linear fitting procedures.Comment: 8 pages, 1 figur
The numerical simulation tool for the MAORY multiconjugate adaptive optics system
The Multiconjugate Adaptive Optics RelaY (MAORY) is and Adaptive Optics
module to be mounted on the ESO European-Extremely Large Telescope (E-ELT). It
is a hybrid Natural and Laser Guide System that will perform the correction of
the atmospheric turbulence volume above the telescope feeding the Multi-AO
Imaging Camera for Deep Observations Near Infrared spectro-imager (MICADO). We
developed an end-to-end Monte- Carlo adaptive optics simulation tool to
investigate the performance of a the MAORY and the calibration, acquisition,
operation strategies. MAORY will implement Multiconjugate Adaptive Optics
combining Laser Guide Stars (LGS) and Natural Guide Stars (NGS) measurements.
The simulation tool implements the various aspect of the MAORY in an end to end
fashion. The code has been developed using IDL and uses libraries in C++ and
CUDA for efficiency improvements. Here we recall the code architecture, we
describe the modeled instrument components and the control strategies
implemented in the code.Comment: 6 pages, 1 figure, Proceeding 9909 310 of the conference SPIE
Astronomical Telescopes + Instrumentation 2016, 26 June 1 July 2016
Edinburgh, Scotland, U
Towards leading isospin breaking effects in mesonic masses with improved Wilson fermions
We present an exploratory study of leading isospin breaking effects in
mesonic masses using improved Wilson fermions. Isospin symmetry is
explicitly broken by distinct masses and electric charges of the up and down
quarks. In order to be able to make use of existing isosymmetric QCD gauge
ensembles we apply reweighting techniques. The path integral describing QCD+QED
is expanded perturbatively in powers of the light quarks' mass deviations and
the electromagnetic coupling. We employ QED as a finite volume
formulation of QED.Comment: 8 pages, 1 figure, proceedings to poster presented at 35th annual
International Symposium on Lattice Field Theory, 18-24 June 2017, Granada,
Spai
End to end numerical simulations of the MAORY multiconjugate adaptive optics system
MAORY is the adaptive optics module of the E-ELT that will feed the MICADO
imaging camera through a gravity invariant exit port. MAORY has been foreseen
to implement MCAO correction through three high order deformable mirrors driven
by the reference signals of six Laser Guide Stars (LGSs) feeding as many
Shack-Hartmann Wavefront Sensors. A three Natural Guide Stars (NGSs) system
will provide the low order correction. We develop a code for the end-to-end
simulation of the MAORY adaptive optics (AO) system in order to obtain
high-delity modeling of the system performance. It is based on the IDL language
and makes extensively uses of the GPUs. Here we present the architecture of the
simulation tool and its achieved and expected performance.Comment: 8 pages, 4 figures, presented at SPIE Astronomical Telescopes +
Instrumentation 2014 in Montr\'eal, Quebec, Canada, with number 9148-25
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