6,160 research outputs found
Density Matrix Renormalization Group Study of the Disorder Line in the Quantum ANNNI Model
We apply Density Matrix Renormalization Group methods to study the phase
diagram of the quantum ANNNI model in the region of low frustration where the
ferromagnetic coupling is larger than the next-nearest-neighbor
antiferromagnetic one. By Finite Size Scaling on lattices with up to 80 sites
we locate precisely the transition line from the ferromagnetic phase to a
paramagnetic phase without spatial modulation. We then measure and analyze the
spin-spin correlation function in order to determine the disorder transition
line where a modulation appears. We give strong numerical support to the
conjecture that the Peschel-Emery one-dimensional line actually coincides with
the disorder line. We also show that the critical exponent governing the
vanishing of the modulation parameter at the disorder transition is .Comment: 4 pages, 5 eps figure
Inter-Coder Agreement for Computational Linguistics
This article is a survey of methods for measuring agreement among corpus annotators. It exposes the mathematics and underlying assumptions of agreement coefficients, covering Krippendorff's alpha as well as Scott's pi and Cohen's kappa; discusses the use of coefficients in several annotation tasks; and argues that weighted, alpha-like coefficients, traditionally less used than kappa-like measures in computational linguistics, may be more appropriate for many corpus annotation tasks—but that their use makes the interpretation of the value of the coefficient even harder. </jats:p
Clustering of vertically constrained passive particles in homogeneous, isotropic turbulence
We analyze the dynamics of small particles vertically confined, by means of a
linear restoring force, to move within a horizontal fluid slab in a
three-dimensional (3D) homogeneous isotropic turbulent velocity field. The
model that we introduce and study is possibly the simplest description for the
dynamics of small aquatic organisms that, due to swimming, active regulation of
their buoyancy, or any other mechanism, maintain themselves in a shallow
horizontal layer below the free surface of oceans or lakes. By varying the
strength of the restoring force, we are able to control the thickness of the
fluid slab in which the particles can move. This allows us to analyze the
statistical features of the system over a wide range of conditions going from a
fully 3D incompressible flow (corresponding to the case of no confinement) to
the extremely confined case corresponding to a two-dimensional slice. The
background 3D turbulent velocity field is evolved by means of fully resolved
direct numerical simulations. Whenever some level of vertical confinement is
present, the particle trajectories deviate from that of fluid tracers and the
particles experience an effectively compressible velocity field. Here, we have
quantified the compressibility, the preferential concentration of the
particles, and the correlation dimension by changing the strength of the
restoring force. The main result is that there exists a particular value of the
force constant, corresponding to a mean slab depth approximately equal to a few
times the Kolmogorov length scale, that maximizes the clustering of the
particles
Baryon Magnetic Moments and Proton Spin: A Model with Collective Quark Rotation
We analyse the baryon magnetic moments in a model that relates them to the
parton spins , , , and includes a contribution
from orbital angular momentum. The specific assumption is the existence of a
3-quark correlation (such as a flux string) that rotates with angular momentum
around the proton spin axis. A fit to the baryon magnetic
moments, constrained by the measured values of the axial vector coupling
constants , , yields , , where the error is a theoretical
estimate. A second fit, under slightly different assumptions, gives , with no constraint on . The
model provides a consistent description of axial vector couplings, magnetic
moments and the quark polarization measured in deep
inelastic scattering. The fits suggest that a significant part of the angular
momentum of the proton may reside in a collective rotation of the constituent
quarks.Comment: 16 pages, 3 ps-figures, uses RevTeX. Abstract, Sec. II, III and IV
have been expande
A simple trapped-ion architecture for high-fidelity Toffoli gates
We discuss a simple architecture for a quantum Toffoli gate implemented using
three trapped ions. The gate, which in principle can be implemented with a
single laser-induced operation, is effective under rather general conditions
and is strikingly robust (within any experimentally realistic range of values)
against dephasing, heating and random fluctuations of the Hamiltonian
parameters. We provide a full characterization of the unitary and
noise-affected gate using three-qubit quantum process tomography
Comment on ``Spin Polarization and Magnetic Circular Dichroism in Photoemission from the 2p Core Level of Ferromagnetic Ni''
Although the Ni_4 cluster includes more information regarding the Ni band
structure with respect to the Anderson impurity model, it also favors very
peculiar ground states which are incompatible with a coherent picture of all
dichroism experiments.Comment: 1 page, RevTeX, 1 epsf figur
Infrared Observations of the Helix Planetary Nebula
We have mapped the Helix (NGC 7293) planetary nebula (PN) with the IRAC instrument on the Spitzer Space Telescope. The Helix is one of the closest bright PNs and therefore provides an opportunity to resolve the small-scale structure in the nebula. The emission from this PN in the 5.8 and 8 μm IRAC bands is dominated by the pure rotational lines of molecular hydrogen, with a smaller contribution from forbidden line emission such as [Ar III] in the ionized region. The IRAC images resolve the "cometary knots," which have been previously studied in this PN. The "tails" of the knots and the radial rays extending into the outer regions of the PN are seen in emission in the IRAC bands. IRS spectra on the main ring and the emission in the IRAC bands are consistent with shock-excited H_2 models, with a small (~10%) component from photodissociation regions. In the northeast arc, the H_2 emission is located in a shell outside the Hα emission
Genetic diversity in the env V1-V2 region of proviral quasispecies from long-term controller MHC-typed cynomolgus macaques infected with SHIVSF162P4cy
Intra-host evolution of human immunodeficiency virus (HIV) and simian immunodeficiency virus (SIV) has been shown by viral RNA analysis in subjects who naturally suppress plasma viremia to low levels, known as controllers. However, little is known about the variability of proviral DNA and the inter-relationships among contained systemic viremia, rate of reservoir reseeding and specific major histocompatibility complex (MHC) genotypes, in controllers. Here, we analysed the proviral DNA quasispecies of the env V1-V2 region, in PBMCs and in anatomical compartments of 13 long-term controller monkeys after 3.2 years of infection with simian/human immunodeficiency virus (SHIV)SF162P4cy. A considerable variation in the genetic diversity of proviral quasispecies was present among animals. Seven monkeys exhibited env V1-V2 proviral populations composed of both clusters of identical ancestral sequences and new variants, whereas the other six monkeys displayed relatively high env V1-V2 genetic diversity with a large proportion of diverse novel sequences. Our results demonstrate that in SHIVSF162P4cy-infected monkeys there exists a disparate pattern of intra-host viral diversity and that reseeding of the proviral reservoir occurs in some animals. Moreover, even though no particular association has been observed between MHC haplotypes and the long-term control of infection, a remarkably similar pattern of intra-host viral diversity and divergence was found within animals carrying the M3 haplotype. This suggests that in animals bearing the same MHC haplotype and infected with the same virus, viral diversity follows a similar pattern with similar outcomes and control of infection
Multitrace Deformations of Vector and Adjoint Theories and their Holographic Duals
We present general methods to study the effect of multitrace deformations in
conformal theories admitting holographic duals in Anti de Sitter space. In
particular, we analyse the case that these deformations introduce an
instability both in the bulk AdS space and in the boundary CFT. We also argue
that multitrace deformations of the O(N) linear sigma model in three dimensions
correspond to nontrivial time-dependent backgrounds in certain theories of
infinitely many interacting massless fields on AdS_4, proposed years ago by
Fradkin and Vasiliev. We point out that the phase diagram of a truly marginal
large-N deformation has an infrared limit in which only an O(N) singlet field
survives. We draw from this case lessons on the full string-theoretical
interpretation of instabilities of the dual boundary theory and exhibit a toy
model that resolves the instability of the O(N) model, generated by a marginal
multitrace deformation. The resolution suggests that the instability may not
survive in an appropriate UV completion of the CFT.Comment: 18 pages, minor changes, references added. Version accepted by JHE
Supersymmetric Sum Rules for Electromagnetic Multipoles
We derive model independent, non-perturbative supersymmetric sum rules for
the magnetic and electric multipole moments of any theory with N=1
supersymmetry. We find that in any irreducible N=1 supermultiplet the diagonal
matrix elements of the l-multipole moments are completely fixed in terms of
their off-diagonal matrix elements and the diagonal (l-1)-multipole moments.Comment: 10 pages, plain Te
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