237 research outputs found
Segregation and charge-density-wave order in the spinless Falicov-Kimball model
The spinless Falicov-Kimball model is solved exactly in the limit of
infinite-dimensions on both the hypercubic and Bethe lattices. The competition
between segregation, which is present for large U, and charge-density-wave
order, which is prevalent at moderate U, is examined in detail. We find a rich
phase diagram which displays both of these phases. The model also shows
nonanalytic behavior in the charge-density-wave transition temperature when U
is large enough to generate a correlation-induced gap in the single-particle
density of states.Comment: 10 pages, 10 figure
Structure and Magnetization of Two-Dimensional Vortex Arrays in the Presence of Periodic Pinning
Ground-state properties of a two-dimensional system of superconducting
vortices in the presence of a periodic array of strong pinning centers are
studied analytically and numerically. The ground states of the vortex system at
different filling ratios are found using a simple geometric argument under the
assumption that the penetration depth is much smaller than the spacing of the
pin lattice. The results of this calculation are confirmed by numerical studies
in which simulated annealing is used to locate the ground states of the vortex
system. The zero-temperature equilibrium magnetization as a function of the
applied field is obtained by numerically calculating the energy of the ground
state for a large number of closely spaced filling ratios. The results show
interesting commensurability effects such as plateaus in the B-H diagram at
simple fractional filling ratios.Comment: 12 pages, 19 figures, submitted for publicatio
Relativistically invariant analysis of polarization effects in exclusive deuteron electrodisintegration process
A general formalism for the calculation of the differential cross section and
polarization observables, for the process of deuteron electrodisintegration, is
developed in the framework of relativistic impulse approximation. A detailed
analysis of the general structure of the differential cross section and
polarization observables for the reaction is derived, using
the formalism of the structure functions. The obtained expressions have a
general nature and they hold in the one--photon--exchange mechanism, assuming
P--invariance of the hadron electromagnetic interaction. The model of
relativistic impulse approximation is introduced and the final state
interaction is taken into account by means of the unitarization of the helicity
amplitudes. A detailed description of the unitarization procedure is also
presented.
Using different parametrizations of the deuteron wave functions, the
following polarization observables are calculated in the kinematical region of
quasi--elastic deuteron electrodisintegration: the asymmetry for the scattering
of longitudinally polarized electrons on a polarized deuteron target, the
proton and neutron polarizations (for longitudinally polarized electron beam or
vector--polarized deuteron target). The sensitivity to the neutron electric
form factor is also thorougly investigated.
The predictions of the model are compared with the results of recent
polarization measurements and a good agreement with the existing experimental
data has been obtained.Comment: 90 pages, 17 figure
Concurrence and entanglement entropy in a dimerized spin-1/2 two-leg ladder
We consider the isotropic spin-1/2 two-leg ladders with the dominant
spatially modulated rung exchanges.
We study the effect of a uniform magnetic field on the ground state phase
diagram of the model using the perturbation theory and the numerical Lanczos
method. The ground state phase diagram consists of two gapless Luttinger liquid
(LL) and three gapped phases. Numerically, we calculate the concurrence between
two spins and entanglement entropy between legs. Numerical experiment shows
that in principle the gapless LL phases are different. In the first LL phase,
only spins on rungs are entangled, but in the second LL phase the spins on legs
are long-distance entangled. Therefore the concurrence between spins on legs
can be considered as a function to distinguish the LL phases.Comment: 7 pages, 7 figure
Weak-coupling Treatment of Electronic (Anti-)Ferroelectricity in the Extended Falicov-Kimball Model
We study the (spinless) Falicov-Kimball model extended by a finite band width
(hopping ) of the localized (f-) electrons in infinite dimensions in the
weak-coupling limit of a small local interband Coulomb correlation for half
filling. In the case of overlapping conduction- and f-bands different kinds of
ordered solutions are possible, namely charge-density wave (CDW) order,
electronic ferroelectricity (EFE) and electronic antiferroelectricity (EAFE).
The order parameters are calculated as a function of the model parameters and
of the temperature. There is a first-order phase transition from the CDW-phase
to the EFE- or EAFE-phase. The total energy is calculated to determine the
thermodynamically stable solution. The quantum phase diagrams are calculated.Comment: 7 pages, 8 figure
Critical Currents and Vortex States at Fractional Matching Fields in Superconductors with Periodic Pinning
We study vortex states and dynamics in 2D superconductors with periodic
pinning at fractional sub-matching fields using numerical simulations. For
square pinning arrays we show that ordered states form at 1/1, 1/2, and 1/4
filling fractions while only partially ordered states form at other filling
fractions, such as 1/3 and 1/5, in agreement with recent imaging experiments.
For triangular pinning arrays we observe matching effects at filling fractions
of 1/1, 6/7, 2/3, 1/3, 1/4, 1/6, and 1/7. For both square and triangular
pinning arrays we also find that, for certian sub-matching fillings, vortex
configurations depend on pinning strength. For weak pinning, ordering in which
a portion of the vortices are positioned between pinning sites can occur.
Depinning of the vortices at the matching fields, where the vortices are
ordered, is elastic while at the incommensurate fields the motion is plastic.
At the incommensurate fields, as the applied driving force is increased, there
can be a transition to elastic flow where the vortices move along the pinning
sites in 1D channels and a reordering transition to a triangular or distorted
triangular lattice. We also discuss the current-voltage curves and how they
relate to the vortex ordering at commensurate and incommensurate fields.Comment: 14 figure
Ergonomics and sustainability: Towards and embrace of complexity and emergence
Technology offers a promising route to a sustainable future, and ergonomics can serve a vital role. The argument of this article is that the lasting success of sustainability initiatives in ergonomics hinges on an examination of ergonomics' own epistemology and ethics. The epistemology of ergonomics is fundamentally empiricist and positivist. This places practical constraints on its ability to address important issues such as sustainability, emergence and complexity. The implicit ethical position of ergonomics is one of neutrality, and its positivist epistemology generally puts value-laden questions outside the parameters of what it sees as scientific practice. We argue, by contrast, that a discipline that deals with both technology and human beings cannot avoid engaging with questions of complexity and emergence and seeking innovative ways of addressing these issues.No Full Tex
Cohort Randomised Controlled Trial of a Multifaceted Podiatry Intervention for the Prevention of Falls in Older People (The REFORM Trial)
BACKGROUND: Falls are a major cause of morbidity among older people. A multifaceted podiatry intervention may reduce the risk of falling. This study evaluated such an intervention. DESIGN: Pragmatic cohort randomised controlled trial in England and Ireland. 1010 participants were randomised (493 to the Intervention group and 517 to Usual Care) to either: a podiatry intervention, including foot and ankle exercises, foot orthoses and, if required, new footwear, and a falls prevention leaflet or usual podiatry treatment plus a falls prevention leaflet. The primary outcome was the incidence rate of self-reported falls per participant in the 12 months following randomisation. Secondary outcomes included: proportion of fallers and those reporting multiple falls, time to first fall, fear of falling, Frenchay Activities Index, Geriatric Depression Scale, foot pain, health related quality of life, and cost-effectiveness. RESULTS: In the primary analysis were 484 (98.2%) intervention and 507 (98.1%) control participants. There was a small, non statistically significant reduction in the incidence rate of falls in the intervention group (adjusted incidence rate ratio 0.88, 95% CI 0.73 to 1.05, p = 0.16). The proportion of participants experiencing a fall was lower (49.7 vs 54.9%, adjusted odds ratio 0.78, 95% CI 0.60 to 1.00, p = 0.05) as was the proportion experiencing two or more falls (27.6% vs 34.6%, adjusted odds ratio 0.69, 95% CI 0.52 to 0.90, p = 0.01). There was an increase (p = 0.02) in foot pain for the intervention group. There were no statistically significant differences in other outcomes. The intervention was more costly but marginally more beneficial in terms of health-related quality of life (mean quality adjusted life year (QALY) difference 0.0129, 95% CI -0.0050 to 0.0314) and had a 65% probability of being cost-effective at a threshold of £30,000 per QALY gained. CONCLUSION: There was a small reduction in falls. The intervention may be cost-effective. TRIAL REGISTRATION: ISRCTN ISRCTN68240461
Gluon polarization in the nucleon from quasi-real photoproduction of high-pT hadron pairs
We present a determination of the gluon polarization Delta G/G in the
nucleon, based on the helicity asymmetry of quasi-real photoproduction events,
Q^2<1(GeV/c)^2, with a pair of large transverse-momentum hadrons in the final
state. The data were obtained by the COMPASS experiment at CERN using a 160 GeV
polarized muon beam scattered on a polarized 6-LiD target. The helicity
asymmetry for the selected events is = 0.002 +- 0.019(stat.) +-
0.003(syst.). From this value, we obtain in a leading-order QCD analysis Delta
G/G=0.024 +- 0.089(stat.) +- 0.057(syst.) at x_g = 0.095 and mu^2 =~ 3
(GeV}/c)^2.Comment: 10 pages, 3 figure
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