900 research outputs found
Information propagation for interacting particle systems
We show that excitations of interacting quantum particles in lattice models
always propagate with a finite speed of sound. Our argument is simple yet
general and shows that by focusing on the physically relevant observables one
can generally expect a bounded speed of information propagation. The argument
applies equally to quantum spins, bosons such as in the Bose-Hubbard model,
fermions, anyons, and general mixtures thereof, on arbitrary lattices of any
dimension. It also pertains to dissipative dynamics on the lattice, and
generalizes to the continuum for quantum fields. Our result can be seen as a
meaningful analogue of the Lieb-Robinson bound for strongly correlated models.Comment: 4 pages, 1 figure, minor change
Constructing topological models by symmetrization: A PEPS study
Symmetrization of topologically ordered wavefunctions is a powerful method
for constructing new topological models. Here, we study wavefunctions obtained
by symmetrizing quantum double models of a group in the Projected Entangled
Pair States (PEPS) formalism. We show that symmetrization naturally gives rise
to a larger symmetry group which is always non-abelian. We prove
that by symmetrizing on sufficiently large blocks, one can always construct
wavefunctions in the same phase as the double model of . In order to
understand the effect of symmetrization on smaller patches, we carry out
numerical studies for the toric code model, where we find strong evidence that
symmetrizing on individual spins gives rise to a critical model which is at the
phase transitions of two inequivalent toric codes, obtained by anyon
condensation from the double model of .Comment: 10 pages. v2: accepted versio
Edge theories in Projected Entangled Pair State models
We study the edge physics of gapped quantum systems in the framework of
Projected Entangled Pair State (PEPS) models. We show that the effective
low-energy model for any region acts on the entanglement degrees of freedom at
the boundary, corresponding to physical excitations located at the edge. This
allows us to determine the edge Hamiltonian in the vicinity of PEPS models, and
we demonstrate that by choosing the appropriate bulk perturbation, the edge
Hamiltonian can exhibit a rich phase diagram and phase transitions. While for
models in the trivial phase any Hamiltonian can be realized at the edge, we
show that for topological models, the edge Hamiltonian is constrained by the
topological order in the bulk which can e.g. protect a ferromagnetic Ising
chain at the edge against spontaneous symmetry breaking.Comment: 5 pages, 4 figure
Coherent states of a charged particle in a uniform magnetic field
The coherent states are constructed for a charged particle in a uniform
magnetic field based on coherent states for the circular motion which have
recently been introduced by the authors.Comment: 2 eps figure
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Physiology of flowering in Coffea arabica L. : role of growth regulators and water relations
Coffee trees growing in the Kona district of Hawaii
were treated with different concentrations of either BA,
GA3, or Promalin, and pruned at the time of flowering. No
significant differences in the flowering and fruit ripening
pattern were found. High concentrations (100 mg/1) of
growth regulators decreased fruit fresh weight. Pruning
yielded the lowest number of fruit and flowers, and caused
branch dieback. Fruit set was between 46 and 83% for the
two years studied.
Flower buds from trees growing in the field that were
>4 mm, but not developed to the candle stage at the time of
GAâ treatment, reached anthesis 20 days earlier than the
controls. Their development was independent of rainfall,
unlike the controls. Fruit from buds that had been treated
at the >4 mm or the candle stage ripened more synchronously
and earlier than the control. Buds smaller than 4 mm did
not respond to GAâ treatment.
A threshold leaf water potential of -2.7 MPa, and
flower bud water potential of about -4.0 MPa was necessary
to overcome dormancy of flower buds in greenhouse grown
trees. GAâ stimulated anthesis in some plants that were
stressed to leaf water potentials of -2.1 MPa. Ethylene
evolution of flower buds where dormancy had been broken
with water stress was low, compared to dormant flower buds.
At anthesis, ethylene evolution reached highest levels.
Free and conjugated IAA levels in flower buds changed
markedly after the dormancy breaking stimulus. Doubling
of fresh and dry weight of flower buds occurred 3 to 5 days
after water stressed plants were rehydrated. It was
preceded by an increase in free IAA, and a tripling in the
rate of water uptake from 1 to 3 days after rehydrating
water stressed plants. Throughout the development, the
largest percentage of IAA was present in the conjugated
form
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Frost hardiness of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) seedlings raised in three nurseries
Two-year-old Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] seedlings of two seed sources raised in three nurseries in Oregon and Washington were tested for differences in frost hardiness from September 1985 to March 1986. The objective of the study was to determine whether nursery location had an influence on seedling acclimation, deacclimation , budburst and first-year field performance.
Frost hardiness was determined five times from September to December in the nurseries. In January 1986 seedlings were lifted at the three nurseries. Dehardening of potted trees was observed under outdoor and growth chamber conditions. Budburst of the trees dehardening outdoors was recorded from March to June. Hardiness was determined with a whole plant freezing test. Seedlings were frozen at each sampling date to 4 test
temperatures to evaluate needle, bud and stem tissue damage. In general, trees raised in the highest elevation nursery or the most northern nursery had hardier tissue than seedlings raised in the coastal nursery. However, hardiness varied for each tissue and among nurseries and seed sources. In February and March seedlings from the Cascade (975m) seed source were less hardy than seedlings from the coastal source (450m). A growth chamber experiment confirmed the outdoor dehardening pattern.
With a 16 hour photoperiod a constant temperature of +5°C maintained cold hardiness, whereas +10°C and 15°C promoted rapid dehardening after 20 days. To predict hardening, a regression equation with nursery
weather data and elevation was calculated. Photoperiod, number of frost days, and elevation were the most important independent
factors predicting hardening (RÂČ=0.29).
The environment of the three nurseries seemed to have a strong influence on budburst. Trees raised in the coastal nursery
burst bud significantly earlier than trees from the other two nurseries. Mean terminal budburst of potted trees from both seed
sources in each nursery occurred only 2 days apart. Frost hardiness in January and first-year field growth were not
correlated. A correlation between the electrolytic conductivity of shoot tips and the damage of needles, buds, and stems as determined by the whole plant freezing test was poor. The highest correlation coefficient (r) was 0.69. Different hardening rates of tissues and plant to plant variability may have contributed to the low correlation. The conductivity method as outlined in this study does not provide a satisfactory estimation of frost hardiness of
two-year-old bareroot Douglas-fir seedlings
Cosmic ray short burst observed with the Global Muon Detector Network (GMDN) on June 22, 2015
We analyze the short cosmic ray intensity increase ("cosmic ray burst": CRB)
on June 22, 2015 utilizing a global network of muon detectors and derive the
global anisotropy of cosmic ray intensity and the density (i.e. the
omnidirectional intensity) with 10-minute time resolution. We find that the CRB
was caused by a local density maximum and an enhanced anisotropy of cosmic rays
both of which appeared in association with Earth's crossing of the heliospheric
current sheet (HCS). This enhanced anisotropy was normal to the HCS and
consistent with a diamagnetic drift arising from the spatial gradient of cosmic
ray density, which indicates that cosmic rays were drifting along the HCS from
the north of Earth. We also find a significant anisotropy along the HCS,
lasting a few hours after the HCS crossing, indicating that cosmic rays
penetrated into the inner heliosphere along the HCS. Based on the latest
geomagnetic field model, we quantitatively evaluate the reduction of the
geomagnetic cut-off rigidity and the variation of the asymptotic viewing
direction of cosmic rays due to a major geomagnetic storm which occurred during
the CRB and conclude that the CRB is not caused by the geomagnetic storm, but
by a rapid change in the cosmic ray anisotropy and density outside the
magnetosphere.Comment: accepted for the publication in the Astrophysical Journa
Computational Complexity of interacting electrons and fundamental limitations of Density Functional Theory
One of the central problems in quantum mechanics is to determine the ground
state properties of a system of electrons interacting via the Coulomb
potential. Since its introduction by Hohenberg, Kohn, and Sham, Density
Functional Theory (DFT) has become the most widely used and successful method
for simulating systems of interacting electrons, making their original work one
of the most cited in physics. In this letter, we show that the field of
computational complexity imposes fundamental limitations on DFT, as an
efficient description of the associated universal functional would allow to
solve any problem in the class QMA (the quantum version of NP) and thus
particularly any problem in NP in polynomial time. This follows from the fact
that finding the ground state energy of the Hubbard model in an external
magnetic field is a hard problem even for a quantum computer, while given the
universal functional it can be computed efficiently using DFT. This provides a
clear illustration how the field of quantum computing is useful even if quantum
computers would never be built.Comment: 8 pages, 3 figures. v2: Version accepted at Nature Physics; differs
significantly from v1 (including new title). Includes an extra appendix (not
contained in the journal version) on the NP-completeness of Hartree-Fock,
which is taken from v
The role of depression in the association between mobilisation timing and live discharge after hip fracture surgery: Secondary analysis of the UK National Hip Fracture Database
Purpose
The aim was to compare the probability of discharge after hip fracture surgery conditional on being alive and in hospital between patients mobilised within and beyond 36-hours of surgery across groups defined by depression.
Methods
Data were taken from the National Hip Fracture Database and included patients 60 years of age or older who underwent hip fracture surgery in England and Wales between 2014 and 2016. The conditional probability of postsurgical live discharge was estimated for patients mobilised early and for patients mobilised late across groups with and without depression. The association between mobilisation timing and the conditional probability of live discharge were also estimated separately through adjusted generalized linear models.
Results
Data were analysed for 116,274 patients. A diagnosis of depression was present in 8.31% patients. In those with depression, 7,412 (76.7%) patients mobilised early. In those without depression, 84,085 (78.9%) patients mobilised early. By day 30 after surgery, the adjusted odds ratio of discharge among those who mobilised early compared to late was 1.79 (95% CI: 1.56â2.05, p<0.001) and 1.92 (95% CI: 1.84â2.00, p<0.001) for those with and without depression, respectively.
Conclusion
A similar proportion of patients with depression mobilised early after hip fracture surgery when compared to those without a diagnosis of depression. The association between mobilisation timing and time to live discharge was observed for patients with and without depression
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