364 research outputs found
Low-Temperature Dynamical Structure Factor of the Two-Leg Spin-1/2 Heisenberg Ladder
We determine the dynamical structure factor of the two-leg spin-1/2
Heisenberg ladder at low temperatures in the regime of strong rung coupling.
The dominant feature at zero temperature is the coherent triplon mode. We show
that the lineshape of this mode broadens in a non-symmetric way at finite
temperatures and that the degree of asymmetry increases with temperature. We
also show that at low frequencies a temperature induced resonance akin to the
Villain mode in the spin-1/2 Heisenberg Ising chain emerges.Comment: 13 pages, 7 figures, revte
Are there localized saddles behind the heterogeneous dynamics of supercooled liquids?
We numerically study the interplay between heterogeneous dynamics and
properties of negatively curved regions of the potential energy surface in a
model glassy system. We find that the unstable modes of saddles and
quasi-saddles undergo a localization transition close to the Mode-Coupling
critical temperature. We also find evidence of a positive spatial correlation
between clusters of particles having large displacements in the unstable modes
and dynamical heterogeneities.Comment: 7 pages, 3 figures, submitted to Europhys. Let
Finite Temperature Dynamical Structure Factor of the Heisenberg-Ising Chain
We consider the spin-1/2 Heisenberg XXZ chain in the regime of large
Ising-like anisotropy . By a combination of duality and Jordan-Wigner
transformations we derive a mapping to weakly interacting spinless fermions,
which represent domain walls between the two degenerate ground states. We
develop a perturbative expansion in for the transverse dynamical spin
structure factor at finite temperatures and in an applied transverse magnetic
field. We present a unified description for both the low-energy
temperature-activated response and the temperature evolution of the T=0
two-spinon continuum. We find that the two-spinon continuum narrows in energy
with increasing temperature. At the same time spectral weight is transferred
from the two-spinon continuum to the low energy intraband scattering continuum,
which is strongly peaked around the position of the (single) spinon dispersion
(`Villain mode').Comment: 23 pages, 19 eps figures (now improved), uses feynm
Effect on gastric function and symptoms of drinking wine, black tea, or schnapps with a Swiss cheese fondue: randomised controlled crossover trial
OBJECTIVE: To compare the effects of drinking white wine or black tea with Swiss cheese fondue followed by a shot of cherry schnapps on gastric emptying, appetite, and abdominal symptoms.
DESIGN: Randomised controlled crossover study.
PARTICIPANTS: 20 healthy adults (14 men) aged 23-58.
INTERVENTIONS: Cheese fondue (3260 kJ, 32% fat) labelled with 150 mg sodium (13)Carbon-octanoate was consumed with 300 ml of white wine (13%, 40 g alcohol) or black tea in randomised order, followed by 20 ml schnapps (40%, 8 g alcohol) or water in randomised order.
MAIN OUTCOME MEASURES: Cumulative percentage dose of (13)C substrate recovered over four hours (higher values indicate faster gastric emptying) and appetite and dyspeptic symptoms (visual analogue scales).
RESULTS: Gastric emptying was significantly faster when fondue was consumed with tea or water than with wine or schnapps (cumulative percentage dose of (13)C recovered 18.1%, 95% confidence interval 15.2% to 20.9% v 7.4%, 4.6% to 10.3%; P<0.001). An inverse dose-response relation between alcohol intake and gastric emptying was evident. Appetite was similar with consumption of wine or tea (difference 0.11, -0.12 to 0.34; P=0.35), but reduced if both wine and schnapps were consumed (difference -0.40, -0.01 to -0.79; P<0.046). No difference in dyspeptic symptoms was present.
CONCLUSIONS: Gastric emptying after a Swiss cheese fondue is noticeably slower and appetite suppressed if consumed with higher doses of alcohol. This effect was not associated with dyspeptic symptoms.
TRIAL REGISTRATION: ClinicalTrials.gov NCT00943696
Inherent-Structure Dynamics and Diffusion in Liquids
The self-diffusion constant D is expressed in terms of transitions among the
local minima of the potential (inherent structure, IS) and their correlations.
The formulae are evaluated and tested against simulation in the supercooled,
unit-density Lennard-Jones liquid. The approximation of uncorrelated
IS-transition (IST) vectors, D_{0}, greatly exceeds D in the upper temperature
range, but merges with simulation at reduced T ~ 0.50. Since uncorrelated IST
are associated with a hopping mechanism, the condition D ~ D_{0} provides a new
way to identify the crossover to hopping. The results suggest that theories of
diffusion in deeply supercooled liquids may be based on weakly correlated IST.Comment: submitted to PR
Dilatancy, Jamming, and the Physics of Granulation
Granulation is a process whereby a dense colloidal suspension is converted
into pasty granules (surrounded by air) by application of shear. Central to the
stability of the granules is the capillary force arising from the interfacial
tension between solvent and air. This force appears capable of maintaining a
solvent granule in a jammed solid state, under conditions where the same amount
of solvent and colloid could also exist as a flowable droplet. We argue that in
the early stages of granulation the physics of dilatancy, which requires that a
powder expand on shearing, is converted by capillary forces into the physics of
arrest. Using a schematic model of colloidal arrest under stress, we speculate
upon various jamming and granulation scenarios. Some preliminary experimental
results on aspects of granulation in hard-sphere colloidal suspensions are also
reported.Comment: Original article intended for J Phys Cond Mat special issue on
Granular Materials (M Nicodemi, Ed.
The Potential Energy Landscape and Mechanisms of Diffusion in Liquids
The mechanism of diffusion in supercooled liquids is investigated from the
potential energy landscape point of view, with emphasis on the crossover from
high- to low-T dynamics. Molecular dynamics simulations with a time dependent
mapping to the associated local mininum or inherent structure (IS) are
performed on unit-density Lennard-Jones (LJ). New dynamical quantities
introduced include r2_{is}(t), the mean-square displacement (MSD) within a
basin of attraction of an IS, R2(t), the MSD of the IS itself, and g_{loc}(t)
the mean waiting time in a cooperative region. At intermediate T, r2_{is}(t)
posesses an interval of linear t-dependence allowing calculation of an
intrabasin diffusion constant D_{is}. Near T_{c} diffusion is intrabasin
dominated with D = D_{is}. Below T_{c} the local waiting time tau_{loc} exceeds
the time, tau_{pl}, needed for the system to explore the basin, indicating the
action of barriers. The distinction between motion among the IS below T_{c} and
saddle, or border dynamics above T_{c} is discussed.Comment: submitted to pr
Violation of the fluctuation-dissipation theorem in glassy systems: basic notions and the numerical evidence
This review reports on the research done during the past years on violations
of the fluctuation-dissipation theorem (FDT) in glassy systems. It is focused
on the existence of a quasi-fluctuation-dissipation theorem (QFDT) in glassy
systems and the currently supporting knowledge gained from numerical simulation
studies. It covers a broad range of non-stationary aging and stationary driven
systems such as structural-glasses, spin-glasses, coarsening systems,
ferromagnetic models at criticality, trap models, models with entropy barriers,
kinetically constrained models, sheared systems and granular media. The review
is divided into four main parts: 1) An introductory section explaining basic
notions related to the existence of the FDT in equilibrium and its possible
extension to the glassy regime (QFDT), 2) A description of the basic analytical
tools and results derived in the framework of some exactly solvable models, 3)
A detailed report of the current evidence in favour of the QFDT and 4) A brief
digression on the experimental evidence in its favour. This review is intended
for inexpert readers who want to learn about the basic notions and concepts
related to the existence of the QFDT as well as for the more expert readers who
may be interested in more specific results.Comment: 120 pages, 37 figures. Topical review paper . Several typos and
misprints corrected, new references included and others updated. to be
published in J. Phys. A (Math. Gen.
Moray eels are more common on coral reefs subject to higher human pressure in the greater Caribbean
Proximity and size of the nearest market (‘market gravity’) have been shown to have strong negative effects on coral reef fish communities that can be mitigated by the establishment of closed areas. However, moray eels are functionally unique predators that are generally not subject to targeted fishing and should therefore not directly be affected by these factors. We used baited remote underwater video systems to investigate associations between morays and anthropogenic, habitat, and ecological factors in the Caribbean region. Market gravity had a positive effect on morays, while the opposite pattern was observed in a predator group subject to exploitation (sharks). Environmental DNA analyses corroborated the positive effect of market gravity on morays. We hypothesize that the observed pattern could be the indirect result of the depletion of moray competitors and predators near humans. Environmental science; ecology; biological sciences; zoology; animals; etholog
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