306 research outputs found
Chaoticity without thermalisation in disordered lattices
We study chaoticity and thermalization in Bose-Einstein condensates in
disordered lattices, described by the discrete nonlinear Schr\"odinger equation
(DNLS). A symplectic integration method allows us to accurately obtain both the
full phase space trajectories and their maximum Lyapunov exponents (mLEs),
which characterize their chaoticity. We find that disorder destroys ergodicity
by breaking up phase space into subsystems that are effectively disjoint on
experimentally relevant timescales, even though energetically, classical
localisation cannot occur. This leads us to conclude that the mLE is a very
poor ergodicity indicator, since it is not sensitive to the trajectory being
confined to a subregion of phase space. The eventual thermalization of a BEC in
a disordered lattice cannot be predicted based only on the chaoticity of its
phase space trajectory
Supersolid phases of dipolar bosons in optical lattices with a staggered flux
We present the theoretical mean-field zero-temperature phase diagram of a
Bose-Einstein condensate (BEC) with dipolar interactions loaded into an optical
lattice with a staggered flux. Apart from uniform superfluid, checkerboard
supersolid and striped supersolid phases, we identify several supersolid phases
with staggered vortices, which can be seen as combinations of supersolid phases
found in earlier work on dipolar BECs and a staggered-vortex phase found for
bosons in optical lattices with staggered flux. By allowing for different
phases and densities on each of the four sites of the elementary plaquette,
more complex phase patterns are found.Comment: 11 pages; added references, minor changes in tex
Finite-momentum Bose-Einstein condensates in shaken 2D square optical lattices
We consider ultracold bosons in a 2D square optical lattice described by the
Bose-Hubbard model. In addition, an external time-dependent sinusoidal force is
applied to the system, which shakes the lattice along one of the diagonals. The
effect of the shaking is to renormalize the nearest-neighbor hopping
coefficients, which can be arbitrarily reduced, can vanish, or can even change
sign, depending on the shaking parameter. It is therefore necessary to account
for higher-order hopping terms, which are renormalized differently by the
shaking, and introduce anisotropy into the problem. We show that the
competition between these different hopping terms leads to finite-momentum
condensates, with a momentum that may be tuned via the strength of the shaking.
We calculate the boundaries between the Mott-insulator and the different
superfluid phases, and present the time-of-flight images expected to be
observed experimentally. Our results open up new possibilities for the
realization of bosonic analogs of the FFLO phase describing inhomogeneous
superconductivity.Comment: 7 pages, 7 figure
Pokrovsky-Talapov Model at finite temperature: a renormalization-group analysis
We calculate the finite-temperature shift of the critical wavevector
of the Pokrovsky-Talapov model using a renormalization-group analysis.
Separating the Hamiltonian into a part that is renormalized and one that is
not, we obtain the flow equations for the stiffness and an arbitrary potential.
We then specialize to the case of a cosine potential, and compare our results
to well-known results for the sine-Gordon model, to which our model reduces in
the limit of vanishing driving wavevector Q=0. Our results may be applied to
describe the commensurate-incommensurate phase transition in several physical
systems and allow for a more realistic comparison with experiments, which are
always carried out at a finite temperature
Liquid-vapor oscillations of water in hydrophobic nanopores
Water plays a key role in biological membrane transport. In ion channels and
water-conducting pores (aquaporins), one dimensional confinement in conjunction
with strong surface effects changes the physical behavior of water. In
molecular dynamics simulations of water in short (0.8 nm) hydrophobic pores the
water density in the pore fluctuates on a nanosecond time scale. In long
simulations (460 ns in total) at pore radii ranging from 0.35 nm to 1.0 nm we
quantify the kinetics of oscillations between a liquid-filled and a
vapor-filled pore. This behavior can be explained as capillary evaporation
alternating with capillary condensation, driven by pressure fluctuations in the
water outside the pore. The free energy difference between the two states
depends linearly on the radius. The free energy landscape shows how a
metastable liquid state gradually develops with increasing radius. For radii
larger than ca. 0.55 nm it becomes the globally stable state and the vapor
state vanishes. One dimensional confinement affects the dynamic behavior of the
water molecules and increases the self diffusion by a factor of two to three
compared to bulk water. Permeabilities for the narrow pores are of the same
order of magnitude as for biological water pores. Water flow is not continuous
but occurs in bursts. Our results suggest that simulations aimed at collective
phenomena such as hydrophobic effects may require simulation times longer than
50 ns. For water in confined geometries, it is not possible to extrapolate from
bulk or short time behavior to longer time scales.Comment: 20 pages, 4 figures, 3 tables; to be published in Proc. Natl. Acad.
Sci. US
Intermittent permeation of cylindrical nanopores by water
Molecular Dynamics simulations of water molecules in nanometre sized
cylindrical channels connecting two reservoirs show that the permeation of
water is very sensitive to the channel radius and to electric polarization of
the embedding material. At threshold, the permeation is {\emph{intermittent}}
on a nanosecond timescale, and strongly enhanced by the presence of an ion
inside the channel, providing a possible mechanism for gating. Confined water
remains surprisingly fluid and bulk-like. Its behaviour differs strikingly from
that of a reference Lennard-Jones fluid, which tends to contract into a highly
layered structure inside the channel.Comment: 4 pages, 4 figure
Atrial Fibrillation in Africa-An Underreported and Unrecognized Risk Factor for Stroke:A Systematic Review
Over three-quarters of deaths from cardiovascular disease and diabetes occur in low- and middle-income countries, which include many African countries. Global studies showed that the prevalence of the cardiac arrhythmia atrial fibrillation (AF) appeared to be lower in Africa. A systematic search of PubMed and African Journals Online was conducted to determine the prevalence of AF and associated stroke risk factors in Africa and to quantify the need for screening. The publications search yielded a total of 840 articles of which 41 were included. AF was often not identified as the disease of primary interest with its own risks. Data on prevalence in the general population was scarce. The prevalence of stroke risk factors showed a large variation between studies, as well as within clustered subpopulations. AF in Africa is under-reported in published reports. The study types and populations are highly heterogeneous, making it difficult to draw a definitive conclusion on AF prevalence
Screening for Atrial Fibrillation in Sub-Saharan Africa:A Health Economic Evaluation to Assess the Feasibility in Nigeria
Background: Cardiovascular disease reflects a major burden of non-communicable disease in Sub-Saharan Africa (SSA). Early detection and treatment of atrial fibrillation (AF), as a preventive measure against stroke, is currently not in the scope of the World Health Organization recommendation to reduce cardiovascular disease. Objective: We hypothesized that screening for AF would be an important approach to determine the true AF prevalence in the general population in African countries and to identify asymptomatic AF patients at risk for stroke to optimize prevention. Methods: A decision analytic model was developed to study the health-economic impact of AF screening in Nigeria over a life-time horizon. The patient population explored in the model was a population of newly detected AF cases that would be diagnosed with a one-time systematic screening for AF with a single lead ECG device in community health centres across Nigeria. Conclusions: The health gain per newly detected AF patient (N = 31,687) was 0.41 QALY at a cost of 12,587 per QALY gained. The intervention was cost-effective with a 99.9% warfarin use with an ICER of 7.3 million for the total screened population in Nigeria or $1.60 per patient screened. Screening for AF to detect AF patients in need for stroke prevention can be a cost-effective intervention in the Sub-Saharan region, depending on type of anticoagulant used and drug costs
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