1,124 research outputs found
Diagrammatic Monte Carlo study of the Fermi polaron in two dimensions
We study the properties of the two-dimensional Fermi polaron model in which
an impurity attractively interacts with a Fermi sea of particles in the
zero-range limit. We use a diagrammatic Monte Carlo (DiagMC) method which
allows us to sample a Feynman diagrammatic series to very high order. The
convergence properties of the series and the role of multiple particle-hole
excitations are discussed. We study the polaron and molecule energy as a
function of the coupling strength, revealing a transition from a polaron to a
molecule in the ground state. We find a value for the critical interaction
strength which complies with the experimentally measured one and predictions
from variational methods. For all considered interaction strengths, the polaron
factor from the full diagrammatic series almost coincides with the
one-particle-hole result. We also formally link the DiagMC and the variational
approaches for the polaron problem at hand.Comment: 7 pages, 5 figure
Quasiparticle properties of an impurity in a Fermi gas
We report on a study of a spin-down impurity strongly coupled to a spin-up
Fermi sea (a so-called Fermi polaron) with the diagrammatic Monte-Carlo
(DiagMC) technique. Conditions of zero temperature and three dimensions are
considered for an ultracold atomic gas with resonant interactions in the
zero-range limit. A Feynman diagrammatic series is developed for the one-body
and two-body propagators providing information about the polaron and molecule
channel respectively. The DiagMC technique allows us to reach diagram orders
that are high enough for extrapolation to infinite order. The robustness of the
extracted results is examined by checking various resummation techniques and by
running the simulations with various choices for the propagators and vertex
functions. It turns out that dressing the lines in the diagrams as much as
possible is not always the optimal choice. We also identify classes of dominant
diagrams for the one-body and two-body self-energy in the region of strong
interaction. These dominant diagrams turn out to be the leading processes of
the strong-coupling limit. The quasiparticle energies and -factor are
obtained as a function of the interaction strength. We find that the DiagMC
results for the molecule and polaron properties are very similar to those
obtained with a variational ansatz. Surprisingly, this variational ansatz gives
very good predictions for the quasiparticle residue even when this residue is
significantly smaller than one.Comment: 11 pages, 15 figure
Diagrammatic Monte Carlo study of the acoustic and the BEC polaron
We consider two large polaron systems that are described by a Fr\"{o}hlich
type of Hamiltonian, namely the Bose-Einstein condensate (BEC) polaron in the
continuum and the acoustic polaron in a solid. We present ground-state energies
of these two systems calculated with the Diagrammatic Monte Carlo (DiagMC)
method and with a Feynman all-coupling approach. The DiagMC method evaluates up
to very high order a diagrammatic series for the polaron Green's function. The
Feynman all-coupling approach is a variational method that has been used for a
wide range of polaronic problems. For the acoustic and BEC polaron both methods
provide remarkably similar non-renormalized ground-state energies that are
obtained after introducing a finite momentum cutoff. For the renormalized
ground-state energies of the BEC polaron, there are relatively large
discrepancies between the DiagMC and the Feynman predictions. These differences
can be attributed to the renormalization procedure for the contact interaction.Comment: 9 pages, 10 figure
Genetic analysis of physical activity in twins
BACKGROUND: The reduced contribution of physical activity (PA) to daily energy expenditure contributes to the increased prevalence of obesity. A genetic control of activity-induced energy expenditure (AEE) may contribute to a genetic susceptibility to obesity. OBJECTIVE: Our aim was to investigate the relative contribution of genetic and environmental factors to the variation and covariation in AEE and PA. DESIGN: Twelve monozygotic and 8 same-sex dizygotic (including 2 same-sex sibling pairs; age differences: 2 and 2.5 y) twin pairs aged between 18 and 39 y participated. AEE was measured in a respiration chamber for 24 h and with doubly labeled water in daily life for 2 wk. PA was recorded simultaneously with a triaxial accelerometer. Structural equation modeling was used to separate and quantify the observed variance into sex-adjusted additive genetic and common and unique environmental contributions. RESULTS: In the respiration chamber, common and unique environmental factors explained the variance in AEE and PA, and no genetic contribution was found. In daily life, genetic factors explained 72% of the variance in AEE and 78% of the variance in PA. Unique environmental factors explained the remaining variance. The same additive genetic factors explained 67% of the covariance in AEE and PA in daily life. CONCLUSIONS: In the present exploratory study that used gold standard measurements for AEE and PA but a limited sample size, genetic influence explained a large part of the variation in AEE and PA in daily life, whereas both AEE and PA were influenced by environment only within the confined area of the respiration chamber
Het oude Oostende en zijne driejarige belegering (1601-1604): opkomst bloei en ondergang met de beroerten der XVIe eeuw
Press release for the exhibition "Magdalena Abakanowicz," June 24-August 26, 1984, held at the Dallas Museum of Art. The press release announces and describes the exhibition
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