851 research outputs found
Mapping out the quasicondensate transition through the dimensional crossover from one to three dimensions
By measuring the density fluctuations in a highly elongated weakly interacting Bose gas, we observe and quantify the transition from the ideal gas to a quasicondensate regime throughout the dimensional crossover from a purely one-dimensional (1D) to an almost three-dimensional (3D) gas. We show that that the entire transition region and the dimensional crossover are described surprisingly well by the modified Yang-Yang model. Furthermore, we find that at low temperatures the linear density at the quasicondensate transition scales according to an interaction-driven scenario of a longitudinally uniform 1D Bose gas, whereas at high temperatures it scales according to the degeneracy-driven critical scenario of transverse condensation of a 3D ideal gas
Engineering spin-orbit coupling for photons and polaritons in microstructures
One of the most fundamental properties of electromagnetism and special
relativity is the coupling between the spin of an electron and its orbital
motion. This is at the origin of the fine structure in atoms, the spin Hall
effect in semiconductors, and underlies many intriguing properties of
topological insulators, in particular their chiral edge states. Configurations
where neutral particles experience an effective spin-orbit coupling have been
recently proposed and realized using ultracold atoms and photons. Here we use
coupled micropillars etched out of a semiconductor microcavity to engineer a
spin-orbit Hamiltonian for photons and polaritons in a microstructure. The
coupling between the spin and orbital momentum arises from the polarisation
dependent confinement and tunnelling of photons between micropillars arranged
in the form of a hexagonal photonic molecule. Dramatic consequences of the
spin-orbit coupling are experimentally observed in these structures in the
wavefunction of polariton condensates, whose helical shape is directly visible
in the spatially resolved polarisation patterns of the emitted light. The
strong optical nonlinearity of polariton systems suggests exciting perspectives
for using quantum fluids of polaritons11 for quantum simulation of the
interplay between interactions and spin-orbit coupling.Comment: main text: pages 1-11 (4 figures); supplementary material: pages
12-28 (9 figures
Momentum distribution of 1D Bose gases at the quasi-condensation crossover: theoretical and experimental investigation
We investigate the momentum distribution of weakly interacting 1D Bose gases
at thermal equilibrium both experimentally and theoretically. Momentum
distribution of single 1D Bose gases is measured using a focusing technique,
whose resolution we improve via a guiding scheme. The momentum distribution
compares very well with quantum Monte Carlo calculations for the Lieb-Liniger
model at finite temperature, allowing for an accurate thermometry of the gas
that agrees with (and improves upon) the thermometry based on in situ density
fluctuation measurements. The quasi-condensation crossover is investigated via
two different experimental parameter sets, corresponding to the two different
sides of the crossover. Classical field theory is expected to correctly
describe the quasi-condensation crossover of weakly interacting gases. We
derive the condition of validity of the classical field theory, and find that,
in typical experiments, interactions are too strong for this theory to be
accurate. This is confirmed by a comparison between the classical field
predictions and the numerically exact quantum Monte Carlo calculations.Comment: 8 page
Mapping out the quasicondensate transition through the dimensional crossover from one to three dimensions Phys.
By measuring the density fluctuations in a highly elongated weakly interacting Bose gas, we observe and quantify the transition from the ideal gas to a quasicondensate regime throughout the dimensional crossover from a purely one-dimensional (1D) to an almost three-dimensional (3D) gas. We show that that the entire transition region and the dimensional crossover are described surprisingly well by the modified Yang-Yang model. Furthermore, we find that at low temperatures the linear density at the quasicondensate transition scales according to an interaction-driven scenario of a longitudinally uniform 1D Bose gas, whereas at high temperatures it scales according to the degeneracy-driven critical scenario of transverse condensation of a 3D ideal gas. Low-dimensional (one-or two-dimensional) systems can have physical properties dramatically different from their three-dimensional (3D) counterparts. Experimental realizations of such systems in recent years has been particularly exciting in the field of ultracold atomic gases In this paper we address this question for a weakly interacting Bose gas that is confined transversely by a harmonic trap of frequency ω ⊥ /2π but is homogeneous in the thermodynamic limit with respect to the longitudinal direction. The one-dimensional (1D) regime is obtained when the thermal energy k B T and the chemical potential µ become much smaller than the transverse excitation energyhω ⊥ . In the absence of interatomic interactions, the homogeneous 1D gas is characterized by the absence of Bose-Einstein condensation. In the 3D limit, however, for k B T hω ⊥ , a sharp transverse condensation is expected: The atoms accumulate in the transverse ground state due to the saturation of population in the transversally excited states, yet the resulting 1D gas is still uncondensed with respect to the longitudinal states Our study relies on the measurement of atomic density fluctuations, previously used to identify the two limiting regimes-the ideal gas and the quasicondensat
Molecular scale contact line hydrodynamics of immiscible flows
From extensive molecular dynamics simulations on immiscible two-phase flows,
we find the relative slipping between the fluids and the solid wall everywhere
to follow the generalized Navier boundary condition, in which the amount of
slipping is proportional to the sum of tangential viscous stress and the
uncompensated Young stress. The latter arises from the deviation of the
fluid-fluid interface from its static configuration. We give a continuum
formulation of the immiscible flow hydrodynamics, comprising the generalized
Navier boundary condition, the Navier-Stokes equation, and the Cahn-Hilliard
interfacial free energy. Our hydrodynamic model yields interfacial and velocity
profiles matching those from the molecular dynamics simulations at the
molecular-scale vicinity of the contact line. In particular, the behavior at
high capillary numbers, leading to the breakup of the fluid-fluid interface, is
accurately predicted.Comment: 33 pages for text in preprint format, 10 pages for 10 figures with
captions, content changed in this resubmissio
Disjoining Potential and Spreading of Thin Liquid Layers in the Diffuse Interface Model Coupled to Hydrodynamics
The hydrodynamic phase field model is applied to the problem of film
spreading on a solid surface. The disjoining potential, responsible for
modification of the fluid properties near a three-phase contact line, is
computed from the solvability conditions of the density field equation with
appropriate boundary conditions imposed on the solid support. The equation
describing the motion of a spreading film are derived in the lubrication
approximation. In the case of quasi-equilibrium spreading, is shown that the
correct sharp-interface limit is obtained, and sample solutions are obtained by
numerical integration. It is further shown that evaporation or condensation may
strongly affect the dynamics near the contact line, and accounting for kinetic
retardation of the interphase transport is necessary to build up a consistent
theory.Comment: 14 pages, 5 figures, to appear in PR
Coordinated analysis of age, sex, and education effects on change in MMSE scores
Objectives. We describe and compare the expected performance trajectories of older adults on the Mini-Mental Status Examination (MMSE) across six independent studies from four countries in the context of a collaborative network of longitudinal studies of aging. A coordinated analysis approach is used to compare patterns of change conditional on sample composition differences related to age, sex, and education. Such coordination accelerates evaluation of particular hypotheses. In particular, we focus on the effect of educational attainment on cognitive decline.Method. Regular and Tobit mixed models were fit to MMSE scores from each study separately. The effects of age, sex, and education were examined based on more than one centering point.Results. Findings were relatively consistent across studies. On average, MMSE scores were lower for older individuals and declined over time. Education predicted MMSE score, but, with two exceptions, was not associated with decline in MMSE over time.Conclusion. A straightforward association between educational attainment and rate of cognitive decline was not supported. Thoughtful consideration is needed when synthesizing evidence across studies, as methodologies adopted and sample characteristics, such as educational attainment, invariably differ. © 2012 The Author
On the Interface Formation Model for Dynamic Triple Lines
This paper revisits the theory of Y. Shikhmurzaev on forming interfaces as a
continuum thermodynamical model for dynamic triple lines. We start with the
derivation of the balances for mass, momentum, energy and entropy in a
three-phase fluid system with full interfacial physics, including a brief
review of the relevant transport theorems on interfaces and triple lines.
Employing the entropy principle in the form given in [Bothe & Dreyer, Acta
Mechanica, doi:10.1007/s00707-014-1275-1] but extended to this more general
case, we arrive at the entropy production and perform a linear closure, except
for a nonlinear closure for the sorption processes. Specialized to the
isothermal case, we obtain a thermodynamically consistent mathematical model
for dynamic triple lines and show that the total available energy is a strict
Lyapunov function for this system
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