9,541 research outputs found
A unified operator splitting approach for multi-scale fluid-particle coupling in the lattice Boltzmann method
A unified framework to derive discrete time-marching schemes for coupling of
immersed solid and elastic objects to the lattice Boltzmann method is
presented. Based on operator splitting for the discrete Boltzmann equation,
second-order time-accurate schemes for the immersed boundary method, viscous
force coupling and external boundary force are derived. Furthermore, a modified
formulation of the external boundary force is introduced that leads to a more
accurate no-slip boundary condition. The derivation also reveals that the
coupling methods can be cast into a unified form, and that the immersed
boundary method can be interpreted as the limit of force coupling for vanishing
particle mass. In practice, the ratio between fluid and particle mass
determines the strength of the force transfer in the coupling. The integration
schemes formally improve the accuracy of first-order algorithms that are
commonly employed when coupling immersed objects to a lattice Boltzmann fluid.
It is anticipated that they will also lead to superior long-time stability in
simulations of complex fluids with multiple scales
Prediction of a surface state and a related surface insulator-metal transition for the (100) surface of stochiometric EuO
We calculate the temperature and layer-dependent electronic structure of a
20-layer EuO(100)-film using a combination of first-principles and model
calculation based on the ferromagnetic Kondo-lattice model. The results suggest
the existence of a EuO(100) surface state which can lead to a surface
insulator-metal transition.Comment: 9 pages, 5 figures, Phys. Rev. Lett. (in press
Photoemission study of the spin-density wave state in thin films of Cr
Angle-resolved photoemission (PE) was used to characterize the spin-density
wave (SDW) state in thin films of Cr grown on W(110). The PE data were analysed
using results of local spin density approximation layer-Korringa-Kohn-Rostoker
calculations. It is shown that the incommensurate SDW can be monitored and
important parameters of SDW-related interactions, such as coupling strength and
energy of collective magnetic excitations, can be determined from the
dispersion of the renormalized electronic bands close to the Fermi energy. The
developed approach can readily be applied to other SDW systems including
magnetic multilayer structures.Comment: 4 figure
Collective waves in dense and confined microfluidic droplet arrays
Excitation mechanisms for collective waves in confined dense one-dimensional
microfluidic droplet arrays are investigated by experiments and computer
simulations. We demonstrate that distinct modes can be excited by creating
specific `defect' patterns in flowing droplet trains. Excited longitudinal
modes exhibit a short-lived cascade of pairs of laterally displacing droplets.
Transversely excited modes obey the dispersion relation of microfluidic phonons
and induce a coupling between longitudinal and transverse modes, whose origin
is the hydrodynamic interaction of the droplets with the confining walls.
Moreover, we investigate the long-time behaviour of the oscillations and
discuss possible mechanisms for the onset of instabilities. Our findings
demonstrate that the collective dynamics of microfluidic droplet ensembles can
be studied particularly well in dense and confined systems. Experimentally, the
ability to control microfluidic droplets may allow to modulate the refractive
index of optofluidic crystals which is a promising approach for the production
of dynamically programmable metamaterials.Comment: 13 pages, 17 figure
Behavior in normal and reduced gravity of an enclosed liquid/gas system with nonuniform heating from above
The temperature and velocity fields have been investigated for a single-phase gas system and a two-layer gas-and-liquid system enclosed in a circular cylinder being heated suddenly and nonuniformly from above. The transient response of the gas, liquid, and container walls was modelled numerically in normal and reduced gravity (10 to the -5 g). Verification of the model was accomplished via flow visualization experiments in 10 cm high by 10 cm diameter plexiglass cylinders
Translational cooling and storage of protonated proteins in an ion trap at subkelvin temperatures
Gas-phase multiply charged proteins have been sympathetically cooled to
translational temperatures below 1 K by Coulomb interaction with laser-cooled
barium ions in a linear ion trap. In one case, an ensemble of 53 cytochrome c
molecules (mass ~ 12390 amu, charge +17 e) was cooled by ~ 160 laser-cooled
barium ions to less than 0.75 K. Storage times of more than 20 minutes have
been observed and could easily be extended to more than an hour. The technique
is applicable to a wide variety of complex molecules.Comment: same version as published in Phys. Rev.
Adiabatic pumping through a quantum dot in the Kondo regime: Exact results at the Toulouse limit
Transport properties of ultrasmall quantum dots with a single unpaired
electron are commonly modeled by the nonequilibrium Kondo model, describing the
exchange interaction of a spin-1/2 local moment with two leads of
noninteracting electrons. Remarkably, the model possesses an exact solution
when tuned to a special manifold in its parameter space known as the Toulouse
limit. We use the Toulouse limit to exactly calculate the adiabatically pumped
spin current in the Kondo regime. In the absence of both potential scattering
and a voltage bias, the instantaneous charge current is strictly zero for a
generic Kondo model. However, a nonzero spin current can be pumped through the
system in the presence of a finite magnetic field, provided the spin couples
asymmetrically to the two leads. Tunneling through a Kondo impurity thus offers
a natural mechanism for generating a pure spin current. We show, in particular,
that one can devise pumping cycles along which the average spin pumped per
cycle is closely equal to . By analogy with Brouwer's formula for
noninteracting systems with two driven parameters, the pumped spin current is
expressed as a geometrical property of a scattering matrix. However, the
relevant %Alex: I replaced topological with geometrical in the sentence above
scattering matrix that enters the formulation pertains to the Majorana fermions
that appear at the Toulouse limit rather than the physical electrons that carry
the current. These results are obtained by combining the nonequilibrium Keldysh
Green function technique with a systematic gradient expansion, explicitly
exposing the small parameter controlling the adiabatic limit.Comment: 14 pages, 3 figures, revised versio
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