4,260 research outputs found
Three flow regimes of viscous jet falling onto a moving surface
A stationary viscous jet falling from an oriented nozzle onto a moving
surface is studied, both theoretically and experimentally. We distinguish three
flow regimes and classify them by the convexity of the jet shape (concave,
vertical and convex). The fluid is modeled as a Newtonian fluid, and the model
for the flow includes viscous effects, inertia and gravity. By studying the
characteristics of the conservation of momentum for a dynamic jet, the boundary
conditions for each flow regime are derived, and the flow regimes are
characterized in terms of the process and material parameters. The model is
solved by a transformation into an algebraic equation. We make a comparison
between the model and experiments, and obtain qualitative agreement
Colossal proximity effect in a superconducting triplet spin valve based on halfmetallic ferromagnetic CrO2
Ferromagnets can sustain supercurrents through the formation of equal spin
triplet Cooper pairs and the mechanism of odd-frequency pairing. Since such
pairs are not broken by the exchange energy of the ferromagnet, superconducting
triplet correlations are long-ranged and spin-polarized, with promises for
superconducting spintronics devices. The main challenge is to understand how
triplets are generated at the superconductor (S)/ ferromagnet (F) interface.
Here we use the concept of a so-called triplet spin valve (TSV) to investigate
the conversion of singlets in a conventional superconductor to triplets in the
halfmetallic ferromagnet CrO_2. TSV's are composed of two ferromagnetic layers
(separated by a thin normal metal (N) layer) and a superconductor
(F_1/N/F_2/S). The package F_1/N/F_2 generates triplets in F_1 when the
magnetization directions of the F_{1,2}-layers are not collinear. This drains
singlet pairs from the S-layer, and triplet generation is therefore signalled
by a decrease of the critical temperature . Recently, experiments with
TSV's were reported with Co draining layers, using in-plane fields, and finding
T_c-shifts up to 100~mK. Using CrO_2 instead of Co and rotating a magnetic
field from in-plane to out-of-plane, we find strong T_c variations of almost a
Kelvin up to fields of the order of a Tesla. Such strong drainage is consistent
with the large lengths over which supercurrents can flow in CrO_2, which are
significantly larger than in conventional ferromagnets. Our results point to
the special interest of halfmetals for superconducting spintronics.Comment: 6 pages, 5 figures; supplementary information separat
Falling of a viscous jet onto a moving surface
We analyze the stationary flow of a jet of Newtonian fluid that is drawn by
gravity onto a moving surface. The situation is modeled by a third-order ODE on
a domain of unknown length and with an additional integral condition; by
solving part of the equation explicitly we can reformulate the problem as a
first-order ODE, again with an integral constraint. We show that there are two
flow regimes, and characterize the associated regions in the three-dimensional
parameter space in terms of an easily calculable quantity. In a qualitative
sense the results from the model are found to correspond with experimental
observations.Comment: 16 pages, 11 figure
Inhomogeneous superconductivity induced in a weak ferromagnet
Under certain conditions, the order parameter induced by a superconductor (S)
in a ferromagnet (F) can be inhomogeneous and oscillating, which results e.g.
in the so-called pi-coupling in S/F/S junctions. In principle, the
inhomogeneous state can be induced at T_c as function of the F-layer thickness
d_F in S/F bilayers and multilayers, which should result in a dip-like
characteristic of T_c(d_F). We show the results of measurements on the S/F
system Nb/Cu_{1-x}Ni_x, for Ni-concentrations in the range x = 0.5-0.7, where
such effects might be expected. We find that the critical thickness for the
occurrence of superconductivity is still relatively high, even for these weak
ferromagnets. The resulting dip then is intrinsically shallow and difficult to
observe, which explains the lack of a clear signature in the T_c(d_F) data.Comment: 4 pages, 4 figures. To be publishedin Physica C (proceedings of the
Second Euroconference on Vortex Matter in Superconductors, Crete, 2001
Adaptive stepsize and instabilities in complex Langevin dynamics
Stochastic quantization offers the opportunity to simulate field theories
with a complex action. In some theories unstable trajectories are prevalent
when a constant stepsize is employed. We construct algorithms for generating an
adaptive stepsize in complex Langevin simulations and find that unstable
trajectories are completely eliminated. To illustrate the generality of the
approach, we apply it to the three-dimensional XY model at nonzero chemical
potential and the heavy dense limit of QCD.Comment: 12 pages, several eps figures; clarification and minor corrections
added, to appear in PL
Exact and Truncated Dynamics in Nonequilibrium Field Theory
Nonperturbative dynamics of quantum fields out of equilibrium is often
described by the time evolution of a hierarchy of correlation functions, using
approximation methods such as Hartree, large N, and nPI-effective action
techniques. These truncation schemes can be implemented equally well in a
classical statistical system, where results can be tested by comparison with
the complete nonlinear evolution obtained by numerical methods. For a 1+1
dimensional scalar field we find that the early-time behaviour is reproduced
qualitatively by the Hartree dynamics. The inclusion of direct scattering
improves this to the quantitative level. We show that the emergence of
nonthermal temperature profiles at intermediate times can be understood in
terms of the fixed points of the evolution equations in the Hartree
approximation. The form of the profile depends explicitly on the initial
ensemble. While the truncated evolution equations do not seem to be able to get
away from the fixed point, the full nonlinear evolution shows thermalization
with a (surprisingly) slow relaxation.Comment: 30 pages with 12 eps figures, minor changes; to appear in Phys.Rev.
Transport of a colloidal particle driven across a temporally oscillating optical potential energy landscape
A colloidal particle is driven across a temporally oscillating one-dimensional optical potential energy landscape and its particle motion is analysed. Different modes of dynamic mode locking are observed and are confirmed with the use of phase portraits. The effect of the oscillation frequency on the mode locked step width is addressed and the results are discussed in light of a high-frequency theory and compared to simulations. Furthermore, the influence of the coupling between the particle and the optical landscape on mode locking is probed by increasing the maximum depth of the optical landscape. Stronger coupling is seen to increase the width of mode locked steps. Finally, transport across the temporally oscillating landscape is studied by measuring the effective diffusion coefficient of a mobile particle, which is seen to be highly sensitive to the driving velocity and mode locking
Thermal effects on slow-roll dynamics
A description of the transition from the inflationary epoch to radiation
domination requires the understanding of quantum fields out of thermal
equilibrium, particle creation and thermalisation. This can be studied from
first principles by solving a set of truncated real-time Schwinger-Dyson
equations, written in terms of the mean field (inflaton) and the field
propagators, derived from the two-particle irreducible effective action. We
investigate some aspects of this problem by considering the dynamics of a
slow-rolling mean field coupled to a second quantum field, using a \phi^2\chi^2
interaction. We focus on thermal effects. It is found that interactions lead to
an earlier end of slow-roll and that the evolution afterwards depends on
details of the heatbath.Comment: 25 pages, 11 eps figures. v2: paper reorganized, title changed,
conclusions unchanged, to appear in PR
- …