1,585 research outputs found
Three- and four-body interactions in colloidal systems
Three-body and four-body interactions have been directly measured in a
colloidal system comprised of three (or four) charged colloidal particles. Two
of the particles have been confined by means of a scanned laser tweezers to a
line-shaped optical trap where they diffused due to thermal fluctuations. By
means of an additional focused optical trap a third particle has been
approached and attractive three-body interactions have been observed. These
observations are in qualitative agreement with additionally performed nonlinear
Poissson-Boltzmann calculations. Two configurations of four particles have been
studied experimentally as well and in both cases a repulsive four-body
interaction term has been observed
Measuring the equation of state of a hard-disc fluid
We use video microscopy to study a two-dimensional (2D) model fluid of
charged colloidal particles suspended in water and compute the pressure from
the measured particle configurations. Direct experimental control over the
particle density by means of optical tweezers allows the precise measurement of
pressure as a function of density. We compare our data with theoretical
predictions for the equation of state, the pair-correlation function and the
compressibility of a hard-disc fluid and find good agreement, both for the
fluid and the solid phase. In particular the location of the transition point
agrees well with results from Monte Carlo simulations.Comment: 7 pages, to appear in EPL, slightly corrected versio
Current driven switching of magnetic layers
The switching of magnetic layers is studied under the action of a spin
current in a ferromagnetic metal/non-magnetic metal/ferromagnetic metal spin
valve. We find that the main contribution to the switching comes from the
non-equilibrium exchange interaction between the ferromagnetic layers. This
interaction defines the magnetic configuration of the layers with minimum
energy and establishes the threshold for a critical switching current.
Depending on the direction of the critical current, the interaction changes
sign and a given magnetic configuration becomes unstable. To model the time
dependence of the switching process, we derive a set of coupled Landau-Lifshitz
equations for the ferromagnetic layers. Higher order terms in the
non-equilibrium exchange coupling allow the system to evolve to its
steady-state configuration.Comment: 8 pages, 2 figure. Submitted to Phys. Rev.
Injection of photoelectrons into dense argon gas
The injection of photoelectrons in a gaseous or liquid sample is a widespread
technique to produce a cold plasma in a weakly--ionized system in order to
study the transport properties of electrons in a dense gas or liquid. We report
here the experimental results of photoelectron injection into dense argon gas
at the temperatureT=142.6 K as a function of the externally applied electric
field and gas density. We show that the experimental data can be interpreted in
terms of the so called Young-Bradbury model only if multiple scattering effects
due to the dense environment are taken into account when computing the
scattering properties and the energetics of the electrons.Comment: 18 pages, 10 figures, figure nr. 10 has been redrawn, to be submitted
to Plasma Sources Science and Technolog
The Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction across a tunneling junction out of equilibrium
The Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between two magnetic
- spin impurities across a tunneling junction is studied when the system
is driven out of equilibrium through biasing the junction. The nonequilibrium
situation is handled with the Keldysh time-loop perturbation formalism in
conjunction with appropriate coupling methods for tunneling systems due to
Caroli and Feuchtwang. We find that the presence of a nonequilibrium bias
across the junction leads to an interference of several fundamental
oscillations, such that in this tunneling geometry, it is possible to tune the
interaction between ferromagnetic and antiferromagnetic coupling at a fixed
impurity configuration, simply by changing the bias across the junction.
Furthermore, it is shown that the range of the RKKY interaction is altered out
of equilibrium, such that in particular the interaction energy between two
slabs of spins scales extensively with the thickness of the slabs in the
presence of an applied bias.Comment: 38 pages revtex preprint; 5 postscript figures; submitted to Phys.
Rev.
Testing the relevance of effective interaction potentials between highly charged colloids in suspension
Combining cell and Jellium model mean-field approaches, Monte Carlo together
with integral equation techniques, and finally more demanding many-colloid
mean-field computations, we investigate the thermodynamic behavior, pressure
and compressibility of highly charged colloidal dispersions, and at a more
microscopic level, the force distribution acting on the colloids. The
Kirkwood-Buff identity provides a useful probe to challenge the
self-consistency of an approximate effective screened Coulomb (Yukawa)
potential between colloids. Two effective parameter models are put to the test:
cell against renormalized Jellium models
Self-similar magnetoresistance of Fibonacci ultrathin magnetic films
We study numerically the magnetic properties (magnetization and
magnetoresistance) of ultra-thin magnetic films (Fe/Cr) grown following the
Fibonacci sequence. We use a phenomenological model which includes Zeeman,
cubic anisotropy, bilinear and biquadratic exchange energies. Our physical
parameters are based on experimental data recently reported, which contain
biquadratic exchange coupling with magnitude comparable to the bilinear
exchange coupling. When biquadratic exchange coupling is sufficiently large a
striking self-similar pattern emerges.Comment: 5 pages, 5 EPS figures, REVTeX, accepted for publication in Phys.
Rev.
Coexistence of hexatic and isotropic phases in two-dimensional Yukawa systems
We have performed Brownian dynamics simulations on melting of two-dimensional
colloidal crystal in which particles interact with Yukawa potential. The pair
correlation function and bond-orientational correlation function was calculated
in the Yukawa system. An algebraic decay of the bond orientational correlation
function was observed. By ruling out the coexistence region, only a unstable
hexatic phase was found in the Yukawa systems. But our work shows that the
melting of the Yukawa systems is a two-stage melting not consist with the KTHNY
theory and the isotropic liquid and the hexatic phase coexistence region was
found. Also we have studied point defects in two-dimensional Yukawa systems.Comment: 9 pages, 8 figures. any comments are welcom
Electronic Theory for the Nonlinear Magneto-Optical Response of Transition-Metals at Surfaces and Interfaces: Dependence of the Kerr-Rotation on Polarization and on the Magnetic Easy Axis
We extend our previous study of the polarization dependence of the nonlinear
optical response to the case of magnetic surfaces and buried magnetic
interfaces. We calculate for the longitudinal and polar configuration the
nonlinear magneto-optical Kerr rotation angle. In particular, we show which
tensor elements of the susceptibilities are involved in the enhancement of the
Kerr rotation in nonlinear optics for different configurations and we
demonstrate by a detailed analysis how the direction of the magnetization and
thus the easy axis at surfaces and buried interfaces can be determined from the
polarization dependence of the nonlinear magneto-optical response, since the
nonlinear Kerr rotation is sensitive to the electromagnetic field components
instead of merely the intensities. We also prove from the microscopic treatment
of spin-orbit coupling that there is an intrinsic phase difference of
90 between tensor elements which are even or odd under magnetization
reversal in contrast to linear magneto-optics. Finally, we compare our results
with several experiments on Co/Cu films and on Co/Au and Fe/Cr multilayers. We
conclude that the nonlinear magneto-optical Kerr-effect determines uniquely the
magnetic structure and in particular the magnetic easy axis in films and at
multilayer interfaces.Comment: 23 pages Revtex, preprintstyle, 2 uuencoded figure
Theory for Spin-Polarized Oscillations in Nonlinear Magneto-Optics due to Quantum Well States
Using an electronic tight-binding theory we calculate the nonlinear
magneto-optical response from an x-Cu/1Fe/Cu(001) film as a function of
frequency and Cu overlayer thickness (x=3 ... 25). We find very strong
spin-polarized quantum well oscillations in the nonlinear magneto-optical Kerr
effect (NOLIMOKE). These are enhanced by the large density of Fe states
close to the Fermi level acting as intermediate states for frequency doubling.
In good agreement with experiment we find two oscillation periods of 6-7 and 11
monolayers the latter being more pronounced.Comment: 12 pages, Revtex, 3 postscript figure
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