2,483 research outputs found
Characterizing Potentials by a Generalized Boltzmann Factor
Based on the concept of a nonequilibrium steady state, we present a novel
method to experimentally determine energy landscapes acting on colloidal
systems. By measuring the stationary probability distribution and the current
in the system, we explore potential landscapes with barriers up to several
hundred \kT. As an illustration, we use this approach to measure the
effective diffusion coefficient of a colloidal particle moving in a tilted
potential
Half Cycle Pulse Train Induced State Redistribution of Rydberg Atoms
Population transfer between low lying Rydberg states independent of the
initial state is realized using a train of half-cycle pulses with pulse
durations much less than the classical orbit period. We demonstrate
experimentally the transfer of population from initial states around n=50 down
to n<40 as well as up to the continuum. The measured population transfer
matches well to a model of the process for 1D atoms.Comment: V2: discussion extended, version accepted for publication in Physical
Review
Mobility and Diffusion of a Tagged Particle in a Driven Colloidal Suspension
We study numerically the influence of density and strain rate on the
diffusion and mobility of a single tagged particle in a sheared colloidal
suspension. We determine independently the time-dependent velocity
autocorrelation functions and, through a novel method, the response functions
with respect to a small force. While both the diffusion coefficient and the
mobility depend on the strain rate the latter exhibits a rather weak
dependency. Somewhat surprisingly, we find that the initial decay of response
and correlation functions coincide, allowing for an interpretation in terms of
an 'effective temperature'. Such a phenomenological effective temperature
recovers the Einstein relation in nonequilibrium. We show that our data is well
described by two expansions to lowest order in the strain rate.Comment: submitted to EP
Probability density functions of work and heat near the stochastic resonance of a colloidal particle
We study experimentally and theoretically the probability density functions
of the injected and dissipated energy in a system of a colloidal particle
trapped in a double well potential periodically modulated by an external
perturbation. The work done by the external force and the dissipated energy are
measured close to the stochastic resonance where the injected power is maximum.
We show a good agreement between the probability density functions exactly
computed from a Langevin dynamics and the measured ones. The probability
density function of the work done on the particle satisfies the fluctuation
theorem
Tagged particle in a sheared suspension: effective temperature determines density distribution in a slowly varying external potential beyond linear response
We consider a sheared colloidal suspension under the influence of an external
potential that varies slowly in space in the plane perpendicular to the flow
and acts on one selected (tagged) particle of the suspension. Using a
Chapman-Enskog type expansion we derive a steady state equation for the tagged
particle density distribution. We show that for potentials varying along one
direction only, the tagged particle distribution is the same as the equilibrium
distribution with the temperature equal to the effective temperature obtained
from the violation of the Einstein relation between the self-diffusion and
tagged particle mobility coefficients. We thus prove the usefulness of this
effective temperature for the description of the tagged particle behavior
beyond the realm of linear response. We illustrate our theoretical predictions
with Brownian dynamics computer simulations.Comment: Accepted for publication in Europhys. Let
Irreversible effects of memory
The steady state of a Langevin equation with short ranged memory and coloured
noise is analyzed. When the fluctuation-dissipation theorem of second kind is
not satisfied, the dynamics is irreversible, i.e. detailed balance is violated.
We show that the entropy production rate for this system should include the
power injected by ``memory forces''. With this additional contribution, the
Fluctuation Relation is fairly verified in simulations. Both dynamics with
inertia and overdamped dynamics yield the same expression for this additional
power. The role of ``memory forces'' within the fluctuation-dissipation
relation of first kind is also discussed.Comment: 6 pages, 1 figure, publishe
Automated assembly of large double-sided microstrip detector modules of the CBM Silicon Tracking System at FAIR
The detector modules of the Silicon Tracking System of the Compressed Baryonic Matter experiment at FAIR comprise double-sided silicon microstrip sensors with a size of up to 124 mm x 62 mm. Due to tight material budget constraints, the sensors are connected to the read-out electronics by long flexible microcables. As manual assembly of the modules is time-consuming and difficult, a fully customized in-house bonder machine has been developed which allows for a highly automated detector module assembly. We present the requirements and the setup of the bonder machine together with the achieved alignment accuracy and first assemblies
The Multitude of Molecular Hydrogen Knots in the Helix Nebula
We present HST/NICMOS imaging of the H_2 2.12 \mu m emission in 5 fields in
the Helix Nebula ranging in radial distance from 250-450" from the central
star. The images reveal arcuate structures with their apexes pointing towards
the central star. Comparison of these images with comparable resolution ground
based images reveals that the molecular gas is more highly clumped than the
ionized gas line tracers. From our images, we determine an average number
density of knots in the molecular gas ranging from 162 knots/arcmin^2 in the
denser regions to 18 knots/arcmin^2 in the lower density outer regions. Using
this new number density, we estimate that the total number of knots in the
Helix to be ~23,000 which is a factor of 6.5 larger than previous estimates.
The total neutral gas mass in the Helix is 0.35 M_\odot assuming a mass of
\~1.5x10^{-5} M_\odot for the individual knots. The H_2 intensity, 5-9x10^{-5}
erg s^{-1} cm^{-2} sr^{-1}, remains relatively constant with projected distance
from the central star suggesting a heating mechanism for the molecular gas that
is distributed almost uniformly in the knots throughout the nebula. The
temperature and H_2 2.12 \mu m intensity of the knots can be approximately
explained by photodissociation regions (PDRs) in the individual knots; however,
theoretical PDR models of PN under-predict the intensities of some knots by a
factor of 10.Comment: 26 pages, 3 tables, 10 figures; AJ accepte
Effective Confinement as Origin of the Equivalence of Kinetic Temperature and Fluctuation-Dissipation Ratio in a Dense Shear Driven Suspension
We study response and velocity autocorrelation functions for a tagged
particle in a shear driven suspension governed by underdamped stochastic
dynamics. We follow the idea of an effective confinement in dense suspensions
and exploit a time-scale separation between particle reorganization and
vibrational motion. This allows us to approximately derive the
fluctuation-dissipation theorem in a "hybrid" form involving the kinetic
temperature as an effective temperature and an additive correction term. We
show numerically that even in a moderately dense suspension the latter is
negligible. We discuss similarities and differences with a simple toy model, a
single trapped particle in shear flow
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