5,182 research outputs found
Organizational change within a maintenance department:antecedents and outcome characteristics of willingness to change
Analytical Expression for the RKKY Interaction in Doped Graphene
We obtain an analytical expression for the Ruderman-Kittel-Kasuya-Yosida
(RKKY) interaction in electron or hole doped graphene for linear Dirac
bands. The results agree very well with the numerical calculations for the full
tight-binding band structure in the regime where the linear band structure is
valid. The analytical result, expressed in terms of the Meijer G-function,
consists of a product of two oscillatory terms, one coming from the
interference between the two Dirac cones and the second coming from the finite
size of the Fermi surface. For large distances, the Meijer G-function behaves
as a sinusoidal term, leading to the result for moments located on the same sublattice. The
dependence, which is the same for the standard two-dimensional electron gas, is
universal irrespective of the sublattice location and the distance direction of
the two moments except when (undoped case), where it reverts to the
dependence. These results correct several inconsistencies found in the
literature.Comment: 5 pages, 5 figure
Operation of a Stark decelerator with optimum acceptance
With a Stark decelerator, beams of neutral polar molecules can be
accelerated, guided at a constant velocity, or decelerated. The effectiveness
of this process is determined by the 6D volume in phase space from which
molecules are accepted by the Stark decelerator. Couplings between the
longitudinal and transverse motion of the molecules in the decelerator can
reduce this acceptance. These couplings are nearly absent when the decelerator
operates such that only every third electric field stage is used for
deceleration, while extra transverse focusing is provided by the intermediate
stages. For many applications, the acceptance of a Stark decelerator in this
so-called mode significantly exceeds that of a decelerator in the
conventionally used () mode. This has been experimentally verified by
passing a beam of OH radicals through a 2.6 meter long Stark decelerator. The
experiments are in quantitative agreement with the results of trajectory
calculations, and can qualitatively be explained with a simple model for the 6D
acceptance. These results imply that the 6D acceptance of a Stark decelerator
in the mode of operation approaches the optimum value, i.e. the value
that is obtained when any couplings are neglected.Comment: 13 pages, 11 figure
Loading Stark-decelerated molecules into electrostatic quadrupole traps
Beams of neutral polar molecules in a low-field seeking quantum state can be
slowed down using a Stark decelerator, and can subsequently be loaded and
confined in electrostatic quadrupole traps. The efficiency of the trap loading
process is determined by the ability to couple the decelerated packet of
molecules into the trap without loss of molecules and without heating. We
discuss the inherent difficulties to obtain ideal trap loading, and describe
and compare different trap loading strategies. A new "split-endcap" quadrupole
trap design is presented that enables improved trap loading efficiencies. This
is experimentally verified by comparing the trapping of OH radicals using the
conventional and the new quadrupole trap designs
Accurate effective pair potentials for polymer solutions
Dilute or semi-dilute solutions of non-intersecting self-avoiding walk (SAW)
polymer chains are mapped onto a fluid of ``soft'' particles interacting via an
effective pair potential between their centers of mass. This mapping is
achieved by inverting the pair distribution function of the centers of mass of
the original polymer chains, using integral equation techniques from the theory
of simple fluids. The resulting effective pair potential is finite at all
distances, has a range of the order of the radius of gyration, and turns out to
be only moderately concentration-dependent. The dependence of the effective
potential on polymer length is analyzed in an effort to extract the scaling
limit. The effective potential is used to derive the osmotic equation of state,
which is compared to simulation data for the full SAW segment model, and to the
predictions of renormalization group calculations. A similar inversion
procedure is used to derive an effective wall-polymer potential from the center
of mass density profiles near the wall, obtained from simulations of the full
polymer segment model. The resulting wall-polymer potential turns out to depend
strongly on bulk polymer concentration when polymer-polymer correlations are
taken into account, leading to a considerable enhancement of the effective
repulsion with increasing concentration. The effective polymer-polymer and
wall-polymer potentials are combined to calculate the depletion interaction
induced by SAW polymers between two walls. The calculated depletion interaction
agrees well with the ``exact'' results from much more computer-intensive direct
simulation of the full polymer-segment model, and clearly illustrates the
inadequacy -- in the semi-dilute regime -- of the standard Asakura-Oosawa
approximation based on the assumption of non-interacting polymer coils.Comment: 18 pages, 24 figures, ReVTeX, submitted to J. Chem. Phy
Deceleration and electrostatic trapping of OH radicals
A pulsed beam of ground state OH radicals is slowed down using a Stark
decelerator and is subsequently loaded into an electrostatic trap.
Characterization of the molecular beam production, deceleration and trap
loading process is performed via laser induced fluorescence detection inside
the quadrupole trap. Depending on details of the trap loading sequence,
typically OH () radicals are trapped at a density
of around cm and at temperatures in the 50-500 mK range. The 1/e
trap lifetime is around 1.0 second.Comment: 4 pages, 3 figure
Direct measurement of the radiative lifetime of vibrationally excited OH radicals
Neutral molecules, isolated in the gas-phase, can be prepared in a long-lived
excited state and stored in a trap. The long observation time afforded by the
trap can then be exploited to measure the radiative lifetime of this state by
monitoring the temporal decay of the population in the trap. This method is
demonstrated here and used to benchmark the Einstein -coefficients in the
Meinel system of OH. A pulsed beam of vibrationally excited OH radicals is
Stark decelerated and loaded into an electrostatic quadrupole trap. The
radiative lifetime of the upper -doublet component of the level is determined as ms, in good
agreement with the calculated value of ms.Comment: 4 pages, 3 figures, submitted to Phys. Rev. Let
Multiple packets of neutral molecules revolving for over a mile
The level of control that one has over neutral molecules in beams dictates
their possible applications. Here we experimentally demonstrate that
state-selected, neutral molecules can be kept together in a few mm long packet
for a distance of over one mile. This is accomplished in a circular arrangement
of 40 straight electrostatic hexapoles through which the molecules propagate
over 1000 times. Up to 19 packets of molecules have simultaneously been stored
in this ring structure. This brings the realization of a molecular low-energy
collider within reach
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