16,143 research outputs found
Wave packet dynamics of potassium dimers attached to helium nanodroplets
The dynamics of vibrational wave packets excited in K dimers attached to
superfluid helium nanodroplets is investigated by means of femtosecond
pump-probe spectroscopy. The employed resonant three-photon-ionization scheme
is studied in a wide wavelength range and different pathways leading to
K-formation are identified. While the wave packet dynamics of the
electronic ground state is not influenced by the helium environment,
perturbations of the electronically excited states are observed. The latter
reveal a strong time dependence on the timescale 3-8 ps which directly reflects
the dynamics of desorption of K off the helium droplets
Generalized Green'S Equivalences on the Subsemigroups of the Bicyclic Monoid
We study generalized Green's equivalences on all subsemigroups of the bicyclic monoid B and determine the abundant (and adequate) subsemigroups of B. © 2010 Copyright Taylor and Francis Group, LLC
Metastable states of a flux line lattice studied by transport and Small Angle Neutron Scattering
Flux Lines Lattice (FLL) states have been studied using transport
measurements and Small Angle Neutron Scattering in low T materials. In
Pb-In, the bulk dislocations in the FLL do not influence the transport
properties. In Fe doped NbSe, transport properties can differ after a
Field Cooling (FC) or a Zero Field Cooling (ZFC) procedure, as previously
reported. The ZFC FLL is found ordered with narrow Bragg Peaks and is linked to
a linear V(I) curve and to a superficial critical current. The FC FLL pattern
exhibits two Bragg peaks and the corresponding V(I) curve shows a S-shape. This
can be explained by the coexistence of two ordered FLL slightly tilted from the
applied field direction by different superficial currents. These currents are
wiped out when the transport current is increased.Comment: accepted for publication in Phys. Rev.
Structure factor and thermodynamics of rigid dendrimers in solution
The ''polymer reference interaction site model'' (PRISM) integral equation
theory is used to determine the structure factor of rigid dendrimers in
solution. The theory is quite successful in reproducing experimental structure
factors for various dendrimer concentrations. In addition, the structure factor
at vanishing scattering vector is calculated via the compressibility equation
using scaled particle theory and fundamental measure theory. The results as
predicted by both theories are systematically smaller than the experimental and
PRISM data for platelike dendrimers.Comment: 7 pages, 5 figures, submitte
Signed zeros of Gaussian vector fields-density, correlation functions and curvature
We calculate correlation functions of the (signed) density of zeros of
Gaussian distributed vector fields. We are able to express correlation
functions of arbitrary order through the curvature tensor of a certain abstract
Riemann-Cartan or Riemannian manifold. As an application, we discuss one- and
two-point functions. The zeros of a two-dimensional Gaussian vector field model
the distribution of topological defects in the high-temperature phase of
two-dimensional systems with orientational degrees of freedom, such as
superfluid films, thin superconductors and liquid crystals.Comment: 14 pages, 1 figure, uses iopart.cls, improved presentation, to appear
in J. Phys.
Phase Separation in Binary Fluid Mixtures with Continuously Ramped Temperature
We consider the demixing of a binary fluid mixture, under gravity, which is
steadily driven into a two phase region by slowly ramping the temperature. We
assume, as a first approximation, that the system remains spatially isothermal,
and examine the interplay of two competing nonlinearities. One of these arises
because the supersaturation is greatest far from the meniscus, creating
inversion of the density which can lead to fluid motion; although isothermal,
this is somewhat like the Benard problem (a single-phase fluid heated from
below). The other is the intrinsic diffusive instability which results either
in nucleation or in spinodal decomposition at large supersaturations.
Experimental results on a simple binary mixture show interesting oscillations
in heat capacity and optical properties for a wide range of ramp parameters. We
argue that these oscillations arise under conditions where both nonlinearities
are important
Slow plasmon modes in polymeric salt solutions
The dynamics of polymeric salt solutions are presented. The salt consists of
chains and , which are chemically different and interact with a
Flory-interaction parameter , the chain ends carry a positive
charge whereas the chain ends are modified by negative charges. The
static structure factor shows a peak corresponding to a micro phase separation.
At low momentum transfer, the interdiffusion mode is driven by electrostatics
and is of the plasmon-type, but with an unusually low frequency, easily
accessible by experiments. This is due to the polymer connectivity that
introduces high friction and amplifies the charge scattering thus allowing for
low charge densities. The interdiffusion mode shows a minimum (critical slowing
down) at finite when the interaction parameter increases we find then a low
frequency quasi-plateau.Comment: accepted in Europhys. Let
The distribution of extremal points of Gaussian scalar fields
We consider the signed density of the extremal points of (two-dimensional)
scalar fields with a Gaussian distribution. We assign a positive unit charge to
the maxima and minima of the function and a negative one to its saddles. At
first, we compute the average density for a field in half-space with Dirichlet
boundary conditions. Then we calculate the charge-charge correlation function
(without boundary). We apply the general results to random waves and random
surfaces. Furthermore, we find a generating functional for the two-point
function. Its Legendre transform is the integral over the scalar curvature of a
4-dimensional Riemannian manifold.Comment: 22 pages, 8 figures, corrected published versio
Why polymer chains in a melt are not random walks
A cornerstone of modern polymer physics is the `Flory ideality hypothesis'
which states that a chain in a polymer melt adopts `ideal' random-walk-like
conformations. Here we revisit theoretically and numerically this pivotal
assumption and demonstrate that there are noticeable deviations from ideality.
The deviations come from the interplay of chain connectivity and the
incompressibility of the melt, leading to an effective repulsion between chain
segments of all sizes . The amplitude of this repulsion increases with
decreasing where chain segments become more and more swollen. We illustrate
this swelling by an analysis of the form factor , i.e. the scattered
intensity at wavevector resulting from intramolecular interferences of a
chain. A `Kratky plot' of {\em vs.} does not exhibit the plateau
for intermediate wavevectors characteristic of ideal chains. One rather finds a
conspicuous depression of the plateau, ,
which increases with and only depends on the monomer density .Comment: 4 pages, 4 figures, EPL, accepted January 200
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