5,066 research outputs found
Solidification in soft-core fluids: disordered solids from fast solidification fronts
Using dynamical density functional theory we calculate the speed of
solidification fronts advancing into a quenched two-dimensional model fluid of
soft-core particles. We find that solidification fronts can advance via two
different mechanisms, depending on the depth of the quench. For shallow
quenches, the front propagation is via a nonlinear mechanism. For deep
quenches, front propagation is governed by a linear mechanism and in this
regime we are able to determine the front speed via a marginal stability
analysis. We find that the density modulations generated behind the advancing
front have a characteristic scale that differs from the wavelength of the
density modulation in thermodynamic equilibrium, i.e., the spacing between the
crystal planes in an equilibrium crystal. This leads to the subsequent
development of disorder in the solids that are formed. For the one-component
fluid, the particles are able to rearrange to form a well-ordered crystal, with
few defects. However, solidification fronts in a binary mixture exhibiting
crystalline phases with square and hexagonal ordering generate solids that are
unable to rearrange after the passage of the solidification front and a
significant amount of disorder remains in the system.Comment: 18 pages, 14 fig
Thin film evolution equations from (evaporating) dewetting liquid layers to epitaxial growth
In the present contribution we review basic mathematical results for three
physical systems involving self-organising solid or liquid films at solid
surfaces. The films may undergo a structuring process by dewetting,
evaporation/condensation or epitaxial growth, respectively. We highlight
similarities and differences of the three systems based on the observation that
in certain limits all of them may be described using models of similar form,
i.e., time evolution equations for the film thickness profile. Those equations
represent gradient dynamics characterized by mobility functions and an
underlying energy functional.
Two basic steps of mathematical analysis are used to compare the different
system. First, we discuss the linear stability of homogeneous steady states,
i.e., flat films; and second the systematics of non-trivial steady states,
i.e., drop/hole states for dewetting films and quantum dot states in epitaxial
growth, respectively. Our aim is to illustrate that the underlying solution
structure might be very complex as in the case of epitaxial growth but can be
better understood when comparing to the much simpler results for the dewetting
liquid film. We furthermore show that the numerical continuation techniques
employed can shed some light on this structure in a more convenient way than
time-stepping methods.
Finally we discuss that the usage of the employed general formulation does
not only relate seemingly not related physical systems mathematically, but does
as well allow to discuss model extensions in a more unified way
Experimental observation of nanoscale radiative heat flow due to surface plasmons in graphene and doped silicon
Owing to its two dimensional electronic structure, graphene exhibits many
unique properties. One of them is a wave vector and temperature dependent
plasmon in the infrared range. Theory predicts that due to these plasmons,
graphene can be used as a universal material to enhance nanoscale radiative
heat exchange for any dielectric substrate. Here we report on radiative heat
transfer experiments between SiC and a SiO2 sphere which have non matching
phonon polariton frequencies, and thus only weakly exchange heat in near field.
We observed that the heat flux contribution of graphene epitaxially grown on
SiC dominates at short distances. The influence of plasmons on radiative heat
transfer is further supported with measurements for doped silicon. These
results highlight graphenes strong potential in photonic nearfield and energy
conversion devices.Comment: 4 pages, 3 figure
A lattice of microtraps for ultracold atoms based on patterned magnetic films
We have realized a two dimensional permanent magnetic lattice of
Ioffe-Pritchard microtraps for ultracold atoms. The lattice is formed by a
single 300 nm magnetized layer of FePt, patterned using optical lithography.
Our magnetic lattice consists of more than 15000 tightly confining microtraps
with a density of 1250 traps/mm. Simple analytical approximations for the
magnetic fields produced by the lattice are used to derive relevant trap
parameters. We load ultracold atoms into at least 30 lattice sites at a
distance of approximately 10 m from the film surface. The present result
is an important first step towards quantum information processing with neutral
atoms in magnetic lattice potentials.Comment: 7 pages, 7 figure
Damping by slow relaxing rare earth impurities in Ni80Fe20
Doping NiFe by heavy rare earth atoms alters the magnetic relaxation
properties of this material drastically. We show that this effect can be well
explained by the slow relaxing impurity mechanism. This process is a
consequence of the anisotropy of the on site exchange interaction between the
4f magnetic moments and the conduction band. As expected from this model the
magnitude of the damping effect scales with the anisotropy of the exchange
interaction and increases by an order of magnitude at low temperatures. In
addition our measurements allow us to determine the relaxation time of the 4f
electrons as a function of temperature
Demonstration of a state-insensitive, compensated nanofiber trap
We report the experimental realization of an optical trap that localizes single Cs atoms ≃ 215
nm from surface of a dielectric nanober. By operating at magic wavelengths for pairs of counterpropagating
red- and blue-detuned trapping beams, dierential scalar light shifts are eliminated, and
vector shifts are suppressed by ≈ 250. We thereby measure an absorption linewidth Γ/2π = 5.7 ± 0.1
MHz for the Cs 6S_(1/2), F = 4 → 6P_(3/2), F' = 5 transition, where Γ_0/2π = 5.2 MHz in free space.
Optical depth d ≃ 66 is observed, corresponding to an optical depth per atom d_1 ≃ 0.08. These
advances provide an important capability for the implementation of functional quantum optical
networks and precision atomic spectroscopy near dielectric surfaces
Modelling the evaporation of nanoparticle suspensions from heterogeneous surfaces
We present a Monte Carlo (MC) grid-based model for the drying of drops of a
nanoparticle suspension upon a heterogeneous surface. The model consists of a
generalised lattice-gas in which the interaction parameters in the Hamiltonian
can be varied to model different properties of the materials involved. We show
how to choose correctly the interactions, to minimise the effects of the
underlying grid so that hemispherical droplets form. We also include the
effects of surface roughness to examine the effects of contact-line pinning on
the dynamics. When there is a `lid' above the system, which prevents
evaporation, equilibrium drops form on the surface, which we use to determine
the contact angle and how it varies as the parameters of the model are changed.
This enables us to relate the interaction parameters to the materials used in
applications. The model has also been applied to drying on heterogeneous
surfaces, in particular to the case where the suspension is deposited on a
surface consisting of a pair of hydrophilic conducting metal surfaces that are
either side of a band of hydrophobic insulating polymer. This situation occurs
when using inkjet printing to manufacture electrical connections between the
metallic parts of the surface. The process is not always without problems,
since the liquid can dewet from the hydrophobic part of the surface, breaking
the bridge before the drying process is complete. The MC model reproduces the
observed dewetting, allowing the parameters to be varied so that the conditions
for the best connection can be established. We show that if the hydrophobic
portion of the surface is located at a step below the height of the
neighbouring metal, the chance of dewetting of the liquid during the drying
process is significantly reduced.Comment: 14 pages, 14 figure
Dewetting of thin films on heterogeneous substrates: Pinning vs. coarsening
We study a model for a thin liquid film dewetting from a periodic
heterogeneous substrate (template). The amplitude and periodicity of a striped
template heterogeneity necessary to obtain a stable periodic stripe pattern,
i.e. pinning, are computed. This requires a stabilization of the longitudinal
and transversal modes driving the typical coarsening dynamics during dewetting
of a thin film on a homogeneous substrate. If the heterogeneity has a larger
spatial period than the critical dewetting mode, weak heterogeneities are
sufficient for pinning. A large region of coexistence between coarsening
dynamics and pinning is found.Comment: 4 pages, 4 figure
Enhancement of laser-driven ion acceleration in non-periodic nanostructured targets
Using particle-in-cell simulations, we demonstrate an improvement of the
target normal sheath acceleration (TNSA) of protons in non-periodically
nanostructured targets with micron-scale thickness. Compared to standard flat
foils, an increase in the proton cutoff energy by up to a factor of two is
observed in foils coated with nanocones or perforated with nanoholes. The
latter nano-perforated foils yield the highest enhancement, which we show to be
robust over a broad range of foil thicknesses and hole diameters. The
improvement of TNSA performance results from more efficient hot-electron
generation, caused by a more complex laser-electron interaction geometry and
increased effective interaction area and duration. We show that TNSA is
optimized for a nanohole distribution of relatively low areal density and that
is not required to be periodic, thus relaxing the manufacturing constraints.Comment: 11 pages, 8 figure
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