13,331 research outputs found
Micellar Crystals in Solution from Molecular Dynamics Simulations
Polymers with both soluble and insoluble blocks typically self-assemble into
micelles, aggregates of a finite number of polymers where the soluble blocks
shield the insoluble ones from contact with the solvent. Upon increasing
concentration, these micelles often form gels that exhibit crystalline order in
many systems. In this paper, we present a study of both the dynamics and the
equilibrium properties of micellar crystals of triblock polymers using
molecular dynamics simulations. Our results show that equilibration of single
micelle degrees of freedom and crystal formation occurs by polymer transfer
between micelles, a process that is described by transition state theory. Near
the disorder (or melting) transition, bcc lattices are favored for all
triblocks studied. Lattices with fcc ordering are also found, but only at lower
kinetic temperatures and for triblocks with short hydrophilic blocks. Our
results lead to a number of theoretical considerations and suggest a range of
implications to experimental systems with a particular emphasis on Pluronic
polymers.Comment: 12 pages, 11 figures. Note that some figures are extremely low
quality to meet arXiv's file size limit
Magnetic shape-memory effect in SrRuO
Like most perovskites, SrRuO exhibits structural phase transitions
associated with rotations of the RuO octahedra. The application of moderate
magnetic fields in the ferromagnetically ordered state allows one to fully
control these structural distortions, although the ferromagnetic order occurs
at six times lower temperature than the structural distortion. Our neutron
diffraction and macroscopic measurements unambiguously show that magnetic
fields rearrange structural domains, and that for the field along a cubic
[110] direction a fully detwinned crystal is obtained. Subsequent heating
above the Curie temperature causes a magnetic shape-memory effect, where the
initial structural domains recover
Radiation Generated by Charge Migration Following Ionization
Electronic many-body effects alone can be the driving force for an ultrafast
migration of a positive charge created upon ionization of molecular systems.
Here we show that this purely electronic phenomenon generates a characteristic
IR radiation. The situation when the initial ionic wave packet is produced by a
sudden removal of an electron is also studied. It is shown that in this case a
much stronger UV emission is generated. This emission appears as an ultrafast
response of the remaining electrons to the perturbation caused by the sudden
ionization and as such is a universal phenomenon to be expected in every
multielectron system.Comment: 5 pages, 4 figure
Nonequilibrium processes from Generalised Langevin Equations: realistic nanoscale systems connected to two thermal baths
We extend the Generalised Langevin Equation (GLE) method [Phys. Rev. B 89,
134303 (2014)] to model a central classical region connected to two realistic
thermal baths at two different temperatures. In such nonequilibrium conditions
a heat flow is established, via the central system, in between the two baths.
The GLE-2B (GLE two baths) scheme permits us to have a realistic description of
both the dissipative central system and its surrounding baths. Following the
original GLE approach, the extended Langevin dynamics scheme is modified to
take into account two sets of auxiliary degrees of freedom corresponding to the
mapping of the vibrational properties of each bath. These auxiliary variables
are then used to solve the non-Markovian dissipative dynamics of the central
region. The resulting algorithm is used to study a model of a short Al nanowire
connected to two baths. The results of the simulations using the GLE-2B
approach are compared to the results of other simulations that were carried out
using standard thermostatting approaches (based on Markovian Langevin and
Nose-Hoover thermostats). We concentrate on the steady state regime and study
the establishment of a local temperature profile within the system. The
conditions for obtaining a flat profile or a temperature gradient are examined
in detail, in agreement with earlier studies. The results show that the GLE-2B
approach is able to treat, within a single scheme, two widely different thermal
transport regimes, i.e. ballistic systems, with no temperature gradient, and
diffusive systems with a temperature gradient.Comment: present version accepted for publication in Phys. Rev. B (Apr 2016
Hyperextended Scalar-Tensor Gravity
We study a general Scalar-Tensor Theory with an arbitrary coupling funtion
but also an arbitrary dependence of the ``gravitational
constant'' in the cases in which either one of them, or both, do not
admit an analytical inverse, as in the hyperextended inflationary scenario. We
present the full set of field equations and study their cosmological behavior.
We show that different scalar-tensor theories can be grouped in classes with
the same solution for the scalar field.Comment: latex file, To appear in Physical Review
The Active Mirror Control of the MAGIC Telescope
One of the main design goals of the MAGIC telescopes is the very fast
repositioning in case of Gamma Ray Burst (GRB) alarms, implying a low weight of
the telescope dish. This is accomplished by using a space frame made of carbon
fiber epoxy tubes, resulting in a strong but not very rigid support structure.
Therefore it is necessary to readjust the individual mirror tiles to correct
for deformations of the dish under varying gravitational load while tracking an
object. We present the concept of the Active Mirror Control (AMC) as
implemented in the MAGIC telescopes and the actual performance reached.
Additionally we show that also telescopes using a stiff structure can benefit
from using an AMC.Comment: Contribution to the 30th ICRC, Merida, Mexico, July 2007 on behalf of
the MAGIC Collaboratio
Ricci Collineations of the Bianchi Type II, VIII, and IX Space-times
Ricci and contracted Ricci collineations of the Bianchi type II, VIII, and IX
space-times, associated with the vector fields of the form (i) one component of
is different from zero and (ii) two components of are
different from zero, for , are presented. In subcase (i.b), which
is , some known solutions are found, and in subcase
(i.d), which is , choosing ,
the Bianchi type II, VIII, and IX space-times is reduced to the
Robertson-Walker metric.Comment: 12 Pages, LaTeX, 1 Table, no figure
Some exact non-vacuum Bianchi VI0 and VII0 instantons
We report some new exact instantons in general relativity. These solutions
are K\"ahler and fall into the symmetry classes of Bianchi types VI0 and VII0,
with matter content of a stiff fluid. The qualitative behaviour of the
solutions is presented, and we compare it to the known results of the
corresponding self-dual Bianchi solutions. We also give axisymmetric Bianchi
VII0 solutions with an electromagnetic field.Comment: latex, 15 pages with 3 eps figure
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