611 research outputs found
Metallurgy of soft spheres with hard core: from BCC to Frank-Kasper phases
Understanding how soft particles can fill the space is still an open
question. Structures far from classical FCC or BCC phases are now commonly
experimentally observed in many different systems. Models based on pair
interaction between soft particle are at present much studied in 2D. Pair
interaction with two different lengths have been shown to lead to
quasicrystalline architectures. It is also the case for a hard core with a
square repulsive shoulder potential. In 3D, global approaches have been
proposed for instance by minimizing the interface area between the deformed
objects in the case of foams or micellar systems or using self-consistent mean
field theory in copolymer melts. In this paper we propose to compare a strong
van der Waals attraction between spherical hard cores and an elastic energy
associated to the deformation of the soft corona. This deformation is measured
as the shift between the deformed shell compared to a corona with a perfect
spherical symmetry. The two main parameters in this model are: the hard core
volume fraction and the weight of the elastic energy compared to the van der
Waals one. The elastic energy clearly favours the BCC structure but large van
der Waals forces favors Frank and Kasper phases. This result opens a route
towards controlling the building of nanoparticle superlattices with complex
structures and thus original physical properties.Comment: To appear in EPJ
Geometrical approach to SU(2) navigation with Fibonacci anyons
Topological quantum computation with Fibonacci anyons relies on the
possibility of efficiently generating unitary transformations upon
pseudoparticles braiding. The crucial fact that such set of braids has a dense
image in the unitary operations space is well known; in addition, the
Solovay-Kitaev algorithm allows to approach a given unitary operation to any
desired accuracy. In this paper, the latter task is fulfilled with an
alternative method, in the SU(2) case, based on a generalization of the
geodesic dome construction to higher dimension.Comment: 12 pages, 5 figure
Forever Young: Aging Control For Smartphones In Hybrid Networks
The demand for Internet services that require frequent updates through small
messages, such as microblogging, has tremendously grown in the past few years.
Although the use of such applications by domestic users is usually free, their
access from mobile devices is subject to fees and consumes energy from limited
batteries. If a user activates his mobile device and is in range of a service
provider, a content update is received at the expense of monetary and energy
costs. Thus, users face a tradeoff between such costs and their messages aging.
The goal of this paper is to show how to cope with such a tradeoff, by devising
\emph{aging control policies}. An aging control policy consists of deciding,
based on the current utility of the last message received, whether to activate
the mobile device, and if so, which technology to use (WiFi or 3G). We present
a model that yields the optimal aging control policy. Our model is based on a
Markov Decision Process in which states correspond to message ages. Using our
model, we show the existence of an optimal strategy in the class of threshold
strategies, wherein users activate their mobile devices if the age of their
messages surpasses a given threshold and remain inactive otherwise. We then
consider strategic content providers (publishers) that offer \emph{bonus
packages} to users, so as to incent them to download updates of advertisement
campaigns. We provide simple algorithms for publishers to determine optimal
bonus levels, leveraging the fact that users adopt their optimal aging control
strategies. The accuracy of our model is validated against traces from the
UMass DieselNet bus network.Comment: See also http://www-net.cs.umass.edu/~sadoc/agecontrol
Signature of nearly icosahedral structures in liquid and supercooled liquid Copper
A growing body of experiments display indirect evidence of icosahedral
structures in supercooled liquid metals. Computer simulations provide more
direct evidence but generally rely on approximate interatomic potentials of
unproven accuracy. We use first-principles molecular dynamics simulations to
generate realistic atomic configurations, providing structural detail not
directly available from experiment, based on interatomic forces that are more
reliable than conventional simulations. We analyze liquid copper, for which
recent experimental results are available for comparison, to quantify the
degree of local icosahedral and polytetrahedral order
Large increase of the Curie temperature by orbital ordering control
Using first principle calculations we showed that the Curie temperature of
manganites thin films can be increased by far more than an order of magnitude
by applying appropriate strains. Our main breakthrough is that the control of
the orbital ordering responsible for the spectacular increase cannot be
imposed by the substrate only. Indeed, the strains, first applied by the
substrate, need to be maintained over the growth direction by the alternation
of the manganite layers with another appropriate material. Following these
theoretical findings, we synthesized such super-lattices and verified our
theoretical predictions
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