1,172 research outputs found
Cation composition effects on oxide conductivity in the Zr_2Y_2O_7-Y_3NbO_7 system
Realistic, first-principles-based interatomic potentials have been used in
molecular dynamics simulations to study the effect of cation composition on the
ionic conductivity in the Zr2Y2O7-Y3NbO7 system and to link the dynamical
properties to the degree of lattice disorder. Across the composition range,
this system retains a disordered fluorite crystal structure and the vacancy
concentration is constant. The observed trends of decreasing conductivity and
increasing disorder with increasing Nb5+ content were reproduced in simulations
with the cations randomly assigned to positions on the cation sublattice. The
trends were traced to the influences of the cation charges and relative sizes
and their effect on vacancy ordering by carrying out additional calculations in
which, for example, the charges of the cations were equalised. The simulations
did not, however, reproduce all the observed properties, particularly for
Y3NbO7. Its conductivity was significantly overestimated and prominent diffuse
scattering features observed in small area electron diffraction studies were
not always reproduced. Consideration of these deficiencies led to a preliminary
attempt to characterise the consequence of partially ordering the cations on
their lattice, which significantly affects the propensity for vacancy ordering.
The extent and consequences of cation ordering seem to be much less pronounced
on the Zr2Y2O7 side of the composition range.Comment: 22 pages, 8 figures, submitted to Journal of Physics: Condensed
Matte
L-Convex Polyominoes are Recognizable in Real Time by 2D Cellular Automata
A polyomino is said to be L-convex if any two of its cells are connected by a
4-connected inner path that changes direction at most once. The 2-dimensional
language representing such polyominoes has been recently proved to be
recognizable by tiling systems by S. Brocchi, A. Frosini, R. Pinzani and S.
Rinaldi. In an attempt to compare recognition power of tiling systems and
cellular automata, we have proved that this language can be recognized by
2-dimensional cellular automata working on the von Neumann neighborhood in real
time.
Although the construction uses a characterization of L-convex polyominoes
that is similar to the one used for tiling systems, the real time constraint
which has no equivalent in terms of tilings requires the use of techniques that
are specific to cellular automata
Spectral properties of quantum -body systems versus chaotic properties of their mean field approximations
We present numerical evidence that in a system of interacting bosons there
exists a correspondence between the spectral properties of the exact quantum
Hamiltonian and the dynamical chaos of the associated mean field evolution.
This correspondence, analogous to the usual quantum-classical correspondence,
is related to the formal parallel between the second quantization of the mean
field, which generates the exact dynamics of the quantum -body system, and
the first quantization of classical canonical coordinates. The limit of
infinite density and the thermodynamic limit are then briefly discussed.Comment: 15 pages RevTeX, 11 postscript figures included with psfig, uuencoded
gz-compressed .tar fil
TOWARDS FULLY AUTOMATED DIGITAL ALIBIS WITH SOCIAL INTERACTION
Digital traces found on local hard drives as a result of online activities have become very valuable in reconstructing events in digital forensic investigations. This paper demonstrates that forged alibis can be created for online activities and social interactions. In particular, a novel, automated framework is presented that uses social interactions to create false digital alibis. The framework simulates user activity and supports communications via email as well as instant messaging using a chatbot. The framework is evaluated by extracting forensic artifacts and comparing them with the results obtained from a human user study
Phase transition in the Jarzynski estimator of free energy differences
The transition between a regime in which thermodynamic relations apply only
to ensembles of small systems coupled to a large environment and a regime in
which they can be used to characterize individual macroscopic systems is
analyzed in terms of the change in behavior of the Jarzynski estimator of
equilibrium free energy differences from nonequilibrium work measurements.
Given a fixed number of measurements, the Jarzynski estimator is unbiased for
sufficiently small systems. In these systems, the directionality of time is
poorly defined and configurations that dominate the empirical average, but
which are in fact typical of the reverse process, are sufficiently well
sampled. As the system size increases the arrow of time becomes better defined.
The dominant atypical fluctuations become rare and eventually cannot be sampled
with the limited resources that are available. Asymptotically, only typical
work values are measured. The Jarzynski estimator becomes maximally biased and
approaches the exponential of minus the average work, which is the result that
is expected from standard macroscopic thermodynamics. In the proper scaling
limit, this regime change can be described in terms of a phase transition in
variants of the random energy model (REM). This correspondence is explicitly
demonstrated in several examples of physical interest: near-equilibrium
processes in which the work distribution is Gaussian, the sudden compression of
an ideal gas and adiabatic quasi-static volume changes in a dilute real gas.Comment: 29 pages, 5 figures, accepted for publication in Physical Review E
(2012
Immune control of HIV-1 infection after therapy interruption: immediate versus deferred antiretroviral therapy
Abstract Background The optimal stage for initiating antiretroviral therapies in HIV-1 bearing patients is still a matter of debate. Methods We present computer simulations of HIV-1 infection aimed at identifying the pro et contra of immediate as compared to deferred Highly Active Antiretroviral Therapy (HAART). Results Our simulations highlight that a prompt specific CD8+ cytotoxic T lymphocytes response is detected when therapy is delayed. Compared to very early initiation of HAART, in deferred treated patients CD8+ T cells manage to mediate the decline of viremia in a shorter time and, at interruption of therapy, the virus experiences a stronger immune pressure. We also observe, however, that the immunological effects of the therapy fade with time in both therapeutic regimens. Thus, within one year from discontinuation, viral burden recovers to the value at which it would level off in the absence of therapy. In summary, simulations show that immediate therapy does not prolong the disease-free period and does not confer a survival benefit when compared to treatment started during the chronic infection phase. Conclusion Our conclusion is that, since there is no therapy to date that guarantees life-long protection, deferral of therapy should be preferred in order to minimize the risk of adverse effects, the occurrence of drug resistances and the costs of treatment.</p
Chaos in effective classical and quantum dynamics
We investigate the dynamics of classical and quantum N-component phi^4
oscillators in the presence of an external field. In the large N limit the
effective dynamics is described by two-degree-of-freedom classical Hamiltonian
systems. In the classical model we observe chaotic orbits for any value of the
external field, while in the quantum case chaos is strongly suppressed. A
simple explanation of this behaviour is found in the change in the structure of
the orbits induced by quantum corrections. Consistently with Heisenberg's
principle, quantum fluctuations are forced away from zero, removing in the
effective quantum dynamics a hyperbolic fixed point that is a major source of
chaos in the classical model.Comment: 6 pages, RevTeX, 5 figures, uses psfig, changed indroduction and
conclusions, added reference
Novel Results on the Number of Runs of the Burrows-Wheeler-Transform
The Burrows-Wheeler-Transform (BWT), a reversible string transformation, is
one of the fundamental components of many current data structures in string
processing. It is central in data compression, as well as in efficient query
algorithms for sequence data, such as webpages, genomic and other biological
sequences, or indeed any textual data. The BWT lends itself well to compression
because its number of equal-letter-runs (usually referred to as ) is often
considerably lower than that of the original string; in particular, it is well
suited for strings with many repeated factors. In fact, much attention has been
paid to the parameter as measure of repetitiveness, especially to evaluate
the performance in terms of both space and time of compressed indexing data
structures.
In this paper, we investigate , the ratio of and of the number
of runs of the BWT of the reverse of . Kempa and Kociumaka [FOCS 2020] gave
the first non-trivial upper bound as , for any string
of length . However, nothing is known about the tightness of this upper
bound. We present infinite families of binary strings for which holds, thus giving the first non-trivial lower bound on
, the maximum over all strings of length .
Our results suggest that is not an ideal measure of the repetitiveness of
the string, since the number of repeated factors is invariant between the
string and its reverse. We believe that there is a more intricate relationship
between the number of runs of the BWT and the string's combinatorial
properties.Comment: 14 pages, 2 figue
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