32,215 research outputs found
In-flight dissipation as a mechanism to suppress Fermi acceleration
Some dynamical properties of time-dependent driven elliptical-shaped billiard
are studied. It was shown that for the conservative time-dependent dynamics the
model exhibits the Fermi acceleration [Phys. Rev. Lett. 100, 014103 (2008)]. On
the other hand, it was observed that damping coefficients upon collisions
suppress such phenomenon [Phys. Rev. Lett. 104, 224101 (2010)]. Here, we
consider a dissipative model under the presence of in-flight dissipation due to
a drag force which is assumed to be proportional to the square of the
particle's velocity. Our results reinforce that dissipation leads to a phase
transition from unlimited to limited energy growth. The behaviour of the
average velocity is described using scaling arguments.Comment: 4 pages, 5 figure
Computational Difficulty of Computing the Density of States
We study the computational difficulty of computing the ground state
degeneracy and the density of states for local Hamiltonians. We show that the
difficulty of both problems is exactly captured by a class which we call #BQP,
which is the counting version of the quantum complexity class QMA. We show that
#BQP is not harder than its classical counting counterpart #P, which in turn
implies that computing the ground state degeneracy or the density of states for
classical Hamiltonians is just as hard as it is for quantum Hamiltonians.Comment: v2: Accepted version. 9 pages, 1 figur
Shadowing by non uniformly hyperbolic periodic points and uniform hyperbolicity
We prove that, under a mild condition on the hyperbolicity of its periodic
points, a map which is topologically conjugated to a hyperbolic map
(respectively, an expanding map) is also a hyperbolic map (respectively, an
expanding map). In particular, this result gives a partial positive answer for
a question done by A. Katok, in a related context
Cool stars in NGC 2547 and pre main sequence lithium depletion
We present the results of a spectroscopic survey of X-ray selected, low-mass
candidate members of the young open cluster NGC 2547. Using a combination of
photometry, spectroscopic indices and radial velocities we refine our candidate
list and then use our spectroscopy to study the progression of lithium
depletion in low-mass pre main sequence stars. We derive lithium abundances or
upper limits for all our candidate members, which have effective temperatures
in the range 5000>Teff>3200K, and compare these with predictions for lithium
burning and depletion provided by a number of models and also with the lithium
depletion seen in younger and older stars. We find that some models can
reproduce the lithium abundance pattern of NGC 2547 if the cluster has an age
of ~20-35Myr, which is also indicated by fits to low-mass isochrones in the
Hertzsprung-Russell diagram. But the lack of significant further lithium
depletion between NGC 2547 and older clusters argues for an age of at least
50Myr, more in keeping with the lack of lithium observed in even fainter NGC
2547 candidates. We show that reconciliation of these age estimates may require
additions to the physics incorporated in current generations of pre main
sequence models.Comment: Accepted for publication in MNRAS (better version of Fig.1 available
at http://www.astro.keele.ac.uk/~rdj/
Symmetry, bifurcation and stacking of the central configurations of the planar 1+4 body problem
In this work we are interested in the central configurations of the planar
1+4 body problem where the satellites have different infinitesimal masses and
two of them are diametrically opposite in a circle. We can think this problem
as a stacked central configuration too. We show that the configuration are
necessarily symmetric and the other sattelites has the same mass. Moreover we
proved that the number of central configuration in this case is in general one,
two or three and in the special case where the satellites diametrically
opposite have the same mass we proved that the number of central configuration
is one or two saying the exact value of the ratio of the masses that provides
this bifurcation.Comment: 9 pages, 2 figures. arXiv admin note: text overlap with
arXiv:1103.627
Self-avoiding fractional Brownian motion - The Edwards model
In this work we extend Varadhan's construction of the Edwards polymer model
to the case of fractional Brownian motions in , for any dimension , with arbitrary Hurst parameters .Comment: 14 page
The Cosmological Evolution of Domain Wall Networks
We have studied the cosmological evolution of domain wall networks in two,
three and four spatial dimensions using high-resolution field theory
simulations. The dynamical range and number of our simulations is larger than
in previous works, but does not allow us to exclude previous hints of
deviations to the naively expected scale-invariant evolution. These results
therefore suggest that the approach of domain wall networks to linear scaling
is a much slower process than that of cosmic strings, which has been previously
characterized in detail.Comment: 7 pages, submitted to Phys Rev
Universal 2-local Hamiltonian Quantum Computing
We present a Hamiltonian quantum computation scheme universal for quantum
computation (BQP). Our Hamiltonian is a sum of a polynomial number (in the
number of gates L in the quantum circuit) of time-independent, constant-norm,
2-local qubit-qubit interaction terms. Furthermore, each qubit in the system
interacts only with a constant number of other qubits. The computer runs in
three steps - starts in a simple initial product-state, evolves it for time of
order L^2 (up to logarithmic factors) and wraps up with a two-qubit
measurement. Our model differs from the previous universal 2-local Hamiltonian
constructions in that it does not use perturbation gadgets, does not need large
energy penalties in the Hamiltonian and does not need to run slowly to ensure
adiabatic evolution.Comment: recomputed the necessary number of interactions, new geometric
layout, added reference
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