24,971 research outputs found
Effective Sampling in the Configurational Space by the Multicanonical-Multioverlap Algorithm
We propose a new generalized-ensemble algorithm, which we refer to as the
multicanonical-multioverlap algorithm. By utilizing a non-Boltzmann weight
factor, this method realizes a random walk in the multi-dimensional,
energy-overlap space and explores widely in the configurational space including
specific configurations, where the overlap of a configuration with respect to a
reference state is a measure for structural similarity. We apply the
multicanonical-multioverlap molecular dynamics method to a penta peptide,
Met-enkephalin, in vacuum as a test system. We also apply the multicanonical
and multioverlap molecular dynamics methods to this system for the purpose of
comparisons. We see that the multicanonical-multioverlap molecular dynamics
method realizes effective sampling in the configurational space including
specific configurations more than the other two methods. From the results of
the multicanonical-multioverlap molecular dynamics simulation, furthermore, we
obtain a new local-minimum state of the Met-enkephalin system.Comment: 15 pages, (Revtex4), 9 figure
Entropy-based analysis of the number partitioning problem
In this paper we apply the multicanonical method of statistical physics on
the number-partitioning problem (NPP). This problem is a basic NP-hard problem
from computer science, and can be formulated as a spin-glass problem. We
compute the spectral degeneracy, which gives us information about the number of
solutions for a given cost and cardinality . We also study an extension
of this problem for partitions. We show that a fundamental difference on
the spectral degeneracy of the generalized () NPP exists, which could
explain why it is so difficult to find good solutions for this case. The
information obtained with the multicanonical method can be very useful on the
construction of new algorithms.Comment: 6 pages, 4 figure
The true reinforced random walk with bias
We consider a self-attracting random walk in dimension d=1, in presence of a
field of strength s, which biases the walker toward a target site. We focus on
the dynamic case (true reinforced random walk), where memory effects are
implemented at each time step, differently from the static case, where memory
effects are accounted for globally. We analyze in details the asymptotic
long-time behavior of the walker through the main statistical quantities (e.g.
distinct sites visited, end-to-end distance) and we discuss a possible mapping
between such dynamic self-attracting model and the trapping problem for a
simple random walk, in analogy with the static model. Moreover, we find that,
for any s>0, the random walk behavior switches to ballistic and that field
effects always prevail on memory effects without any singularity, already in
d=1; this is in contrast with the behavior observed in the static model.Comment: to appear on New J. Phy
Moduli Redefinitions and Moduli Stabilisation
Field redefinitions occur in string compactifications at the one loop level.
We review arguments for why such redefinitions occur and study their effect on
moduli stabilisation and supersymmetry breaking in the LARGE volume scenario.
For small moduli, although the effect of such redefinitions can be larger than
that of the corrections in both the K\"ahler and scalar potentials,
they do not alter the structure of the scalar potential. For the less well
motivated case of large moduli, the redefinitions can dominate all other terms
in the scalar potential. We also study the effect of redefinitions on the
structure of supersymmetry breaking and soft terms.Comment: 21 pages, 3 figures; v2. references adde
Monte Carlo simulation and global optimization without parameters
We propose a new ensemble for Monte Carlo simulations, in which each state is
assigned a statistical weight , where is the number of states with
smaller or equal energy. This ensemble has robust ergodicity properties and
gives significant weight to the ground state, making it effective for hard
optimization problems. It can be used to find free energies at all temperatures
and picks up aspects of critical behaviour (if present) without any parameter
tuning. We test it on the travelling salesperson problem, the Edwards-Anderson
spin glass and the triangular antiferromagnet.Comment: 10 pages with 3 Postscript figures, to appear in Phys. Rev. Lett
A Search for Very Low-mass Stars and Brown Dwarfs in the Young sigma Orionis Cluster
We present a CCD-based photometric survey covering 870 sq. arcmin in a young
stellar cluster around the young multiple star sigma Orionis. Our survey
limiting R, I, and Z magnitudes are 23.2, 21.8, and 21.0, respectively. From
our colour-magnitude diagrams, we have selected 49 faint objects, which
smoothly extrapolate the photometric sequence defined by more massive known
members. Adopting the currently accepted age interval of 2-10 Myr for the Orion
1b association and considering recent evolutionary models, our objects may span
a mass range from 0.1 down to 0.02 Msun, well within the substellar regime.
Follow-up low-resolution optical spectroscopy (635-920 nm) for eight of our
candidates (I=16-19.5) shows that they have spectral types M6-M8.5 which are
consistent with the expectations for true members. Compared with their Pleiades
counterparts of similar types, Halpha emission is generally stronger, while NaI
and KI absorption lines appear weaker, as expected for lower surface gravities
and younger ages. Additionally, TiO bands and in particular VO bands appear
clearly enhanced in our candidate with the latest spectral type, SOri 45 (M8.5,
I=19.5), compared to objects of similar types in older clusters and the field.
We have estimated the mass of this candidate at only 0.020-0.040 Msun, hence it
is one of the least massive brown dwarfs yet discovered. We also discuss in
this paper the potential role of deuterium as a tracer of both substellar
nature and age in very young clusters.Comment: Accepted for publication in ApJ Main Journal. 32 pages of text and
tables + 9 pages of figures. Figures 3a and 3b (gif format) provided
separatel
Gravitational diffraction radiation
We show that if the visible universe is a membrane embedded in a
higher-dimensional space, particles in uniform motion radiate gravitational
waves because of spacetime lumpiness. This phenomenon is analogous to the
electromagnetic diffraction radiation of a charge moving near to a metallic
grating. In the gravitational case, the role of the metallic grating is played
by the inhomogeneities of the extra-dimensional space, such as a hidden brane.
We derive a general formula for gravitational diffraction radiation and apply
it to a higher-dimensional scenario with flat compact extra dimensions.
Gravitational diffraction radiation may carry away a significant portion of the
particle's initial energy. This allows to set stringent limits on the scale of
brane perturbations. Physical effects of gravitational diffraction radiation
are briefly discussed.Comment: 5 pages, 2 figures, RevTeX4. v2: References added. Version to appear
in Phys. Rev.
Out of Equilibrium Solutions in the -Hamiltonian Mean Field model
Out of equilibrium magnetised solutions of the -Hamiltonian Mean Field
(-HMF) model are build using an ensemble of integrable uncoupled pendula.
Using these solutions we display an out-of equilibrium phase transition using a
specific reduced set of the magnetised solutions
Coherent Hydrodynamic Coupling for Stochastic Swimmers
A recently developed theory of stochastic swimming is used to study the
notion of coherence in active systems that couple via hydrodynamic
interactions. It is shown that correlations between various modes of
deformation in stochastic systems play the same role as the relative internal
phase in deterministic systems. An example is presented where a simple swimmer
can use these correlations to hunt a non-swimmer by forming a hydrodynamic
bound state of tunable velocity and equilibrium separation. These results
highlight the significance of coherence in the collective behavior of
nano-scale stochastic swimmers.Comment: 6 pages, 3 figure
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