24,286 research outputs found
Monte Carlo study of the two-dimensional site-diluted dipolar Ising model
By tempered Monte Carlo simulations, we study 2D site-diluted dipolar Ising
systems. Dipoles are randomly placed on a fraction x of all L^2 sites in a
square lattice, and point along a common crystalline axis. For x_c< x<=1, where
x_c = 0.79(5), we find an antiferromagnetic phase below a temperature which
vanishes as x approaches x_c from above. At lower values of x, we study (i)
distributions of the spin--glass (SG) overlap q, (ii) their relative mean
square deviation Delta_q^2 and kurtosis and (iii) xi_L/L, where xi_L is a SG
correlation length. From their variation with temperature and system size, we
find that the paramagnetic phase covers the entire T>0 range. Our results
enable us to obtain an estimate of the critical exponent associated to the
correlation length at T=0, 1/nu=0.35(10).Comment: 10 LaTeX pages, 10 figures, 1 table
Lepton flavor violating decays of vector mesons
We estimate the rates of lepton flavor violating decays of the vector mesons
. The theoretical tools are based on an effective
Lagrangian approach without referring to any specific realization of the
physics beyond the standard model responsible for lepton flavor violation
(\Lfv). The effective lepton-vector meson couplings are extracted from the
existing experimental bounds on the nuclear conversion. In
particular, we derive an upper limit for the \Lfv branching ratio which is much more stringent than
the recent experimental result
presented by the SND Collaboration. Very tiny limits on \Lfv decays of vector
mesons derived in this letter make direct experimental observation of these
processes unrealistic.Comment: 3 pages, 1 figure, accepted for publication in Phys. Rev.
New bounds on lepton flavor violating decays of vector mesons and the Z0 boson
We give an estimate for the upper bounds on rates of lepton flavor violating
(LFV) decays M to mu(pm) + e(mp) of vector mesons M = rho0, omega, phi, J/psi,
Upsilon and the Z0 boson in a model independent way, analyzing the
corresponding lowest dimension effective operators. These operators also
contribute to nuclear mu-e-conversion. Based on this observation and using the
existing experimental limits on this LFV nuclear process, we show that the
studied two-body LFV decays of vector bosons are strongly suppressed
independent on the explicit realization of new physics. The upper limits on the
rates of some of these decays are significantly more stringent than similar
limits known in the literature. In view of these results experimental
observation of the two-body LFV decays of vector bosons looks presently
unrealistic.Comment: 4 pages, 1 figur
Forster energy transfer signatures in optically driven quantum dot molecules
The Forster resonant energy transfer mechanism (FRET) is investigated in
optically driven and electrically gated tunnel coupled quantum dot molecules.
Two novel FRET induced optical signatures are found in the dressed excitonic
spectrum. This is constructed from exciton level occupation as function of pump
laser energy and applied bias, resembling a level anticrossing spectroscopy
measurement. We observe a redistribution of spectral weight and splitting of
the exciton spectral lines. FRET among single excitons induces a splitting in
the spatially-direct exciton lines, away from the anticrossing due to charge
tunneling in the molecule. However, near the anticrossing, a novel signature
appears as a weak satellite line following an indirect exciton line. FRET
signatures may also occur among indirect excitons, appearing as split indirect
lines. In that case, the signatures appear also in the direct biexciton states,
as the indirect satellite mixes in near the tunneling anticrossing region
On the nonexistence of conformally flat slices in the Kerr and other stationary spacetimes
It is proved that a stationary solutions to the vacuum Einstein field
equations with non-vanishing angular momentum have no Cauchy slice that is
maximal, conformally flat, and non-boosted. The proof is based on results
coming from a certain type of asymptotic expansions near null and spatial
infinity --which also show that the developments of Bowen-York type of data
cannot have a development admitting a smooth null infinity--, and from the fact
that stationary solutions do admit a smooth null infinity
A sample of relatively unstudied star clusters in the Large Magellanic Cloud: fundamental parameters determined from Washington photometry
To enlarge our growing sample of well-studied star clusters in the Large
Magellanic Cloud (LMC), we present CCD Washington CT1 photometry to T1 ~ 23 in
the fields of twenty-three mostly unstudied clusters located in the inner disc
and outer regions of the LMC. We estimated cluster radii from star counts.
Using the cluster Washington (T1,C-T1) colour-magnitude diagrams, statistically
cleaned from field star contamination, we derived cluster ages and
metallicities from a comparison with theoretical isochrones of the Padova
group. Whenever possible, we also derived ages using delta_T1 - the magnitude
difference between the red giant clump and the main sequence turn off - and
estimated metallicities from the standard giant branch procedure. We enlarged
our sample by adding clusters with published ages and metallicities determined
on a similar scale by applying the same methods. We examined relationships
between their positions in the LMC, ages and metallicities. We find that the
two methods for age and metallicity determination agree well with each other.
Fourteen clusters are found to be intermediate-age clusters (1-2 Gyr), with
[Fe/H] values ranging from -0.4 to -0.7. The remaining nine clusters turn out
to be younger than 1 Gyr, with metallicities between 0.0 and -0.4. Our 23
clusters represent an increase of ~ 30% in the current total amount number of
well-studied LMC clusters using Washington photometry. In agreement with
previous studies, we find no evidence for a metallicity gradient. We also find
that the younger clusters were formed closer to the LMC centre than the older
ones.Comment: 11 pages, 10 figures. A&A, in pres
Aging and intermittency in a p-spin model of a glass
We numerically analyze the statistics of the heat flow between an aging
system and its thermal bath, following a method proposed and tested for a
spin-glass model in a recent Letter (P. Sibani and H.J. Jensen, Europhys.
Lett.69, 563 (2005)). The present system, which lacks quenched randomness,
consists of Ising spins located on a cubic lattice, with each plaquette
contributing to the total energy the product of the four spins located at its
corners. Similarly to our previous findings, energy leaves the system in rare
but large, so called intermittent, bursts which are embedded in reversible and
equilibrium-like fluctuations of zero average. The intermittent bursts, or
quakes, dissipate the excess energy trapped in the initial state at a rate
which falls off with the inverse of the age. This strongly heterogeneous
dynamical picture is explained using the idea that quakes are triggered by
energy fluctuations of record size, which occur independently within a number
of thermalized domains. From the temperature dependence of the width of the
reversible heat fluctuations we surmise that these domains have an exponential
density of states. Finally, we show that the heat flow consists of a
temperature independent term and a term with an Arrhenius temperature
dependence. Microscopic dynamical and structural information can thus be
extracted from numerical intermittency data. This type of analysis seems now
within the reach of time resolved micro-calorimetry techniques.Comment: 9 pages, 6 figures, europhysics letter style, to appear in Physical
Review
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