65,881 research outputs found
Solving the puzzle of an unconventional phase transition for a 2d dimerized quantum Heisenberg model
Motivated by the indication of a new critical theory for the spin-1/2
Heisenberg model with a spatially staggered anisotropy on the square lattice as
suggested in \cite{Wenzel08}, we re-investigate the phase transition of this
model induced by dimerization using first principle Monte Carlo simulations. We
focus on studying the finite-size scaling of and ,
where stands for the spatial box size used in the simulations and
with is the spin-stiffness in the -direction.
Remarkably, while we do observe a large correction to scaling for the
observable as proposed in \cite{Fritz11}, the data for
exhibit a good scaling behavior without any indication of a large
correction. As a consequence, we are able to obtain a numerical value for the
critical exponent which is consistent with the known O(3) result with
moderate computational effort. Specifically, the numerical value of we
determine by fitting the data points of to their expected scaling
form is given by , which agrees quantitatively with the most
accurate known Monte Carlo O(3) result . Finally, while we can
also obtain a result of from the observable second Binder ratio
which is consistent with , the uncertainty of calculated
from is more than twice as large as that of determined from
.Comment: 7 figures, 1 table; brief repor
Magnetization Losses in Multifilament Coated Superconductors
We report the results of a study of the magnetization losses in experimental
multifilament, as well as control (uniform), coated superconductors exposed to
time-varying magnetic field of various frequencies. Both the hysteresis loss,
proportional to the sweep rate of the applied magnetic field, and the coupling
loss, proportional to the square of the sweep rate, have been observed. A
scaling is found that allows us to quantify each of these contributions and
extrapolate the results of the experiment beyond the envelope of accessible
field amplitude and frequency. The combined loss in the multifilament conductor
is reduced by about 90% in comparison with the uniform conductor at full field
penetration at sweep rate as high as 3T/s
Modelling total solar irradiance since 1878 from simulated magnetograms
We present a new model of total solar irradiance (TSI) based on magnetograms
simulated with a surface flux transport model (SFTM) and the SATIRE (Spectral
And Total Irradiance REconstructions) model. Our model provides daily maps of
the distribution of the photospheric field and the TSI starting from 1878. We
first calculate the magnetic flux on the solar surface emerging in active and
ephemeral regions. The evolution of the magnetic flux in active regions is
computed using a surface flux transport model fed with the observed record of
sunspot group areas and positions. The magnetic flux in ephemeral regions is
treated separately using the concept of overlapping cycles. To model the
ephemeral region cycles, we assume that their length and amplitude are related
to that of the sunspot cycles. We then use a version of the SATIRE model to
compute the TSI. The area coverage and the distribution of different magnetic
features as a function of time, which are required by SATIRE, are extracted
from the simulated magnetograms and the modelled ephemeral region magnetic
flux. Previously computed intensity spectra of the various types of magnetic
features are employed. Our model reproduces the PMOD composite of TSI
measurements starting from 1978 at daily and rotational timescales more
accurately than the previous version of the SATIRE model computing TSI over
this period of time. The simulated magnetograms provide a more realistic
representation of the evolution of the magnetic field on the photosphere and
also allow us to make use of information on the spatial distribution of the
magnetic fields before the times when observed magnetograms were available. We
find that the secular increase in TSI since 1878 is fairly stable to
modifications of the treatment of the ephemeral region magnetic flux
Statistical significance of rich-club phenomena in complex networks
We propose that the rich-club phenomena in complex networks should be defined
in the spirit of bootstrapping, in which a null model is adopted to assess the
statistical significance of the rich-club detected. Our method can be served as
a definition of rich-club phenomenon and is applied to analyzing three real
networks and three model networks. The results improve significantly compared
with previously reported results. We report a dilemma with an exceptional
example, showing that there does not exist an omnipotent definition for the
rich-club phenomenon.Comment: 3 Revtex pages + 5 figure
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