8,516 research outputs found
Re-examining the directional-ordering transition in the compass model with screw-periodic boundary conditions
We study the directional-ordering transition in the two-dimensional classical
and quantum compass models on the square lattice by means of Monte Carlo
simulations. An improved algorithm is presented which builds on the Wolff
cluster algorithm in one-dimensional subspaces of the configuration space. This
improvement allows us to study classical systems up to . Based on the
new algorithm we give evidence for the presence of strongly anomalous scaling
for periodic boundary conditions which is much worse than anticipated before.
We propose and study alternative boundary conditions for the compass model
which do not make use of extended configuration spaces and show that they
completely remove the problem with finite-size scaling. In the last part, we
apply these boundary conditions to the quantum problem and present a
considerably improved estimate for the critical temperature which should be of
interest for future studies on the compass model. Our investigation identifies
a strong one-dimensional magnetic ordering tendency with a large correlation
length as the cause of the unusual scaling and moreover allows for a precise
quantification of the anomalous length scale involved.Comment: 10 pages, 8 figures; version as publishe
Ab initio prediction of equilibrium boron isotope fractionation between minerals and aqueous fluids at high P and T
Over the last decade experimental studies have shown a large B isotope
fractionation between materials carrying boron incorporated in trigonally and
tetrahedrally coordinated sites, but the mechanisms responsible for producing
the observed isotopic signatures are poorly known. In order to understand the
boron isotope fractionation processes and to obtain a better interpretation of
the experimental data and isotopic signatures observed in natural samples, we
use first principles calculations based on density functional theory in
conjunction with ab initio molecular dynamics and a new pseudofrequency
analysis method to investigate the B isotope fractionation between B-bearing
minerals (such as tourmaline and micas) and aqueous fluids containing H_3BO_3
and H_4BO_4- species. We confirm the experimental finding that the isotope
fractionation is mainly driven by the coordination of the fractionating boron
atoms and have found in addition that the strength of the produced isotopic
signature is strongly correlated with the B-O bond length. We also demonstrate
the ability of our computational scheme to predict the isotopic signatures of
fluids at extreme pressures by showing the consistency of computed
pressure-dependent beta factors with the measured pressure shifts of the B-O
vibrational frequencies of H_3BO_3 and H_4BO_4- in aqueous fluid. The
comparison of the predicted with measured fractionation factors between
boromuscovite and neutral fluid confirms the existence of the admixture of
tetrahedral boron species in neutral fluid at high P and T found
experimentally, which also explains the inconsistency between the various
measurements on the tourmaline-mica system reported in the literature. Our
investigation shows that the calculated equilibrium isotope fractionation
factors have an accuracy comparable to the experiments.Comment: 19 pages, 11 figures, Accepted for publication in Geochimica et
Cosmochimica Act
Holographic field theory models of dark energy in interaction with dark matter
We discuss two lagrangian interacting dark energy models in the context of
the holographic principle. The potentials of the interacting fields are
constructed. The models are compared with CMB distance information, baryonic
acoustic oscilations, lookback time and the Constitution supernovae sample. For
both models the results are consistent with a non vanishing interaction between
dark sectors - with more than three standard deviations of confidence for one
of them. Moreover, in both cases, the sign of coupling is consistent with dark
energy decaying into dark matter, alleviating the coincidence problem.Comment: arXiv admin note: substantial text overlap with arXiv:0912.399
CMB anisotropies from acausal scaling seeds
We investigate models where structure formation is initiated by scaling
seeds: We consider rapidly expanding relativistic shells of energy and show
that they can fit current CMB and large scale structure data if they expand
with super-luminal velocities. These acausally expanding shells provide a
viable alternative to inflation for cosmological structure formation with the
same minimal number of parameters to characterize the initial fluctuations.
Causally expanding shells alone cannot fit present data. Hybrid models where
causal shells and inflation are mixed also provide good fits.Comment: 9 pages,13 figures, revised version accepted for publication in PR
Metal-insulator transition and strong-coupling spin liquid in the Hubbard model
We study the phase diagram of the frustrated Hubbard model on
the square lattice by using a novel variational wave function. Taking the clue
from the backflow correlations that have been introduced long-time ago by
Feynman and Cohen and have been used for describing various interacting systems
on the continuum (like liquid He, the electron jellium, and metallic
Hydrogen), we consider many-body correlations to construct a suitable
approximation for the ground state of this correlated model on the lattice. In
this way, a very accurate {\it ansatz} can be achieved both at weak and strong
coupling. We present the evidence that an insulating and non-magnetic phase can
be stabilized at strong coupling and sufficiently large frustrating ratio
.Comment: 8 pages, Proceedings of the HFM2008 Conferenc
Metal-insulator transition and strong-coupling spin liquid in the Hubbard model
We study the phase diagram of the frustrated Hubbard model on
the square lattice by using a novel variational wave function. Taking the clue
from the backflow correlations that have been introduced long-time ago by
Feynman and Cohen and have been used for describing various interacting systems
on the continuum (like liquid He, the electron jellium, and metallic
Hydrogen), we consider many-body correlations to construct a suitable
approximation for the ground state of this correlated model on the lattice. In
this way, a very accurate {\it ansatz} can be achieved both at weak and strong
coupling. We present the evidence that an insulating and non-magnetic phase can
be stabilized at strong coupling and sufficiently large frustrating ratio
.Comment: 8 pages, Proceedings of the HFM2008 Conferenc
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