1,983 research outputs found
Single particle spectrum of the flux phase in the FM Kondo Model
We investigate the 2D ferromagnetic Kondo lattice model for manganites with
classical corespins at Hund's rule coupling J_H=6, with antiferromagnetic
superexchange 0.03 < J' < 0.05. We employ canonical and grand canonical
unbiased Monte Carlo simulations and find paramagnetism, weak ferromagnetism
and the Flux phase, depending on doping and on J'. The observed single particle
spectrum in the flux phase differs from the idealized infinite lattice case,
but agrees well with an idealized finite lattice case with thermal
fluctuations.Comment: contribution to the SCES04 conferenc
Phase diagram and superconductivity of calcium borohyrides at extreme pressures
Motivated by the recent discovery of near-room temperature superconductivity in high-pressure superhydrides, we investigate from first principles the high-pressure superconducting phase diagram of the ternary Ca-B-H system, using ab initio evolutionary crystal structure prediction, and Density Functional Perturbation Theory. We find that below 100 GPa all stable and weakly metastable phases are insulating. This pressure marks the appearance of several new chemically-forbidden phases on the hull of stability, and the first onset of metalization in CaBH5. Metallization is then gradually achieved at higher pressure at different compositions. Among the metallic phases stable in the Megabar regime, we predict two high-Tc superconducting phases with CaBH6 and Ca2B2H13 compositions, with critical temperatures of 119 and 89 K at 300 GPa, respectively, surviving to lower pressures. Ternary hydrides will most likely play a major role in superconductivity research in the coming years; our study suggests that, in order to reduce the pressure for the onset of metallicity and superconductivity, further explorations of ternary hydrides should focus on elements less electronegative than boron
Photoemission spectra of many-polaron systems
The cross over from low to high carrier densities in a many-polaron system is
studied in the framework of the one-dimensional spinless Holstein model, using
unbiased numerical methods. Combining a novel quantum Monte Carlo approach and
exact diagonalization, accurate results for the single-particle spectrum and
the electronic kinetic energy on fairly large systems are obtained. A detailed
investigation of the quality of the Monte Carlo data is presented. In the
physically most important adiabatic intermediate electron-phonon coupling
regime, for which no analytical results are available, we observe a
dissociation of polarons with increasing band filling, leading to normal
metallic behavior, while for parameters favoring small polarons, no such
density-driven changes occur. The present work points towards the inadequacy of
single-polaron theories for a number of polaronic materials such as the
manganites.Comment: 15 pages, 13 figures; final version, accepted for publication in
Phys. Rev.
Cosmology and Astrophysics from Relaxed Galaxy Clusters I: Sample Selection
This is the first in a series of papers studying the astrophysics and
cosmology of massive, dynamically relaxed galaxy clusters. Here we present a
new, automated method for identifying relaxed clusters based on their
morphologies in X-ray imaging data. While broadly similar to others in the
literature, the morphological quantities that we measure are specifically
designed to provide a fair basis for comparison across a range of data quality
and cluster redshifts, to be robust against missing data due to point-source
masks and gaps between detectors, and to avoid strong assumptions about the
cosmological background and cluster masses. Based on three morphological
indicators - Symmetry, Peakiness and Alignment - we develop the SPA criterion
for relaxation. This analysis was applied to a large sample of cluster
observations from the Chandra and ROSAT archives. Of the 361 clusters which
received the SPA treatment, 57 (16 per cent) were subsequently found to be
relaxed according to our criterion. We compare our measurements to similar
estimators in the literature, as well as projected ellipticity and other image
measures, and comment on trends in the relaxed cluster fraction with redshift,
temperature, and survey selection method. Code implementing our morphological
analysis will be made available on the web.Comment: MNRAS, in press. 43 pages in total, of which 17 are tables (please
think twice before printing). 18 figures, 4 tables. Machine-readable tables
will be available from the journal and at the url below; code will be posted
at http://www.slac.stanford.edu/~amantz/work/morph14
Consistent Application of Maximum Entropy to Quantum-Monte-Carlo Data
Bayesian statistics in the frame of the maximum entropy concept has widely
been used for inferential problems, particularly, to infer dynamic properties
of strongly correlated fermion systems from Quantum-Monte-Carlo (QMC) imaginary
time data. In current applications, however, a consistent treatment of the
error-covariance of the QMC data is missing. Here we present a closed Bayesian
approach to account consistently for the QMC-data.Comment: 13 pages, RevTeX, 2 uuencoded PostScript figure
Charge ordering in extended Hubbard models: Variational cluster approach
We present a generalization of the recently proposed variational cluster
perturbation theory to extended Hubbard models at half filling with repulsive
nearest neighbor interaction. The method takes into account short-range
correlations correctly by the exact diagonalisation of clusters of finite size,
whereas long-range order beyond the size of the clusters is treated on a
mean-field level. For one dimension, we show that quantum Monte Carlo and
density-matrix renormalization-group results can be reproduced with very good
accuracy. Moreover we apply the method to the two-dimensional extended Hubbard
model on a square lattice. In contrast to the one-dimensional case, a first
order phase transition between spin density wave phase and charge density wave
phase is found as function of the nearest-neighbor interaction at onsite
interactions U>=3t. The single-particle spectral function is calculated for
both the one-dimensional and the two-dimensional system.Comment: 15 pages, 12 figure
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