261 research outputs found
Pattern formation in the dipolar Ising model on a two-dimensional honeycomb lattice
We present Monte Carlo simulation results for a two-dimensional Ising model
with ferromagnetic nearest-neighbor couplings and a competing long-range
dipolar interaction on a honeycomb lattice. Both structural and thermodynamic
properties are very similar to the case of a square lattice, with the exception
that structures reflect the sixfold rotational symmetry of the underlying
honeycomb lattice. To deal with the long-range nature of the dipolar
interaction we also present a simple method of evaluating effective interaction
coefficients, which can be regarded as a more straightforward alternative to
the prevalent Ewald summation techniques.Comment: 5 pages, 5 figure
Effect of dopant atoms on local superexchange in cuprate superconductors: a perturbative treatment
Recent scanning tunneling spectroscopy experiments have provided evidence
that dopant impurities in high- Tc superconductors can strongly modify the
electronic structure of the CuO2 planes nearby, and possibly influence the
pairing. To investigate this connection, we calculate the local magnetic
superexchange J between Cu ions in the presence of dopants within the framework
of the three-band Hubbard model, up to fifth-order in perturbation theory. We
demonstrate that the sign of the change in J depends on the relative
dopant-induced spatial variation of the atomic levels in the CuO2 plane,
contrary to results obtained within the one-band Hubbard model. We discuss some
realistic cases and their relevance for theories of the pairing mechanism in
the cupratesComment: 5 pages, 4 figures, revised versio
Correlation effects on the electronic structure of TiOCl: a NMTO+DMFT study
Using the recently developed N-th order muffin-tin orbital-based downfolding
technique in combination with the Dynamical Mean Field theory, we investigate
the electronic properties of the much discussed Mott insulator TiOCl in the
undimerized phase. Inclusion of correlation effects through this approach
provides a description of the spectral function into an upper and a lower
Hubbard band with broad valence states formed out of the orbitally polarized,
lower Hubbard band. We find that these results are in good agreement with
recent photo-emission spectra.Comment: 4 pages, 3 figure
High-energy emission from jet-cloud interactions in AGNs
Active galactic nuclei present continuum and line emission. The emission
lines are originated by gas located close to the central super-massive black
hole. Some of these lines are broad, and would be produced in a small region
called broad-line region. This region could be formed by clouds surrounding the
central black hole. In this work, we study the interaction of such clouds with
the base of the jets in active galactic nuclei, and we compute the produced
high-energy emission. We focus on sources with low luminosities in the inner
jet regions, to avoid strong gamma-ray absorption. We find that the resulting
high-energy radiation may be significant in Centaurus A. Also, this phenomenon
might be behind the variable gamma-ray emission detected in M87, if very large
dark clouds are present. The detection of jet-cloud interactions in active
galactic nuclei would give information on the properties of the jet base and
the very central regions.Comment: 6 pages, 3 figures. To appear in the proceeding of the conference:
"High Energy Phenomena in Relativistic Outflows (HEPRO) II", held in Buenos
Aires, Argentina, October 26-30 2009
Electronic properties of Fabre charge-transfer salts under various temperature and pressure conditions
Using density functional theory, we determine parameters of tight-binding
Hamiltonians for a variety of Fabre charge transfer salts, focusing in
particular on the effects of temperature and pressure. Besides relying on
previously published crystal structures, we experimentally determine two new
sets of structures; (TMTTF)SbF at different temperatures and
(TMTTF)PF at various pressures. We find that a few trends in the
electronic behavior can be connected to the complex phase diagram shown by
these materials. Decreasing temperature and increasing pressure cause the
systems to become more two-dimensional. We analyze the importance of
correlations by considering an extended Hubbard model parameterized using
Wannier orbital overlaps and show that while charge order is strongly activated
by the inter-site Coulomb interaction, the magnetic order is only weakly
enhanced. Both orders are suppressed when the effective pressure is increased.Comment: 12 pages, 16 figure
Sermón de la aparición de Nuestra Señora de la Saleta : predicado el 21 de octubre de 1894 en ... Ponferrada, para inaugurar ... la imagen y el culto de la Excelsa Aparecida
Copia digital. Valladolid : Junta de Castilla y LeĂłn. ConsejerĂa de Cultura y Turismo, 2010-201
Cluster Dynamical Mean-field calculations for TiOCl
Based on a combination of cluster dynamical mean field theory (DMFT) and
density functional calculations, we calculated the angle-integrated spectral
density in the layered quantum magnet TiOCl. The agreement with recent
photoemission and oxygen K-edge X-ray absorption spectroscopy experiments is
found to be good. Th e improvement achieved with this calculation with respect
to previous single-site DMFT calculations is an indication of the correlated
nature and low-dimensionality of TiOCl.Comment: 9 pages, 3 figures, improved version as publishe
Spin Hall conductivity in the Kane-Mele-Hubbard model at finite temperature
The Kane-Mele model is known to show a quantized spin Hall conductivity at
zero temperature. Including Hubbard interactions at each site leads to a
quantum phase transition to an XY antiferromagnet at sufficiently high
interaction strength. Here, we use the two-particle self-consistent approach
(TPSC), which we extend to include spin-orbit coupling, to investigate the
Kane-Mele-Hubbard model at finite temperature and half-filling. TPSC is a weak
to intermediate coupling approach capable of calculating a frequency- and
momentum-dependent self-energy from spin and charge fluctuations. We present
results for the spin Hall conductivity and correlation lengths for
antiferromagnetic spin fluctuations for different values of temperature,
spin-orbit coupling and Hubbard interaction. The vertex corrections, which here
are analogues of Maki-Thompson contributions, show a strong momentum dependence
and give a large contribution in the vicinity of the phase transition at all
temperatures. Their inclusion is necessary to observe the quantization of the
spin Hall conductivity for the interacting system in the zero temperature
limit. At finite temperature, increasing the Hubbard interaction leads to a
decrease of the spin Hall conductivity. This decrease can be explained by
band-gap renormalization from scattering of electrons on antiferromagnetic spin
fluctuations.Comment: 11 pages, 8 figure
Multiferroic FeTeOBr: Alternating spin chains with frustrated interchain interactions
A combination of density functional theory calculations, many-body model
considerations, magnetization and electron spin resonance measurements shows
that the multiferroic FeTeOBr should be described as a system of
alternating antiferromagnetic chains with strong Fe-O-Te-O-Fe bridges
weakly coupled by two-dimensional frustrated interactions, rather than the
previously reported tetramer models. The peculiar temperature dependence of the
incommensurate magnetic vector can be explained in terms of interchain exchange
striction being responsible for the emergent net electric polarization.Comment: 7 pages, 6 figure
Local moments and symmetry breaking in metallic PrMnSbO
We report a combined experimental and theoretical investigation of the
layered antimonide PrMnSbO which is isostructural to the parent phase of the
iron pnictide superconductors. We find linear resistivity near room temperature
and Fermi liquid-like T^{2} behaviour below 150 K. Neutron powder diffraction
shows that unfrustrated C-type Mn magnetic order develops below \sim 230 K,
followed by a spin-flop coupled to induced Pr order. At T \sim 35 K, we find a
tetragonal to orthorhombic (T-O) transition. First principles calculations show
that the large magnetic moments observed in this metallic compound are of local
origin. Our results are thus inconsistent with either the itinerant or
frustrated models proposed for symmetry breaking in the iron pnictides. We show
that PrMnSbO is instead a rare example of a metal where structural distortions
are driven by f-electron degrees of freedom
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