2,542 research outputs found
Ground state and finite temperature signatures of quantum phase transitions in the half-filled Hubbard model on a honeycomb lattice
We investigate ground state and finite temperature properties of the
half-filled Hubbard model on a honeycomb lattice using quantum monte carlo and
series expansion techniques. Unlike the square lattice, for which magnetic
order exists at T=0 for any non-zero , the honeycomb lattice is known to
have a semi-metal phase at small and an antiferromagnetic one at large .
We investigate the phase transition at T=0 by studying the magnetic
structureU_c/tC(T)U>U_cU <
U_cUC(T)U \approx U_c$.Comment: 11 pages, 19 figure
Limits of the energy-momentum tensor in general relativity
A limiting diagram for the Segre classification of the energy-momentum tensor
is obtained and discussed in connection with a Penrose specialization diagram
for the Segre types. A generalization of the coordinate-free approach to limits
of Paiva et al. to include non-vacuum space-times is made. Geroch's work on
limits of space-times is also extended. The same argument also justifies part
of the procedure for classification of a given spacetime using Cartan scalars.Comment: LaTeX, 21 page
s-wave Superconductivity Phase Diagram in the Inhomogeneous Two-Dimensional Attractive Hubbard Model
We study s-wave superconductivity in the two-dimensional square lattice
attractive Hubbard Hamiltonian for various inhomogeneous patterns of
interacting sites. Using the Bogoliubov-de Gennes (BdG) mean field
approximation, we obtain the phase diagram for inhomogeneous patterns in which
the on-site attractive interaction U_i between the electrons takes on two
values, U_i=0 and -U/(1-f) (with f the concentration of non-interacting sites)
as a function of average electron occupation per site n, and study the
evolution of the phase diagram as f varies. In certain regions of the phase
diagram, inhomogeneity results in a larger zero temperature average pairing
amplitude (order parameter) and also a higher superconducting (SC) critical
temperature T_c, relative to a uniform system with the same mean interaction
strength (U_i=-U on all sites). These effects are observed for stripes,
checkerboard, and even random patterns of the attractive centers, suggesting
that the pattern of inhomogeneity is unimportant. The phase diagrams also
include regions where superconductivity is obliterated due to the formation of
various charge ordered phases. The enhancement of T_{c} due to inhomogeneity is
robust as long as the electron doping per site n is less than twice the
fraction of interacting sites [2(1-f)] regardless of the pattern. We also show
that for certain inhomogeneous patterns, when n = 2(1-f), increasing
temperature can work against the stability of existing charge ordered phases
for large f and as a result, enhance T_{c}.Comment: 16 pages, 11 figure
Effect of inhomogeneity on s-wave superconductivity in the attractive Hubbard model
Inhomogeneous s-wave superconductivity is studied in the two-dimensional,
square lattice attractive Hubbard Hamiltonian using the Bogoliubov-de Gennes
(BdG) mean field approximation. We find that at weak coupling, and for
densities mainly below half-filling, an inhomogeneous interaction in which the
on-site interaction takes on two values, results in a larger
zero temperature pairing amplitude, and that the superconducting can also
be significantly increased, relative to a uniform system with on all
sites. These effects are observed for stripe, checkerboard, and even random
patterns of the attractive centers, suggesting that the pattern of
inhomogeneity is unimportant. Monte Carlo calculations which reintroduce some
of the fluctuations neglected within the BdG approach see the same effect, both
for the attractive Hubbard model and a Hamiltonian with d-wave pairing
symmetry.Comment: 5 pages, 4 figure
A layering model for superconductivity in the borocarbides
We propose a superlattice model to describe superconductivity in layered
materials, such as the borocarbide families with the chemical formul\ae\
BC and BC, with being (essentially) a rare earth, and a
transition metal. We assume a single band in which electrons feel a local
attractive interaction (negative Hubbard-) on sites representing the B
layers, while U=0 on sites representing the C layers; the multi-band
structure is taken into account minimally through a band offset . The
one-dimensional model is studied numerically through the calculation of the
charge gap, the Drude weight, and of the pairing correlation function. A
comparison with the available information on the nature of the electronic
ground state (metallic or superconducting) indicates that the model provides a
systematic parametrization of the whole borocarbide family.Comment: 4 figure
The Schwarzschild-de Sitter solution in five-dimensional general relativity briefly revisited
We briefly revisit the Schwarzschild-de Sitter solution in the context of
five-dimensional general relativity. We obtain a class of five-dimensional
solutions of Einstein vacuum field equations into which the four-dimensional
Schwarzschild-de Sitter space can be locally and isometrically embedded. We
show that this class of solutions is well-behaved in the limit of lambda
approaching zero. Applying the same procedure to the de Sitter cosmological
model in five dimensions we obtain a class of embedding spaces which are
similarly well-behaved in this limit. These examples demonstrate that the
presence of a non-zero cosmological constant does not in general impose a rigid
relation between the (3+1) and (4+1)-dimensional spacetimes, with degenerate
limiting behaviour.Comment: 7 page
Variabilidade para teor de tanino em sorgo (Sorghum bicolor L.) e sua associação com a resistência a pássaros.
O objetivo do presente trabalho foi verificar a variabilidade existente entre diversos materiais geneticos quanto ao teor de tanino e resistencia a passaros nas fases de grao leitoso, grao pastoso, maturacao fisiologicas e de colheita. O experimento foi conduzido no delineamento blocos casualizados em tres repeticoes e semeadura em outubro/89 no Centro Nacional de Pesquisa de Milho e Sorgo (CNPMS/EMBRAPA) em Sete Lagoas, Estado de Minas Gerais. Os resultados mostraram maior teor de tanino na materia seca na fase de grao leitoso. No entanto, quando se considerou o conteudo de tanino na materia verde do grao, os resultados mostraram maior teor nas fases de maturacao fisiologicas ou de colheita para sorgo de alto tanino (valores maiores que 1%). O dano causado por passaro iniciou-se na fase de grao pastoso, sendo que todos os materiais geneticos resistentes apresentaram alto teor de tanino no grao na fase de colheita
Virtual Meson Cloud of the Nucleon and Intrinsic Strangeness and Charm
We have applied the Meson Cloud Model (MCM) to calculate the charm and
strange antiquark distribution in the nucleon. The resulting distribution, in
the case of charm, is very similar to the intrinsic charm momentum distribution
in the nucleon. This seems to corroborate the hypothesis that the intrinsic
charm is in the cloud and, at the same time, explains why other calculations
with the MCM involving strange quark distributions fail in reproducing the low
x region data. From the intrinsic strange distribution in the nucleon we have
extracted the strangeness radius of the nucleon, which is in agreement with
other meson cloud calculations.Comment: 9 pages RevTex, 4 figure
Taub's plane-symmetric vacuum spacetime revisited
The gravitational properties of the {\em only} static plane-symmetric vacuum
solution of Einstein's field equations without cosmological term (Taub's
solution, for brevity) are presented: some already known properties (geodesics,
weak field limit and pertainment to the Schwarzschild family of spacetimes) are
reviewed in a physically much more transparent way, as well as new results
about its asymptotic structure, possible matchings and nature of the source are
furnished. The main results point to the fact that the solution must be
interpreted as representing the exterior gravitational field due to a {\em
negative} mass distribution, confirming previous statements to that effect in
the literature. Some analogies to Kasner's spatially homogeneous cosmological
model are also referred to.Comment: plain LaTex, four Postscript figure
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