20,161 research outputs found
Charge carrier correlation in the electron-doped t-J model
We study the t-t'-t''-J model with parameters chosen to model an
electron-doped high temperature superconductor. The model with one, two and
four charge carriers is solved on a 32-site lattice using exact
diagonalization. Our results demonstrate that at doping levels up to x=0.125
the model possesses robust antiferromagnetic correlation. When doped with one
charge carrier, the ground state has momenta (\pm\pi,0) and (0,\pm\pi). On
further doping, charge carriers are unbound and the momentum distribution
function can be constructed from that of the single-carrier ground state. The
Fermi surface resembles that of small pockets at single charge carrier ground
state momenta, which is the expected result in a lightly doped antiferromagnet.
This feature persists upon doping up to the largest doping level we achieved.
We therefore do not observe the Fermi surface changing shape at doping levels
up to 0.125
Adaptive FE-BE coupling for strongly nonlinear transmission problems with friction II
This article discusses the well-posedness and error analysis of the coupling
of finite and boundary elements for transmission or contact problems in
nonlinear elasticity. It concerns W^{1,p}-monotone Hencky materials with an
unbounded stress-strain relation, as they arise in the modelling of ice sheets,
non-Newtonian fluids or porous media. For 1<p<2 the bilinear form of the
boundary element method fails to be continuous in natural function spaces
associated to the nonlinear operator. We propose a functional analytic
framework for the numerical analysis and obtain a priori and a posteriori error
estimates for Galerkin approximations to the resulting boundary/domain
variational inequality. The a posteriori estimate complements recent estimates
obtained for mixed finite element formulations of friction problems in linear
elasticity.Comment: 20 pages, corrected typos and improved expositio
Description of GADEL
This article describes the first implementation of the GADEL system : a
Genetic Algorithm for Default Logic. The goal of GADEL is to compute extensions
in Reiter's default logic. It accepts every kind of finite propositional
default theories and is based on evolutionary principles of Genetic Algorithms.
Its first experimental results on certain instances of the problem show that
this new approach of the problem can be successful.Comment: System Descriptions and Demonstrations at Nonmonotonic Reasoning
Workshop, 2000 6 pages, 2 figures, 5 table
Hole correlation and antiferromagnetic order in the t-J model
We study the t-J model with four holes on a 32-site square lattice using
exact diagonalization. This system corresponds to doping level x=1/8. At the
``realistic'' parameter J/t=0.3, holes in the ground state of this system are
unbound. They have short range repulsion due to lowering of kinetic energy.
There is no antiferromagnetic spin order and the electron momentum distribution
function resembles hole pockets. Furthermore, we show evidence that in case
antiferromagnetic order exists, holes form d-wave bound pairs and there is
mutual repulsion among hole pairs. This presumably will occur at low doping
level. This scenario is compatible with a checkerboard-type charge density
state proposed to explain the ``1/8 anomaly'' in the LSCO family, except that
it is the ground state only when the system possesses strong antiferromagnetic
order
Complexity of the General Chromatic Art Gallery Problem
In the original Art Gallery Problem (AGP), one seeks the minimum number of
guards required to cover a polygon . We consider the Chromatic AGP (CAGP),
where the guards are colored. As long as is completely covered, the number
of guards does not matter, but guards with overlapping visibility regions must
have different colors. This problem has applications in landmark-based mobile
robot navigation: Guards are landmarks, which have to be distinguishable (hence
the colors), and are used to encode motion primitives, \eg, "move towards the
red landmark". Let , the chromatic number of , denote the minimum
number of colors required to color any guard cover of . We show that
determining, whether is \NP-hard for all . Keeping
the number of colors minimal is of great interest for robot navigation, because
less types of landmarks lead to cheaper and more reliable recognition
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