3,380 research outputs found
Stability of Ferromagnetism in Hubbard models with degenerate single-particle ground states
A Hubbard model with a N_d-fold degenerate single-particle ground state has
ferromagnetic ground states if the number of electrons is less or equal to N_d.
It is shown rigorously that the local stability of ferromagnetism in such a
model implies global stability: The model has only ferromagnetic ground states,
if there are no single spin-flip ground states. If the number of electrons is
equal to N_d, it is well known that the ferromagnetic ground state is unique if
and only if the single-particle density matrix is irreducible. We present a
simplified proof for this result.Comment: accepted for publication in J. Phys.
Bose-Hubbard model on two-dimensional line graphs
We construct a basis for the many-particle ground states of the positive
hopping Bose-Hubbard model on line graphs of finite 2-connected planar
bipartite graphs at sufficiently low filling factors. The particles in these
states are localized on non-intersecting vertex-disjoint cycles of the line
graph which correspond to non-intersecting edge-disjoint cycles of the original
graph. The construction works up to a critical filling factor at which the
cycles are close-packed.Comment: 9 pages, 5 figures, figures and conclusions update
Ferromagnetism in the Hubbard model with Topological/Non-Topological Flat Bands
We introduce and study two classes of Hubbard models with magnetic flux or
with spin-orbit coupling, which have a flat lowest band separated from other
bands by a nonzero gap. We study the Chern number of the flat bands, and find
that it is zero for the first class but can be nontrivial in the second. We
also prove that the introduction of on-site Coulomb repulsion leads to
ferromagnetism in both the classes.Comment: 6 pages, 5 figure
Stability of patterns with arbitrary period for a Ginzburg-Landau equation with a mean field
We consider the following system of equations
A_t= A_{xx} + A - A^3 -AB,\quad x\in R,\,t>0,
B_t = \sigma B_{xx} + \mu (A^2)_{xx}, x\in R, t>0,
where \mu > \sigma >0. It plays an
important role as a Ginzburg-Landau equation with a mean field in
several fields of the applied sciences.
We study the existence and stability of periodic patterns with an
arbitrary minimal period L. Our approach is by combining methods
of nonlinear functional analysis such as nonlocal eigenvalue
problems and the variational characterization of eigenvalues with
Jacobi elliptic integrals. This enables us to give a complete
characterization of existence and stability for all solutions with
A>0, spatial average =0 and an arbitrary minimal period
Medications for chronic pain a practical review
A journal article on medication for chronic pain in HIV/AIDS patients in Sub-Saharan Africa.This review is confined to the drug management of chronic pain, and is specifically adapted to the resource- poor environment and the HIV pandemic of sub-Saharan Africa. A brief classification of chronic pain is followed by a discussion of the different classes of medications in use, including those used in migraine. An approach to the rational drug management of neuropathic pain is presented. In conclusion some general
principles for prescribing in this setting are derived
Flat-band excitonic states in Kagome lattice on semiconductor surface
Excitonic properties in the Kagome lattice system, which is produced by
quantum wires on semiconductor surfaces, are investigated by using the exact
diagonalization of a tight binding model. It is shown that due to the existence
of flat bands the binding energy of exciton becomes remarkably large in the
two-dimensional Kagome lattice compared to that in one-dimensional lattice, and
the exciton Bohr radius is quite small as large as a lattice constant. We also
discuss the magnetic field effects on the exciton binding energy and the
stability of exciton against the creation of charged exciton and biexciton.Comment: 5 pages, 5 figure
Design of multivariable feedback control systems via spectral assignment
The applicability of spectral assignment techniques to the design of multivariable feedback control systems was investigated. A fractional representation design procedure for unstable plants is presented and illustrated with an example. A computer aided design software package implementing eigenvalue/eigenvector design procedures is described. A design example which illustrates the use of the program is explained
Relationship between spiral and ferromagnetic states in the Hubbard model in the thermodynamic limit
We explore how the spiral spin(SP) state, a spin singlet known to accompany
fully-polarized ferromagnetic (F) states in the Hubbard model, is related with
the F state in the thermodynamic limit using the density matrix renormalization
group and exact diagonalization. We first obtain an indication that when the F
state is the ground state the SP state is also eligible as the ground state in
that limit. We then follow the general argument by Koma and Tasaki [J. Stat.
Phys. {\bf 76}, 745 (1994)] to find that: (i) The SP state possesses a kind of
order parameter. (ii) Although the SP state does not break the SU(2) symmetry
in finite systems, it does so in the thermodynamic limit by making a linear
combination with other states that are degenerate in that limit. We also
calculate the one-particle spectral function and dynamical spin and charge
susceptibilities for various 1D finite-size lattices. We find that the
excitation spectrum of the SP state and the F state is almost identical. Our
present results suggest that the SP and the F states are equivalent in the
thermodynamic limit. These properties may be exploited to determine the
magnetic phase diagram from finite-size studies.Comment: 17 figures, to be published in Phys. Rev.
- …