3,670 research outputs found
Flat-Bands on Partial Line Graphs -- Systematic Method for Generating Flat-Band Lattice Structures
We introduce a systematic method for constructing a class of lattice
structures that we call ``partial line graphs''.In tight-binding models on
partial line graphs, energy bands with flat energy dispersions emerge.This
method can be applied to two- and three-dimensional systems. We show examples
of partial line graphs of square and cubic lattices. The method is useful in
providing a guideline for synthesizing materials with flat energy bands, since
the tight-binding models on the partial line graphs provide us a large room for
modification, maintaining the flat energy dispersions.Comment: 9 pages, 4 figure
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.
Effective rate equations for the over-damped motion in fluctuating potentials
We discuss physical and mathematical aspects of the over-damped motion of a
Brownian particle in fluctuating potentials. It is shown that such a system can
be described quantitatively by fluctuating rates if the potential fluctuations
are slow compared to relaxation within the minima of the potential, and if the
position of the minima does not fluctuate. Effective rates can be calculated;
they describe the long-time dynamics of the system. Furthermore, we show the
existence of a stationary solution of the Fokker-Planck equation that describes
the motion within the fluctuating potential under some general conditions. We
also show that a stationary solution of the rate equations with fluctuating
rates exists.Comment: 18 pages, 2 figures, standard LaTeX2
Antiferromagnetism in the Exact Ground State of the Half Filled Hubbard Model on the Complete-Bipartite Graph
As a prototype model of antiferromagnetism, we propose a repulsive Hubbard
Hamiltonian defined on a graph \L={\cal A}\cup{\cal B} with and bonds connecting any element of with all the
elements of . Since all the hopping matrix elements associated with
each bond are equal, the model is invariant under an arbitrary permutation of
the -sites and/or of the -sites. This is the Hubbard model
defined on the so called -complete-bipartite graph,
() being the number of elements in (). In this
paper we analytically find the {\it exact} ground state for at
half filling for any ; the repulsion has a maximum at a critical
-dependent value of the on-site Hubbard . The wave function and the
energy of the unique, singlet ground state assume a particularly elegant form
for N \ra \inf. We also calculate the spin-spin correlation function and show
that the ground state exhibits an antiferromagnetic order for any non-zero
even in the thermodynamic limit. We are aware of no previous explicit analytic
example of an antiferromagnetic ground state in a Hubbard-like model of
itinerant electrons. The kinetic term induces non-trivial correlations among
the particles and an antiparallel spin configuration in the two sublattices
comes to be energetically favoured at zero Temperature. On the other hand, if
the thermodynamic limit is taken and then zero Temperature is approached, a
paramagnetic behavior results. The thermodynamic limit does not commute with
the zero-Temperature limit, and this fact can be made explicit by the analytic
solutions.Comment: 19 pages, 5 figures .ep
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
Modulation Equations: Stochastic Bifurcation in Large Domains
We consider the stochastic Swift-Hohenberg equation on a large domain near
its change of stability. We show that, under the appropriate scaling, its
solutions can be approximated by a periodic wave, which is modulated by the
solutions to a stochastic Ginzburg-Landau equation. We then proceed to show
that this approximation also extends to the invariant measures of these
equations
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
Graduate Nurse Perceptions Of Effectiveness Of Prelicensure Education On Medication Administration
This cross-sectional descriptive survey examined nurse graduates’ perceptions of the efficacy of their educational experiences in preparing them to administer medications safely. Situated cognition provided organization for the study design and analysis. Data were obtained from a cohort of nursing graduates from a community college in south central Michigan using a two-step online and paper survey method. Respondents included 24 nurse graduates from the college of study. Data analysis from the researcher-designed survey revealed learning environments, activities and tools considered to be realistic to nursing practice are considered more effective for learning safe medication practices. Graduate respondents may feel effectively prepared to administer medications safely, however, they do not feel as effectively prepared to anticipate or respond to adverse medication reactions or recognize that a medication error occurred. Needing more practice administering medications was clearly indicated, as was the need to socialize to realistic expectations of nursing practice. Opportunities for nursing educational leaders to improve the effectiveness of graduate preparedness for safe medication administration practices is demonstrated. Situated cognition theory was shown to be an effective tool in evaluating teaching practices. Combined with graduate perceptions, situated cognition can provide a means of developing more effective teaching strategies. Implementing more realistic activities and tools into the learning environments may improve graduate perceptions of preparedness for practice. Graduates whom are better prepared for safe medication administration practices may decrease medication errors and increase patient safety
Asymptotic dynamics in 3D gravity with torsion
We study the nature of boundary dynamics in the teleparallel 3D gravity. The
asymptotic field equations with anti-de Sitter boundary conditions yield only
two non-trivial boundary modes, related to a conformal field theory with
classical central charge. After showing that the teleparallel gravity can be
formulated as a Chern-Simons theory, we identify dynamical structure at the
boundary as the Liouville theory.Comment: 16 pages, RevTeX, no figure
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