56,275 research outputs found
The dynamics of bistable liquid crystal wells
A planar bistable liquid crystal device, reported in Tsakonas et al. [27], is modelled within the Landau-de Gennes theory for nematic liquid crystals. This planar device consists of an array of square micron-sized wells. We obtain six different classes of equilibrium profiles and these profiles are classified as diagonal or rotated solutions. In the strong anchoring case, we propose a Dirichlet boundary condition that mimics the experimentally imposed tangent boundary conditions. In the weak anchoring case, we present a suitable surface energy and study the multiplicity of solutions as a function of the anchoring strength. We find that diagonal solutions exist for all values of the anchoring strength W ≥ 0 while rotated solutions only exist for W ≥ Wc > 0, where Wc is a critical anchoring strength that has been computed numerically. We propose a dynamic model for the switching mechanisms based on only dielectric effects. For sufficiently strong external electric fields, we numerically demonstrate diagonal to rotated and rotated to diagonal switching by allowing for variable anchoring strength across the domain boundary
Tunable "Doniach Phase Diagram" for strongly-correlated nanoclusters
Exact diagonalization calculations reveal that the energy spacing in
the conduction band tunes the interplay between the {\it local} Kondo and {\it
non local} RKKY interactions, giving rise to a "Doniach phase diagram" for a
nanocluster with regions of prevailing Kondo or RKKY correlations. The parity
of the total number of electrons alters the competition between the Kondo and
RKKY correlations. This interplay may be relevant to experimental realizations
of small rings or quantum dots with tunable magnetic properties. Below a
critical value V of the hybridization the susceptibility exhibits a low-T
exponential activation behavior determined by the interplay of the spin gap and
.Comment: 4 pages, 5 figure
Simple model of bouncing ball dynamics: displacement of the table assumed as quadratic function of time
Nonlinear dynamics of a bouncing ball moving in gravitational field and
colliding with a moving limiter is considered. Displacement of the limiter is a
quadratic function of time. Several dynamical modes, such as fixed points, 2 -
cycles and chaotic bands are studied analytically and numerically. It is shown
that chaotic bands appear due to homoclinic structures created from unstable 2
- cycles in a corner-type bifurcation.Comment: 11 pages, 6 figure
Theoretical modeling of spatial and temperature dependent exciton energy in coupled quantum wells
Motivated by a recent experiment of spatial and temperature dependent average
exciton energy distribution in coupled quantum wells [S. Yang \textit{et al.},
Phys. Rev. B \textbf{75}, 033311 (2007)], we investigate the nature of the
interactions in indirect excitons. Based on the uncertainty principle, along
with a temperature and energy dependent distribution which includes both
population and recombination effects, we show that the interplay between an
attractive two-body interaction and a repulsive three-body interaction can lead
to a natural and good account for the nonmonotonic temperature dependence of
the average exciton energy. Moreover, exciton energy maxima are shown to locate
at the brightest regions, in agreement with the recent experiments. Our results
provide an alternative way for understanding the underlying physics of the
exciton dynamics in coupled quantum wells.Comment: 8 pages, 5 figure
The millimeter-wave properties of superconducting microstrip lines
We have developed a novel technique for making high quality measurements of the millimeter-wave properties of superconducting thin-film microstrip transmission lines. Our experimental technique currently covers the 75-100 GHz band. The method is based on standing wave resonances in an open ended transmission line. We obtain information on the phase velocity and loss of the microstrip. Our data for Nb/SiO/Nb lines, taken at 4.2 K and 1.6 K, can be explained by a single set of physical parameters. Our preliminary conclusion is that the loss is dominated by the SiO dielectric, with a temperature-independent loss tangent of 5.3 ± 0.5 x 10^(-3) for our samples
Disordered Kondo Nanoclusters: Effect of Energy Spacing
Exact diagonalization results for Kondo nanoclusters alloyed with mixed
valence impurities show that tuning the {\it energy spacing}, , drives
the system from the Kondo to the RKKY regime. The interplay of and
disorder gives rise to a versus concentration T=0 phase diagram very
rich in structure, where regions with prevailing Kondo or RKKY correlations
alternate with domains of ferromagnetic order. The local Kondo temperatures,
, and RKKY interactions depend strongly on the local environment and are
overall {\it enhanced} by disorder, in contrast to the hypothesis of ``Kondo
disorder'' single-impurity models.Comment: 4pages 4 figuresDisordered Kondo Nanoclusters: Effect of Energy
Spacin
Spin evolution of spin-1 Bose-Einstein condensates
An analytical formula is obtained to describe the evolution of the average
populations of spin components of spin-1 atomic gases. The formula is derived
from the exact time-dependent solution of the Hamiltonian without using approximation. Therefore it goes beyond the mean
field theory and provides a general, accurate, and complete description for the
whole process of non-dissipative evolution starting from various initial
states. The numerical results directly given by the formula coincide
qualitatively well with existing experimental data, and also with other
theoretical results from solving dynamic differential equations. For some
special cases of initial state, instead of undergoing strong oscillation as
found previously, the evolution is found to go on very steadily in a very long
duration.Comment: 7 pages, 3 figures
Generalized seniority for the shell model with realistic interactions
The generalized seniority scheme has long been proposed as a means of
dramatically reducing the dimensionality of nuclear shell model calculations,
when strong pairing correlations are present. However, systematic benchmark
calculations, comparing results obtained in a model space truncated according
to generalized seniority with those obtained in the full shell model space, are
required to assess the viability of this scheme. Here, a detailed comparison is
carried out, for semimagic nuclei taken in a full major shell and with
realistic interactions. The even-mass and odd-mass Ca isotopes are treated in
the generalized seniority scheme, for generalized seniority v<=3. Results for
level energies, orbital occupations, and electromagnetic observables are
compared with those obtained in the full shell model space.Comment: 13 pages, 8 figures; published in Phys. Rev.
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