2,184 research outputs found
Topological Insulators by Topology Optimization
An acoustic topological insulator (TI) is synthesized using topology
optimization, a free material inverse design method. The TI appears
spontaneously from the optimization process without imposing requirements on
the existence of pseudo spin-1/2 states at the TI interface edge, or the Chern
number of the topological phases. The resulting TI is passive; consisting of
acoustically hard members placed in an air background and has an operational
bandwidth of 12.5\% showing high transmission. Further analysis
demonstrates confinement of more than 99\% of the total field intensity in the
TI within at most six lattice constants from the TI interface. The proposed
design hereby outperforms a reference from recent literature regarding energy
transmission, field confinement and operational bandwidth.Comment: 6 pages, 5 figure
Designing Photonic Topological Insulators with Quantum-Spin-Hall Edge States using Topology Optimization
Designing photonic topological insulators is highly non-trivial because it
requires inversion of band symmetries around the band gap, which was so far
done using intuition combined with meticulous trial and error. Here we take a
completely different approach: we consider the design of photonic topological
insulators as an inverse design problem and use topology optimization to
maximize the transmission through an edge mode with a sharp bend. Two design
domains composed of two different, but initially identical,
C-symmetric unit cells define the geometrical design problem.
Remarkably, the optimization results in a photonic topological insulator
reminiscent of the shrink-and-grow approach to quantum-spin-Hall photonic
topological insulators but with notable differences in the topology of the
crystal as well as qualitatively different band structures and with
significantly improved performance as gauged by the band-gap sizes, which are
at least 50 \% larger than previous designs. Furthermore, we find a directional
beta factor exceeding 99 \%, and very low losses for sharp bends. Our approach
allows for the introduction of fabrication limitations by design and opens an
avenue towards designing PTIs with hitherto unexplored symmetry constraints.Comment: 7 pages, 5 figure
Inverse design in photonics by topology optimization: tutorial
Topology optimization methods for inverse design of nano-photonic systems
have recently become extremely popular and are presented in various forms and
under various names. Approaches comprise gradient and non-gradient based
algorithms combined with more or less systematic ways to improve convergence,
discreteness of solutions and satisfaction of manufacturing constraints. We
here provide a tutorial for the systematic and efficient design of
nano-photonic structures by Topology Optimization (TopOpt). The implementation
is based on the advanced and systematic approaches developed in TopOpt for
structural optimization during the last three decades. The tutorial presents a
step-by-step guide for deriving the continuous constrained optimization problem
forming the foundation of the Topology Optimization method, using a cylindrical
metalens design problem as an example. It demonstrates the effect and necessity
of applying a number of auxiliary tools in the design process in order to
ensure good numerical modelling practice and to achieve physically realisable
designs. Application examples also include an optical demultiplexer.Comment: 8 figures, 19 page
Maximizing the quality factor to mode volume ratio for ultra-small photonic crystal cavities
Small manufacturing-tolerant photonic crystal cavities are systematically
designed using topology optimization to enhance the ratio between quality
factor and mode volume, Q/V. For relaxed manufacturing tolerance, a cavity with
bow-tie shape is obtained which confines light beyond the diffraction limit
into a deep-subwavelength volume. Imposition of a small manufacturing tolerance
still results in efficient designs, however, with diffraction-limited
confinement. Inspired by numerical results, an elliptic ring grating cavity
concept is extracted via geometric fitting. Numerical evaluations demonstrate
that for small sizes, topology-optimized cavities enhance the Q/V-ratio by up
to two orders of magnitude relative to standard L1 cavities and more than one
order of magnitude relative to shape-optimized L1 cavities. An increase in
cavity size can enhance the Q/V-ratio by an increase of the Q-factor without
significant increase of V. Comparison between optimized and reference cavities
illustrates that significant reduction of V requires big topological changes in
the cavity
A Century of Psychology as Science
This reissued edition (originally published by McGraw-Hill in 1985) of A Century of Psychology as Science comprehensively assesses the accomplishments, status, and prospects of psychology at the end of its first century as a science, while offering a new postscript. The forty-three contributors are among psychology\u27s foremost authorities. Among the fields addressed are sensory processes and perception, learning, motivation, emotion, cognition, development, personality, and social psychology.https://scholarship.richmond.edu/bookshelf/1168/thumbnail.jp
Phase separation in the vicinity of "quantum critical" doping concentration: implications for high temperature superconductors
A general quantitative measure of the tendency towards phase separation is
introduced for systems exhibiting phase transitions or crossovers controlled by
charge carrier concentration. This measure is devised for the situations when
the quantitative knowledge of various contributions to free energy is
incomplete, and is applied to evaluate the chances of electronic phase
separation associated with the onset of antiferromagnetic correlations in
high-temperature cuprate superconductors. The experimental phenomenology of
lanthanum- and yittrium-based cuprates was used as input to this analysis. It
is also pointed out that Coulomb repulsion between charge carriers separated by
the distances of 1-3 lattice periods strengthens the tendency towards phase
separation by accelerating the decay of antiferromagnetic correlations with
doping. Overall, the present analysis indicates that cuprates are realistically
close to the threshold of phase separation -- nanoscale limited or even
macroscopic with charge density varying between adjacent crystal planes
Effective approach to the Nagaoka regime of the two dimensional t-J model
We argue that the t-J model and the recently proposed Ising version of this
model give the same physical picture of the Nagaoka regime for J/t << 1. In
particular, both models are shown to give compatible results for a single
Nagaoka polaron as well as for a Nagaoka bipolaron. When compared to the
standard t-J or t-Jz models, the Ising version allows for a numerical analysis
on much larger clusters by means of classical Monte Carlo simulations. Taking
the advantage of this fact, we study the low doping regime of t-J model for J/t
<< 1 and show that the ground state exhibits phase separation into hole-rich
ferromagnetic and hole-depleted antiferromagnetic regions. This picture holds
true up to a threshold concentration of holes, \delta < \delta_t ~ 0.44
\sqrt{J/t}. Analytical calculations show that \delta_t=\sqrt{J/2\pi t}.Comment: 10 pages, 10 figures, revte
The Stokes-Einstein Relation in Supercooled Aqueous Solutions of Glycerol
The diffusion of glycerol molecules decreases with decreasing temperature as
its viscosity increases in a manner simply described by the Stokes-Einstein(SE)
relation. Approaching the glass transition, this relation breaks down as it
does with a number of other pure liquid glass formers. We have measured the
diffusion coefficient for binary mixtures of glycerol and water and find that
the Stokes-Einstein relation is restored with increasing water concentration.
Our comparison with theory suggests that addition of water postpones the
formation of frustration domainsComment: 4 Pages and 3 Figure
A Fluid Dynamics Calculation of Sputtering from a Cylindrical Thermal Spike
The sputtering yield, Y, from a cylindrical thermal spike is calculated using
a two dimensional fluid dynamics model which includes the transport of energy,
momentum and mass. The results show that the high pressure built-up within the
spike causes the hot core to perform a rapid expansion both laterally and
upwards. This expansion appears to play a significant role in the sputtering
process. It is responsible for the ejection of mass from the surface and causes
fast cooling of the cascade. The competition between these effects accounts for
the nearly linear dependence of with the deposited energy per unit depth
that was observed in recent Molecular Dynamics simulations. Based on this we
describe the conditions for attaining a linear yield at high excitation
densities and give a simple model for this yield.Comment: 10 pages, 9 pages (including 9 figures), submitted to PR
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