8,532 research outputs found
Formation of Hydrogenated Graphene Nanoripples by Strain Engineering and Directed Surface Self-assembly
We propose a new class of semiconducting graphene-based nanostructures:
hydrogenated graphene nanoripples (HGNRs), based on continuum-mechanics
analysis and first principles calculations. They are formed via a two-step
combinatorial approach: first by strain engineered pattern formation of
graphene nanoripples, followed by a curvature-directed self-assembly of H
adsorption. It offers a high level of control of the structure and morphology
of the HGNRs, and hence their band gaps which share common features with
graphene nanoribbons. A cycle of H adsorption/desorption at/from the same
surface locations completes a reversible metal-semiconductor-metal transition
with the same band gap.Comment: 11 pages, 5 figure
Coupling Matrix Representation of Nonreciprocal Filters Based on Time Modulated Resonators
This paper addresses the analysis and design of non-reciprocal filters based
on time modulated resonators. We analytically show that time modulating a
resonator leads to a set of harmonic resonators composed of the unmodulated
lumped elements plus a frequency invariant element that accounts for
differences in the resonant frequencies. We then demonstrate that harmonic
resonators of different order are coupled through non-reciprocal admittance
inverters whereas harmonic resonators of the same order couple with the
admittance inverter coming from the unmodulated filter network. This coupling
topology provides useful insights to understand and quickly design
non-reciprocal filters and permits their characterization using an
asynchronously tuned coupled resonators network together with the coupling
matrix formalism. Two designed filters, of orders three and four, are
experimentally demonstrated using quarter wavelength resonators implemented in
microstrip technology and terminated by a varactor on one side. The varactors
are biased using coplanar waveguides integrated in the ground plane of the
device. Measured results are found to be in good agreement with numerical
results, validating the proposed theory
Collisionless galaxy simulations
Three-dimensional fully self-consistent computer models were used to determine the evolution of galaxies consisting of 100 000 simulation stars. Comparison of two-dimensional simulations with three-dimensional simulations showed only a very slight stabilizing effect due to the additional degree of freedom. The addition of a fully self-consistent, nonrotating, exponential core/halo component resulted in considerable stabilization. A second series of computer experiments was performed to determine the collapse and relaxation of initially spherical, uniform density and uniform velocity dispersion stellar systems. The evolution of the system was followed for various amounts of angular momentum in solid body rotation. For initally low values of the angular momentum satisfying the Ostriker-Peebles stability criterion, the systems quickly relax to an axisymmetric shape and resemble elliptical galaxies in appearance. For larger values of the initial angular momentum bars develop and the systems undergo a much more drastic evolution
On the soliton width in the incommensurate phase of spin-Peierls systems
We study using bosonization techniques the effects of frustration due to
competing interactions and of the interchain elastic couplings on the soliton
width and soliton structure in spin-Peierls systems. We compare the predictions
of this study with numerical results obtained by exact diagonalization of
finite chains. We conclude that frustration produces in general a reduction of
the soliton width while the interchain elastic coupling increases it. We
discuss these results in connection with recent measurements of the soliton
width in the incommensurate phase of CuGeO_3.Comment: 4 pages, latex, 2 figures embedded in the tex
Low Temperature Magnetic Properties of the Double Exchange Model
We study the {\it ferromagnetic} (FM) Kondo lattice model in the strong
coupling limit (double exchange (DE) model). The DE mechanism proposed by Zener
to explain ferromagnetism has unexpected properties when there is more than one
itinerant electron. We find that, in general, the many-body ground state of the
DE model is {\it not} globally FM ordered (except for special filled-shell
cases). Also, the low energy excitations of this model are distinct from spin
wave excitations in usual Heisenberg ferromagnets, which will result in unusual
dynamic magnetic properties.Comment: 5 pages, RevTeX, 5 Postscript figures include
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