2,236 research outputs found
A Superlens Based on Metal-Dielectric Composites
Pure noble metals are typically considered to be the materials of choice for
a near-field superlens that allows subwavelength resolution by recovering both
propagating and evanescent waves. However, a superlens based on bulk metal can
operate only at a single frequency for a given dielectric host. In this Letter,
it is shown that a composite metal-dielectric film, with an appropriate metal
filling factor, can operate at practically any desired wavelength in the
visible and near-infrared ranges. Theoretical analysis and simulations verify
the feasibility of the proposed lens.Comment: 15 pages, 4 figure
Multicritical Points and Crossover Mediating the Strong Violation of Universality: Wang-Landau Determinations in the Random-Bond Blume-Capel model
The effects of bond randomness on the phase diagram and critical behavior of
the square lattice ferromagnetic Blume-Capel model are discussed. The system is
studied in both the pure and disordered versions by the same efficient
two-stage Wang-Landau method for many values of the crystal field, restricted
here in the second-order phase transition regime of the pure model. For the
random-bond version several disorder strengths are considered. We present phase
diagram points of both pure and random versions and for a particular disorder
strength we locate the emergence of the enhancement of ferromagnetic order
observed in an earlier study in the ex-first-order regime. The critical
properties of the pure model are contrasted and compared to those of the random
model. Accepting, for the weak random version, the assumption of the double
logarithmic scenario for the specific heat we attempt to estimate the range of
universality between the pure and random-bond models. The behavior of the
strong disorder regime is also discussed and a rather complex and yet not fully
understood behavior is observed. It is pointed out that this complexity is
related to the ground-state structure of the random-bond version.Comment: 12 pages, 11 figures, submitted for publicatio
Strong Violation of Critical Phenomena Universality: Wang-Landau Study of the 2d Blume-Capel Model under Bond Randomness
We study the pure and random-bond versions of the square lattice
ferromagnetic Blume-Capel model, in both the first-order and second-order phase
transition regimes of the pure model. Phase transition temperatures, thermal
and magnetic critical exponents are determined for lattice sizes in the range
L=20-100 via a sophisticated two-stage numerical strategy of entropic sampling
in dominant energy subspaces, using mainly the Wang-Landau algorithm. The
second-order phase transition, emerging under random bonds from the
second-order regime of the pure model, has the same values of critical
exponents as the 2d Ising universality class, with the effect of the bond
disorder on the specific heat being well described by double-logarithmic
corrections, our findings thus supporting the marginal irrelevance of quenched
bond randomness. On the other hand, the second-order transition, emerging under
bond randomness from the first-order regime of the pure model, has a
distinctive universality class with \nu=1.30(6) and \beta/\nu=0.128(5). This
amounts to a strong violation of the universality principle of critical
phenomena, since these two second-order transitions, with different sets of
critical exponents, are between the same ferromagnetic and paramagnetic phases.
Furthermore, the latter of these two transitions supports an extensive but weak
universality, since it has the same magnetic critical exponent (but a different
thermal critical exponent) as a wide variety of two-dimensional systems. In the
conversion by bond randomness of the first-order transition of the pure system
to second order, we detect, by introducing and evaluating connectivity spin
densities, a microsegregation that also explains the increase we find in the
phase transition temperature under bond randomness.Comment: Added discussion and references. 10 pages, 6 figures. Published
versio
Cut-wire-pair structures as two-dimensional magnetic metamaterials
We study numerically and experimentally magnetic metamaterials based on
cut-wire pairs instead of split-ring resonators. The cut-wire pair planar
structure is extended in order to create a truly two-dimensional metamaterial
suitable for scaling to optical frequencies. We fabricate the cut-wire
metamaterial operating at microwave frequencies with lattice spacing around 10%
of the free-space wavelength, and find good agreement with direct numerical
simulations. Unlike the structures based on split-ring resonators, the
nearest-neighbor coupling in cut-wire pairs can result in a magnetic stop-band
with propagation in the transverse direction
Alternative approach to all-angle negative refraction in two-dimensional photonic crystals
We show that with an appropriate surface modification, a slab of photonic
crystal can be made to allow wave transmission within the band gap.
Furthermore, negative refraction and all-angle-negative-refraction (AANR) can
be achieved by this surface modification in frequency windows that were not
realized before in two-dimensional photonic crystals [C. Luo et al, Phys. Rev.
B 65, 201104 (2002)]. This approach to AANR leads to new applications in flat
lens imaging. Previous flat lens using photonic crystals requires object-image
distance u+v less than or equal to the lens thickness d, u+v d. Our approach
can be used to design flat lens with u+v=sd with s>>1, thus being able to image
large and/or far away objects. Our results are confirmed by FDTD simulations.Comment: 5 pages, 9 eps figs in RevTex forma
Critical Point Correlation Function for the 2D Random Bond Ising Model
High accuracy Monte Carlo simulation results for 1024*1024 Ising system with
ferromagnetic impurity bonds are presented. Spin-spin correlation function at a
critical point is found to be numerically very close to that of a pure system.
This is not trivial since a critical temperature for the system with impurities
is almost two times lower than pure Ising . Finite corrections to the
correlation function due to combined action of impurities and finite lattice
size are described.Comment: 7 pages, 2 figures after LaTeX fil
Negative-Index Metamaterials: Second-Harmonic Generation, Manley-Rowe Relations and Parametric Amplification
Second harmonic generation and optical parametric amplification in
negative-index metamaterials (NIMs) are studied. The opposite directions of the
wave vector and the Poynting vector in NIMs results in a "backward"
phase-matching condition, causing significant changes in the Manley-Rowe
relations and spatial distributions of the coupled field intensities. It is
shown that absorption in NIMs can be compensated by backward optical parametric
amplification. The possibility of distributed-feedback parametric oscillation
with no cavity has been demonstrated. The feasibility of the generation of
entangled pairs of left- and right-handed counter-propagating photons is
discussed.Comment: 7 pages, 6 figure
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