7,008 research outputs found
Strong absorption and selective thermal emission from a mid-infrared metamaterial
We demonstrate thin-film metamaterials with resonances in the mid-infrared
wavelength range. Our structures are numerically modeled and experimentally
characterized by reflection and angularly-resolved thermal emission
spectroscopy. We demonstrate strong and controllable absorption resonances
across the mid-infrared wavelength range. In addition, the polarized thermal
emission from these samples is shown to be highly selective and largely
independent of emission angles from normal to 45 degrees. Experimental results
are compared to numerical models with excellent agreement. Such structures hold
promise for large-area, low-cost metamaterial coatings for control of gray- or
black-body thermal signatures, as well as for possible mid-IR sensing
applications.Comment: The following article has been submitted to Appl. Phys. Lett. After
it is published, it will be found at http://apl.aip.org/. 14 pages including
4 figure page
Network Evolution Based on Centrality
We study the evolution of networks when the creation and decay of links are
based on the position of nodes in the network measured by their centrality. We
show that the same network dynamics arises under various centrality measures,
and solve analytically the network evolution. During the complete evolution,
the network is characterized by nestedness: the neighbourhood of a node is
contained in the neighbourhood of the nodes with larger degree. We find a
discontinuous transition in the network density between hierarchical and
homogeneous networks, depending on the rate of link decay. We also show that
this evolution mechanism leads to double power-law degree distributions, with
interrelated exponents.Comment: 6 pages, 3 figure
Negative differential Rashba effect in two-dimensional hole systems
We demonstrate experimentally and theoretically that two-dimensional (2D)
heavy hole systems in single heterostructures exhibit a \emph{decrease} in
spin-orbit interaction-induced spin splitting with an increase in perpendicular
electric field. Using front and back gates, we measure the spin splitting as a
function of applied electric field while keeping the density constant. Our
results are in contrast to the more familiar case of 2D electrons where spin
splitting increases with electric field.Comment: 3 pages, 3 figures. To appear in AP
Local modularity measure for network clusterizations
Many complex networks have an underlying modular structure, i.e., structural
subunits (communities or clusters) characterized by highly interconnected
nodes. The modularity has been introduced as a measure to assess the
quality of clusterizations. has a global view, while in many real-world
networks clusters are linked mainly \emph{locally} among each other
(\emph{local cluster-connectivity}). Here, we introduce a new measure,
localized modularity , which reflects local cluster structure. Optimization
of and on the clusterization of two biological networks shows that the
localized modularity identifies more cohesive clusters, yielding a
complementary view of higher granularity.Comment: 5 pages, 4 figures, RevTex4; Changed conten
A dual modelling of evolving political opinion networks
We present the result of a dual modeling of opinion network. The model
complements the agent-based opinion models by attaching to the social agent
(voters) network a political opinion (party) network having its own intrinsic
mechanisms of evolution. These two sub-networks form a global network which can
be either isolated from or dependent on the external influence. Basically, the
evolution of the agent network includes link adding and deleting, the opinion
changes influenced by social validation, the political climate, the
attractivity of the parties and the interaction between them. The opinion
network is initially composed of numerous nodes representing opinions or
parties which are located on a one dimensional axis according to their
political positions. The mechanism of evolution includes union, splitting,
change of position and of attractivity, taken into account the pairwise node
interaction decaying with node distance in power law. The global evolution ends
in a stable distribution of the social agents over a quasi-stable and
fluctuating stationary number of remaining parties. Empirical study on the
lifetime distribution of numerous parties and vote results is carried out to
verify numerical results
Centrality scaling in large networks
Betweenness centrality lies at the core of both transport and structural
vulnerability properties of complex networks, however, it is computationally
costly, and its measurement for networks with millions of nodes is near
impossible. By introducing a multiscale decomposition of shortest paths, we
show that the contributions to betweenness coming from geodesics not longer
than L obey a characteristic scaling vs L, which can be used to predict the
distribution of the full centralities. The method is also illustrated on a
real-world social network of 5.5*10^6 nodes and 2.7*10^7 links
Lensing of Fast Radio Bursts by Plasma Structures in Host Galaxies
Plasma lenses in the host galaxies of fast radio bursts (FRBs) can strongly
modulate FRB amplitudes for a wide range of distances, including the
Gpc distance of the repeater FRB121102. To produce caustics, the lens'
dispersion-measure depth (), scale size (), and distance
from the source () must satisfy . Caustics produce strong
magnifications () on short time scales ( hours to days and
perhaps shorter) along with narrow, epoch dependent spectral peaks (0.1 to
1~GHz). However, strong suppression also occurs in long-duration (
months) troughs. For geometries that produce multiple images, the resulting
burst components will arrive differentially by s to tens of ms and
they will show different apparent dispersion measures, pc cm. Arrival time perturbations may mask any
underlying periodicity with period s. When arrival times differ by
less than the burst width, interference effects in dynamic spectra are
expected. Strong lensing requires source sizes smaller than , which can be satisfied by compact objects such as
neutron star magnetospheres but not by AGNs. Much of the phenomenology of the
repeating fast radio burst source FRB121102 is similar to lensing effects. The
overall picture can be tested by obtaining wideband spectra of bursts (from
to 10 GHz and possibly higher), which can also be used to characterize the
plasma environment near FRB sources. A rich variety of phenomena is expected
from an ensemble of lenses near the FRB source. We discuss constraints on
densities, magnetic fields, and locations of plasma lenses related to
requirements for lensing to occur.Comment: 11 pages, 7 figures, submitted to the Astrophysical Journa
Anomalous Spin Polarization of GaAs Two-Dimensional Hole Systems
We report measurements and calculations of the spin-subband depopulation,
induced by a parallel magnetic field, of dilute GaAs two-dimensional (2D) hole
systems. The results reveal that the shape of the confining potential
dramatically affects the values of in-plane magnetic field at which the upper
spin subband is depopulated. Most surprisingly, unlike 2D electron systems, the
carrier-carrier interaction in 2D hole systems does not significantly enhance
the spin susceptibility. We interpret our findings using a multipole expansion
of the spin density matrix, and suggest that the suppression of the enhancement
is related to the holes' band structure and effective spin j=3/2.Comment: 6 pages, 4 figures, substantially extended discussion of result
Scale Invariance in Road Networks
We study the topological and geographic structure of the national road
networks of the United States, England and Denmark. By transforming these
networks into their dual representation, where roads are vertices and an edge
connects two vertices if the corresponding roads ever intersect, we show that
they exhibit both topological and geographic scale invariance. That is, we show
that for sufficiently large geographic areas, the dual degree distribution
follows a power law with exponent 2.2 < alpha < 2.4, and that journeys,
regardless of their length, have a largely identical structure. To explain
these properties, we introduce and analyze a simple fractal model of road
placement that reproduces the observed structure, and suggests a testable
connection between the scaling exponent alpha and the fractal dimensions
governing the placement of roads and intersections.Comment: 6 pages, 10 figures; revision incorporates more rigorous statistical
analyses; matches journal versio
- …