117,255 research outputs found
Opinion diversity and community formation in adaptive networks
It is interesting and of significant importance to investigate how network
structures co-evolve with opinions. The existing models of such co-evolution
typically lead to the final states where network nodes either reach a global
consensus or break into separated communities, each of which holding its own
community consensus. Such results, however, can hardly explain the richness of
real-life observations that opinions are always diversified with no global or
even community consensus, and people seldom, if not never, totally cut off
themselves from dissenters. In this article, we show that, a simple model
integrating consensus formation, link rewiring and opinion change allows
complex system dynamics to emerge, driving the system into a dynamic
equilibrium with co-existence of diversified opinions. Specifically, similar
opinion holders may form into communities yet with no strict community
consensus; and rather than being separated into disconnected communities,
different communities remain to be interconnected by non-trivial proportion of
inter-community links. More importantly, we show that the complex dynamics may
lead to different numbers of communities at steady state with a given tolerance
between different opinion holders. We construct a framework for theoretically
analyzing the co-evolution process. Theoretical analysis and extensive
simulation results reveal some useful insights into the complex co-evolution
process, including the formation of dynamic equilibrium, the phase transition
between different steady states with different numbers of communities, and the
dynamics between opinion distribution and network modularity, etc.Comment: 12 pages, 8 figures, Journa
All-Optical Manipulation of Electron Spins in Carbon-Nanotube Quantum Dots
We demonstrate theoretically that it is possible to manipulate electron or
hole spins all optically in semiconducting carbon nanotubes. The scheme that we
propose is based on the spin-orbit interaction that was recently measured
experimentally; we show that this interaction, together with an external
magnetic field, can be used to achieve optical electron-spin state preparation
with a fidelity exceeding 99%. Our results also imply that it is possible to
implement coherent spin rotation and measurement using laser fields linearly
polarized along the nanotube axis, as well as to convert spin qubits into
time-bin photonic qubits. We expect that our findings will open up new avenues
for exploring spin physics in one-dimensional systems
Measurement Matrix Design for Compressive Sensing Based MIMO Radar
In colocated multiple-input multiple-output (MIMO) radar using compressive
sensing (CS), a receive node compresses its received signal via a linear
transformation, referred to as measurement matrix. The samples are subsequently
forwarded to a fusion center, where an L1-optimization problem is formulated
and solved for target information. CS-based MIMO radar exploits the target
sparsity in the angle-Doppler-range space and thus achieves the high
localization performance of traditional MIMO radar but with many fewer
measurements. The measurement matrix is vital for CS recovery performance. This
paper considers the design of measurement matrices that achieve an optimality
criterion that depends on the coherence of the sensing matrix (CSM) and/or
signal-to-interference ratio (SIR). The first approach minimizes a performance
penalty that is a linear combination of CSM and the inverse SIR. The second one
imposes a structure on the measurement matrix and determines the parameters
involved so that the SIR is enhanced. Depending on the transmit waveforms, the
second approach can significantly improve SIR, while maintaining CSM comparable
to that of the Gaussian random measurement matrix (GRMM). Simulations indicate
that the proposed measurement matrices can improve detection accuracy as
compared to a GRMM
Information filtering via Iterative Refinement
With the explosive growth of accessible information, expecially on the
Internet, evaluation-based filtering has become a crucial task. Various systems
have been devised aiming to sort through large volumes of information and
select what is likely to be more relevant. In this letter we analyse a new
ranking method, where the reputation of information providers is determined
self-consistently.Comment: 10 pages, 3 figures. Accepted for publication on Europhysics Letter
8-band k·p modelling of mid-infrared intersubband absorption in Ge quantum wells
The 8-band k·p parameters which include the direct band coupling between the conduction and the valence bands are derived and used to model optical intersubband transitions in Ge quantum well heterostructure material grown on Si substrates. Whilst for Si rich quantum wells the coupling between the conduction bands and valence bands is not important for accurate modelling, the present work demonstrates that the inclusion of such coupling is essential to accurately determine intersubband transitions between hole states in Ge and Ge-rich Si1− x Ge x quantum wells. This is due to the direct bandgap being far smaller in energy in Ge compared to Si. Compositional bowing parameters for a range of the key modelling input parameters required for Ge/SiGe heterostructures, including the Kane matrix elements, the effective mass of the Γ2'Γ2′ conduction band, and the Dresselhaus parameters for both 6- and 8-band k·p modelling, have been determined. These have been used to understand valence band intersubband transitions in a range of Ge quantum well intersubband photodetector devices in the mid-infrared wavelength range
Unexpected goings-on in the structure of a neutron star crust
We present a brief account of two phenomena taking place in a neutron star
crust: the Fermionic Casimir effect and the major density depletion of the
cores of the superfluid neutron vortices.Comment: 6 pages, invited talk presented by AB at Tours 2003 Symposium on
Nuclear Physics, August 26-29,Tours, Franc
Calculations of spin induced transport in ferromagnets
Based on first-principles density functional calculations, a general approach
for determining and analyzing the degree of spin polarization (P) in
ferromagnets is presented. The approach employs the so-called tetrahedron
method to evaluate the Fermi surface integrations of P in both ballistic and
diffusive regimes. The validity of the method is examined by comparing the
calculated P values for Fe and Ni with the experiment. The method is shown to
yield highly accurate results with minimal computational effort. Within our
approach, it is also possible to systematically analyze the contributions of
various types of electronic states to the spin induced transport. As a case
study, the transport properties of the soft-ferromagnet CeMnNi4 are
investigated in order to explain the origin of the existing difference between
the experimental and theoretical values of P in this intermetallic compound.Comment: 6 pages, 4 figures; to appear in Physical Review B 75 (2007
Theory of the Stark Effect for P donors in Si
We develop a multi-valley effective mass theory for substitutional donors in
silicon in an inhomogeneous environment. Valley-orbit coupling is treated
perturbatively. We apply the theory to the Stark effect in Si:P. The method
becomes more accurate at high fields, and it is designed to give correct
experimental binding energies at zero field. Unexpectedly, the ground state
energy for the donor electron is found to increase with electric field as a
consequence of spectrum narrowing of the 1s manifold. Our results are of
particular importance for the Kane quantum computer.Comment: published versio
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