7,872 research outputs found
Quantum Topological Excitations: from the Sawtooth Lattice to the Heisenberg Chain
The recently elucidated structure of the delafossite YCuO reveals a
Cu-O network with nearly independent chains having different
interactions between the spins. Motivated by this result, we study the
chain for various ratios of the base-base and
base-vertex interactions. By exact diagonalization and extrapolation, we show
that the elementary excitation spectrum, which (within numerical error) is the
same for total spins and 1, has a gap only in the interval
. The gap is dispersionless
for , but has increasing -dependence as moves away from unity, related to the instability of dimers in
the ground state.Comment: 4 pages, 6 figures (revtex twocolumn
Limits of feedback control in bacterial chemotaxis
Inputs to signaling pathways can have complex statistics that depend on the
environment and on the behavioral response to previous stimuli. Such behavioral
feedback is particularly important in navigation. Successful navigation relies
on proper coupling between sensors, which gather information during motion, and
actuators, which control behavior. Because reorientation conditions future
inputs, behavioral feedback can place sensors and actuators in an operational
regime different from the resting state. How then can organisms maintain proper
information transfer through the pathway while navigating diverse environments?
In bacterial chemotaxis, robust performance is often attributed to the zero
integral feedback control of the sensor, which guarantees that activity returns
to resting state when the input remains constant. While this property provides
sensitivity over a wide range of signal intensities, it remains unclear how
other parameters affect chemotactic performance, especially when considering
that the swimming behavior of the cell determines the input signal. Using
analytical models and simulations that incorporate recent experimental
evidences about behavioral feedback and flagellar motor adaptation we identify
an operational regime of the pathway that maximizes drift velocity for various
environments and sensor adaptation rates. This optimal regime is outside the
dynamic range of the motor response, but maximizes the contrast between run
duration up and down gradients. In steep gradients, the feedback from
chemotactic drift can push the system through a bifurcation. This creates a
non-chemotactic state that traps cells unless the motor is allowed to adapt.
Although motor adaptation helps, we find that as the strength of the feedback
increases individual phenotypes cannot maintain the optimal operational regime
in all environments, suggesting that diversity could be beneficial.Comment: Corrected one typo. First two authors contributed equally. Notably,
there were various typos in the values of the parameters in the model of
motor adaptation. The results remain unchange
Solution to the Isotropy Problem for Cosmological Hidden Vector Models
Gauge bosons associated to new gauge symmetries under which the standard
model particles are not charged are predicted in many extensions of the
standard model of particles and interactions. We show that under very general
conditions, the average energy-momentum tensor of these rapidly oscillating
vector fields is isotropic for any locally inertial observer. This result has a
fundamental importance in order to consider coherent vector fields as a viable
alternative to support models of dark matter, dark energy or inflation.Comment: 4 pages, 1 figure. Contributed to the 9th Patras Workshop on Axions,
WIMPs and WISPs, Mainz, June 24-28, 201
Spherical harmonic decomposition applied to spatial-temporal analysis of human high-density EEG
We demonstrate an application of spherical harmonic decomposition to analysis
of the human electroencephalogram (EEG). We implement two methods and discuss
issues specific to analysis of hemispherical, irregularly sampled data.
Performance of the methods and spatial sampling requirements are quantified
using simulated data. The analysis is applied to experimental EEG data,
confirming earlier reports of an approximate frequency-wavenumber relationship
in some bands.Comment: 12 pages, 8 figures, submitted to Phys. Rev. E, uses APS RevTeX
style
Interference Suppression in Multiple Access Communications Using M-Ary Phase Shift Keying Generated via Spectral Encoding
A conceptual transform domain communication system (TDCS) is shown capable of operating successfully using M-Ary phase shift keying (MPSK) data modulation in a multiple access environment. Using spectral encoding, the conceptual TDCS provides an effective means for mitigating interference affects while achieving multiple access communications. The use of transform domain processing with MPSK data modulation (TD-MPSK) provides higher spectral efficiency relative to other modulation techniques (antipodal signaling and cyclic shift keying) considered previously for TDCS applications. The proposed TD-MPSK technique uses spectral encoding for both data and multiple access phase modulations. Demodulation of the spectrally encoded TD-MPSK communication symbols is accomplished using conventional, multi-channel time domain correlation techniques. Analytic expressions for TD-MPSK probability of symbol error (PE) and probability of bit error (PB) are derived and validated using simulated results over the range of signal-to-noise ratios typically considered for communications. This validation includes scenarios with: 1) multiple access interference, 2) spectral notching, 3) jamming present and 4) combinations of all three. For a J/S of 3.14 dB and a Eb/N0 of 6 dB, PB dropped by up to a factor of 3 for TD-QPSK in a MA environment for the case when spectral notching was present versus the case when spectral notching wasn\u27t present. The cross-correlation between communication symbols of different synchronous users can be made identically zero through proper selection of multiple access phase codes (orthogonal signaling). For a synchronous network containing orthogonal users, PE and PB are unaffected as the number of orthogonal network users increases. For a J/S of 3.14 dB and a Eb/N0 of 6 dB, PB dropped by a factor of 12 for TD-QPSK in a MA environment for the case when spectral notching was present versus the case when spectral notching wasn\u27t present
Permanent exclusion of young men from secondary school and its influence on their offending and social exclusion in Belize City, Belize
A Physical Model of Human Skin and Its Application for Search and Rescue
For this research we created a human skin reflectance model in the VIS and NIR. We then modeled sensor output for an RGB sensor based on output from the skin reflectance model. The model was also used to create a skin detection algorithm and a skin pigmentation level (skin reflectance at 685nm) estimation algorithm. The average root mean square error across the VIS and NIR between the skin reflectance model and measured data was 2%. The skin reflectance model then allowed us to generate qualitatively accurate responses for an RGB sensor for different biological and lighting conditions. To test the accuracy of the skin detection and skin color estimation algorithms, hyperspectral images of a suburban test scene containing people with various skin colors were collected. The skin detection algorithm had a probability of detection as high as 95% with a probability of false alarm of 0.6%. The skin pigmentation level estimation algorithm had a mean absolute error when compared with data measured by a reflectometer of 2.6% where the reflectance of the individuals at 685nm ranged from 14% to 64%
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