15,967 research outputs found
Phase Diagram of the Two-Leg Kondo Ladder
The phase diagram of the two-leg Kondo ladder is investigated using
computational techniques. Ferromagnetism is present, but only at small
conduction electron densities and robust Kondo coupling . For densities
and any Kondo coupling, a paramagnetic phase is found. We also
observed spin dimerization at densities =1/4 and =1/2. The spin structure
factor at small peaks at = for , and at
= for . The charge structure factor suggests
that electrons behave as free particles with spin-1/2 (spin-0) for small
(large) .Comment: 5 pages, 4 fig
A New Method for Generation of Soundings from Phase-Difference Measurements
A desirable feature of bathymetric sonar systems is the production of statistically independent soundings allowing a system to achieve its full capability in resolution and object detection. Moreover gridding algorithms such as the Combined Uncertainty Bathymetric Estimator (CUBE) rely on the statistical independence of soundings to properly estimate depth and discriminate outliers. Common methods of filtering to mitigate uncertainty in the signal processing of both multibeam and phase-differencing sidescan systems (curve fitting in zero-crossing detections and differential phase filtering respectively) can produce correlated soundings. Here we propose an alternative method for the generation of soundings from differential phase measurements made by either sonar type to produce statistically independent soundings. The method extracts individual, non-overlapping and unfiltered, phase-difference measurements (from either sonar type) converting these to sonar-relative receive angle, estimates their uncertainty, fixes the desired depth uncertainty level and combines these individual measurements into an uncertainty-weighted mean to achieve the desired depth uncertainty, and no more. When the signal to noise ratio is sufficiently high such that the desired depth uncertainty is achieved with an individual measurement, bathymetric estimates are produced at the sonar’s full resolution capability. When multiple measurements are required, the filtering automatically adjusts to maintain the desired uncertainty level, degrading the resolution only as necessary. Because no two measurements contribute to a single reported sounding, the resulting estimated soundings are statistically independent and therefore better resolve adjacent objects, increase object detectability and are more suitable for statistical gridding methodologies
Optimizing Resolution and Uncertainty in Bathymetric Sonar Systems
Bathymetric sonar systems (whether multibeam or phase-differencing sidescan) contain an inherent trade-off between resolution and uncertainty. Systems are traditionally designed with a fixed spatial resolution, and the parameter settings are optimized to minimize the uncertainty in the soundings within that constraint. By fixing the spatial resolution of the system, current generation sonars operate sub-optimally when the SNR is high, producing soundings with lower resolution than is supportable by the data, and inefficiently when the SNR is low, producing high-uncertainty soundings of little value. Here we propose fixing the sounding measurement uncertainty instead, and optimizing the resolution of the system within that uncertainty constraint. Fixing the sounding measurement uncertainty produces a swath with a variable number of bathymetric estimates per ping, in which each estimate’s spatial resolution is optimized by combining measurements only until the desired depth uncertainty is achieved. When the signal to noise ratio is sufficiently high such that the desired depth uncertainty is achieved with individual measurements, bathymetric estimates are produced at the sonar’s full resolution capability. Correspondingly, a sonar’s resolution is no-longer only considered as a property of the sonar (based on, for example, beamwidth and bandwidth,) but now incorporates geometrical aspects of the measurements and environmental factors (e.g., seafloor scattering strength). Examples are shown from both multibeam and phase- differencing sonar systems
Partial energies fluctuations and negative heat capacities
We proceed to a critical examination of the method used in nuclear
fragmentation to exhibit signals of negative heat capacity. We show that this
method leads to unsatisfactory results when applied to a simple and well
controlled model. Discrepancies are due to incomplete evaluation of potential
energies.Comment: Modified figures 3 and
Prediction of the derivative discontinuity in density functional theory from an electrostatic description of the exchange and correlation potential
We propose a new approach to approximate the exchange and correlation (XC)
functional in density functional theory. The XC potential is considered as an
electrostatic potential, generated by a fictitious XC density, which is in turn
a functional of the electronic density. We apply the approach to develop a
correction scheme that fixes the asymptotic behavior of any approximated XC
potential for finite systems. Additionally, the correction procedure gives the
value of the derivative discontinuity; therefore it can directly predict the
fundamental gap as a ground-state property.Comment: 5 pages, 4 figure
Is Private Production of Public Services Cheaper than Public Production? A meta-regression analysis of solid waste and water services
Privatization of local government services is assumed to deliver cost savings but empirical evidence for this from around the world is mixed. We conduct a meta-regression analysis of all econometric studies examining privatization for water distribution and solid waste collection services and find no systematic support for lower costs with private production. Differences in study results are explained by differences in time-period of the analyses, service characteristics, and policy environment. We do not find a genuine empirical effect of cost savings resulting from private production. The results suggest that to ensure cost savings, more attention be given to the cost characteristics of the service, the transaction costs involved, and the policy environment stimulating competition, rather than to the debate over public versus private delivery of these services.Privatization, contracting-out, costs, local governments, meta-regression analysis.
Photon locking
A novel observation of photon locking—the optical analog of spin locking—is reported, demonstrating the applicability of phase-coherent pulse sequences. The experiments are reported for the optical transition of iodine gas at 589.7 nm using the pulse sequence XYX-XYX̄. Locking decay rates are presented as a function of pressure and compared with optical dephasing (echo-decay) rates
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