Coherent energy migration in solids: Determination of the average coherence length in one‐dimensional systems using tunable dye lasers

The coherent nature of energy propagation in solids at low temperatures was established from the time resolved response of the crystal to short optical pulses obtained from a dye laser (pumped by a nitrogen gas laser). The trapping and detrapping of the energy by shallow defects (x traps) was evident in the spectra and enabled us to extract the coherence length: l≳700 Å=186 molecules for the one‐dimensional triplet excitons of 1,2,4,5‐tetrachlorobenzene crystals at T<4.2° K. This length which clearly exceeds the stochastic random walk limit is related to the thermalization mechanisms in this coupled exciton–trap system, and its magnitude supports the notion that exciton–phonon coupling is responsible for the loss of coherence on very long molecular chains (trap concentration is 1/256 000)

Inter-frequency Bias Estimation for the GPS Monitor Station Network

The inter-frequency bias (IFB) is present in all dual frequency combinations of GPS pseudorange and carrier phase observables. It is caused by the path dependent signal delays in both the satellite and receiver. That delay can be directly measured for a space vehicle prior to launch, or for a ground based receiver prior to its being used in the field. However the bias is known to drift, and monitoring the delay estimate by direct measurement is time consuming for ground based receivers and impossible for deployed space vehicles. Hansen (2002) examined the observability of IFB through a global model of ionosphere total electron content (TEC). Variation in the receiver portion of the IFB can also be observed in receivers with antennae in a zero-baseline configuration. This is referred to as an inter-receiver bias (IRB). In this study a Kalman filter is formulated to observe IFBs and IRBs. Process noise is used to allow the filter to track changes in the IFBs and IRBs. The filter also implements constraints to reflect the fact that a given IRB is not linearly independent of the IFBs. Because the receivers are distributed on a global scale, the Kalman filter requires a globally observable phenomenon by which to tie the IFBs. In this case ionosphere delay provides such a phenomenon. The filter was applied to observations collected by GPS monitor stations that comprise the National Geospatial- Intelligence Agency Monitor Station Network (MSN). Each monitor station contains two geodetic quality receivers in a zero-baseline configuration and continuously collects GPS observations. The GPS observations collected by this network are used to produce both precise ephemeris and the broadcast ephemeris. GPS observations made through the network are incorporated into the GPS Master Control Station (MCS) Kalman filter of the Operational Control System (OCS) (Wiley, 2006). The Kalman filter in the OCS estimates the orbital parameters that are transmitted via the navigation message. If estimated effectively, knowledge of the receiver portion of the IFB can aid in achieving better ionosphere models. IFBs are made observable using a global ionosphere delay model. A ninth order spherical harmonic model derived by Y.C. Chao (1997) was used in this study for ionosphere delay. Chao used this spherical harmonic model to capture ionospheric variations that occurred over a smaller global region in his IFB estimation process. In this study a similar model was used but was verified using observations that span a global coverage. The receiver portion of the IFB is observed precisely using the IRB. In this study error terms were introduced into the Kalman filter design to realign the IRB estimates to the IFB estimates produced for each of the two receivers in a zero baseline configuration. For a nominal epoch of measurement, there were 198 noisy measurements used each epoch to generate twelve monitor station specific IRBs. The IRB estimates showed small, decimeter level dynamic variation over the period of a day. The quality of the IFB estimate directly affects the quality of the ionospheric model formed during the estimation process. Results verify that the filter is operating properly. The ionosphere model, though simple, demonstrates that the total electron content (TEC) peaks during local noon and is at a minimum during local night. IRB estimates are roughly constant over time and have a magnitude of less than 2.5 meters. Similar estimates are formed for the IFBs, however when processing one day of observations, the IFB estimates are less stable than those of the IRBs. Future effort will involve tuning the filter, and establishing criteria for its convergence

Assessment in Scotland

Assessment practice will follow and reinforce the curriculum and promote high quality learning and teaching approaches. Assessment of children's and young people's progress and achievement during their broad general education to the end of S3 will be based on teachers' assessment of their knowledge and understanding, skills, attributes and capabilities, as described in the experiences and outcomes across the curriculum

On the absence of Shapiro-like steps in certain mesoscopic S-N-S junctions

In DC transport through mesoscopic S-N-S junctions, it is known that the Josephson coupling decreases exponentially with increasing temperature, but the phase dependence of the conductance persists to much higher temperatures and decreases only as 1/T. It is pointed out here that, despite the fact that such a phase-dependent conductance does bring about an AC current for a pure DC voltage, it cannot, by itself, lead to the formation of Shapiro steps.Comment: 1 page, to be published in PRL (as Comment

Avionics test bed development plan

The plan is for a facility for the early investigation and evaluation of new concepts for the control of large space structures, orbiter attached flex body experiments, and orbiter enhancements. This plan outlines a distributed data processing facility that will utilize the current JSC laboratory resources for the test bed development. The future studies required for implementation, the management system for project control, and the baseline system configuration are described

A three dimensional model of the Venusian thermosphere with superrotation

An improved three dimensional spectral model of the thermosphere of Venus is described. The model solves the Navier-Stokes equations and includes nonlinear effects for an arbitrary number of atmospheric species. A two dimensional axisymmetric model of the superrotation of the thermosphere is also presented. This model addresses the Pioneer-Venus mission finding, which suggested the thermospheric rotation rate to be much higher than that of the planet as seen from the asymmetric distribution of hydrogen and helium. Both models include the effects of an anisotropic eddy diffusion that is consistent with atmospheric mixing length theory

Thermal oxidative degradation reactions of perfluoroalklethers

The objective of this contract was to investigate the mechanisms operative in thermal and thermal oxidative degradation of Fomblin Z and hexafluoropropene oxide derived fluids and the effect of alloys and additives upon these processes. The nature of arrangements responsible for the inherent thermal oxidative instability of the Fomblin Z fluids has not been established. It was determined that this behavior was not associated with hydrogen end-groups or peroxy linkages. The degradation rate of these fluids at elevated temperatures in oxidizing atmospheres was found to be dependent on the surface/volume ratio. Once a limiting ratio was reached, a steady rate appeared to be attained. Based on elemental analysis and oxygen consumption data, -CF2OCF2CF2O-, not -CF2CF2O-, is one of the major arrangements present. The action of the M-50 and Ti(4 Al, 4 Mn) alloys was found to be much more drastic in the case of Fomblin Z fluids than that observed for the hexalfuoropropane oxide derived materials. The effectiveness of antioxidation/anticorrosion additives, P-3 and phospha-s-triazine, in the presence of metal alloys was very limited at 316 C; at 288 C the additives arrested almost completely the fluid degradation. The phospha-s-triazine appeared to be at least twice as effective as the P-3 compound; it also protected the coupon better. The Ti(4 Al, 4 Mn) alloy degraded the fluid mainly by chain scission processes; this took place to a much lesser degree with M-50

A solvable non-conservative model of Self-Organized Criticality

We present the first solvable non-conservative sandpile-like critical model of Self-Organized Criticality (SOC), and thereby substantiate the suggestion by Vespignani and Zapperi [A. Vespignani and S. Zapperi, Phys. Rev. E 57, 6345 (1998)] that a lack of conservation in the microscopic dynamics of an SOC-model can be compensated by introducing an external drive and thereby re-establishing criticality. The model shown is critical for all values of the conservation parameter. The analytical derivation follows the lines of Broeker and Grassberger [H.-M. Broeker and P. Grassberger, Phys. Rev. E 56, 3944 (1997)] and is supported by numerical simulation. In the limit of vanishing conservation the Random Neighbor Forest Fire Model (R-FFM) is recovered.Comment: 4 pages in RevTeX format (2 Figures) submitted to PR

Avionics test bed development plan

A development plan for a proposed avionics test bed facility for the early investigation and evaluation of new concepts for the control of large space structures, orbiter attached flex body experiments, and orbiter enhancements is presented. A distributed data processing facility that utilizes the current laboratory resources for the test bed development is outlined. Future studies required for implementation, the management system for project control, and the baseline system configuration are defined. A background analysis of the specific hardware system for the preliminary baseline avionics test bed system is included