The development and control of traffic jams caused by incidents in rectangular grid networks.

Urban traffic congestion is becoming a central issue in transport planning. If the present growth in car ownership and use continues, traffic jams are likely to increase in frequency and extent, particularly within the central areas of major cities. Whilst it is important to study the impact of congestion in the field, there is an urgent need for a fundamental understanding of the causes of congestion and the way in which it propagates. But, although a number of control schemes for controlling traffic congestion exist, no comprehensive rationale for an effective dispersal strategy has been developed. This research is mainly concerned with the properties of incident-induced traffic jams on rectangular grid networks, and possible measures for preventing and controlling them. The research investigates the underlying structure of such jams using a combination of theoretical and simulation models developed for this purpose. Using these models, gridlock is identified as a crucial stage in the evolution of traffic jams. However, most conventional traffic management measures aim to increase capacity and hence postpone the onset of gridlock and are unsuitable when gridlock has already set in. This thesis develops several alternative strategies for protecting networks from gridlock and dissipating traffic jams once they have formed. The treatment focuses on the installation of bans at specific network locations. The bans come in two forms: turn or ahead. Turn bans are imposed on selected links to break gridlock cycles at the nucleus of the traffic jam. By contrast, ahead bans are implemented around the traffic jam envelope to reduce input into critical sections of the road. The control strategies are tested extensively using the simulation model and as a result, some general control principles have emerged. These are not intended to be immediately applicable to real networks since they incorporate some simplifying assumptions. However, they point to certain characteristics of traffic jam growth and dispersal which would not be accessible in any other way

Photoinduced Changes of Reflectivity in Single Crystals of YBa2Cu3O6.5 (Ortho II)

We report measurements of the photoinduced change in reflectivity of an untwinned single crystal of YBa2Cu3O6.5 in the ortho II structure. The decay rate of the transient change in reflectivity is found to decrease rapidly with decreasing temperature and, below Tc, with decreasing laser intensity. We interpret the decay as a process of thermalization of antinodal quasiparticles, whose rate is determined by an inelastic scattering rate of quasiparticle pairs.Comment: 4 pages, 4 figure

Zero temperature optical conductivity of ultra-clean Fermi liquids and superconductors

We calculate the low-frequency optical conductivity sigma(w) of clean metals and superconductors at zero temperature neglecting the effects of impurities and phonons. In general, the frequency and temperature dependences of sigma have very little in common. For small Fermi surfaces in three dimensions (but not in 2D) we find for example that Re sigma(w>0)=const. for low w which corresponds to a scattering rate Gamma proportional to w^2 even in the absence of Umklapp scattering when there is no T^2 contribution to Gamma. In the main part of the paper we discuss in detail the optical conductivity of d-wave superconductors in 2D where Re sigma(w>0) \propto w^4 for the smallest frequencies and the Umklapp processes typically set in smoothly above a finite threshold w_0 smaller than twice the maximal gap Delta. In cases where the nodes are located at (pi/2, pi/2), such that direct Umklapp scattering among them is possible, one obtains Re sigma(w) \propto w^2.Comment: 7 pages, 3 figure

Distribution of phytoplasma in grapevines in the Golan Heights, Israel, and development of a new universal primer

Our survey, made in 1999 and 2000, sampled 4 of the most prevalent grapevine cultivars in northern, central and southern regions of the Golan Heights. There were significant differences in the levels of phytoplasma infection between different sub-regions of the Golan Heights; highest levels were found in the south, followed by the center, and the lowest levels in the north. Stolbur was found to be the predominant phytoplasma (~70 %), although aster yellow (~11 %), western-X (~5 %) and mixtures of two phytoplasmas (13 % of all infections, 90 % of which involved Stol and AY) were also found. Prior research on phytoplasmas of grapevines in Israel involved the use of two sets of universal primers. Primers were developed specifically for phytoplasmas occurring in Israel in which there were homologies of 90,85 and 78 % for Stol,AY and W-X phytoplasmas, respectively.

Nondiffusive spin dynamics in a two-dimensional electron gas

We describe measurements of spin dynamics in the two-dimensional electron gas in GaAs=GaAlAs quantum wells. Optical techniques, including transient spin-grating spectroscopy, are used to probe the relaxation rates of spin polarization waves in the wave vector range from zero to 6 x 104 cm-1. We find that the spin polarization lifetime is maximal at a nonzero wave vector, in contrast with expectations based on ordinary spin diffusion, but in quantitative agreement with recent theories that treat diffusion in the presence of spin-orbit couplin

Observation of ferromagnetic resonance in strontium ruthenate (SrRuO3)

We report the observation of ferromagnetic resonance (FMR) in SrRuO3 using the time-resolved magneto-optical Kerr effect. The FMR oscillations in the time-domain appear in response to a sudden, optically induced change in the direction of easy-axis anistropy. The high FMR frequency, 250 GHz, and large Gilbert damping parameter, alpha ~ 1, are consistent with strong spin-orbit coupling. We find that the parameters associated with the magnetization dynamics, including alpha, have a non-monotonic temperature dependence, suggestive of a link to the anomalous Hall effect.Comment: submitted to Phys. Rev. Let