Kinetic Freeze-out and Radial Flow in 11.6 A GeV Au+Au Collisions

We study the kinetic freeze-out conditions of hadrons in Au+Au collisions at 11.6 A GeV/c using different parametrizations of an expanding thermal fireball. We take into account the available double differential momentum spectra of a variety of particle species, covering a large fraction of the total momentum space. The overall fit to the data is very good and indicates a relatively low kinetic freeze-out temperature of about 90 MeV with an average transverse expansion velocity at midrapidity of about 0.5 c.Comment: 5 pages ReVTeX, incl. 3 figures and 2 tables. Revised version with improved discussion of model and added references. Accepted by Phys. Lett.

Leaf Area, Competition with Grass, and Clover Cultivar: Key Factors to Successful Overwintering and Fast Regrowth of White Clover (Trifolium repens L.) in Spring

The greater sensitivity of white clover (Trifolium repens L.) to low temperature compared with perennial ryegrass (Lolium perenne L.) is a major problem in sustaining the relative contributions to yield of the two species in mixed swards. The objectives of this investigation were to examine the dynamics of leaf development of two white clover cultivars, AberHerald and Grasslands Huia, under field conditions, and to determine the significance of leaf area in winter, and of competition by perennial ryegrass, for the overwintering and regrowth of white clover in spring. Undefoliated white clover plants developed 3·8-6·6 new leaves between late autumn and early spring, and stolon dry matter and total non-structural carbohydrates (TNC) content increased by 262 and by 68% respectively. In contrast, white clover plants that were defoliated frequently during the winter showed a 28% decrease in stolon dry matter and an 82% decrease in the content of TNC. Frequent defoliation in winter caused severe reduction in the rates of emergence of nodes (by 60%) and of buds (by 67%), and the rate of death of nodes and buds increased by a factor of 10, leading to small stolon systems of individual plants in spring. Competition had similar, but weaker, effects to those of winter defoliation, presumably caused by shading of white clover leaves. AberHerald had a higher cumulative leaf emergence (by 22%), a higher mean leaf number (by 23%), a higher stolon DM (by 36%) and a higher TNC content per plant (by 115%) than Grasslands Huia. Results demonstrate the crucial positive role of leaf area during winter, the negative effect of grass competition, and the importance of the clover cultivar, for the overwintering and subsequent spring regrowth of white clove

Ultrafast coherent oscillations reveal a reactive mode in the ring-opening reaction of fulgides

The ultrafast ring-opening reaction of photochromic fulgides proceeds via conical intersections to the ground state isomers involving activation barriers in the excited state. The coherent oscillations observed in the femtosecond transient absorption signal of a methyl-substituted indolylfulgide were analysed in the framework of vibrational wavepackets to expose a dominant low-frequency mode at similar to 80 cm(-1). The quantum chemical calculations in the relaxed excited state geometry of this fulgide revealed that the experimentally observed vibrational normal mode has a dominant contribution to the relevant ring-opening reactive coordinate

Discrimination between FRET and non-FRET quenching in a photochromic CdSe quantum dot/dithienylethene dye system

A photochromic Förster resonance energy transfer (FRET) system was employed to disentangle the fluorescence quenching mechanisms in quantum dot/photochromic dye hybrids. In the off-state of the dye the main quenching mechanism is FRET whereas the moderate quenching in the on-state is due to non-FRET pathways opened up upon assembly

On the driven Frenkel-Kontorova model: II. Chaotic sliding and nonequilibrium melting and freezing

The dynamical behavior of a weakly damped harmonic chain in a spatially periodic potential (Frenkel-Kontorova model) under the subject of an external force is investigated. We show that the chain can be in a spatio-temporally chaotic state called fluid-sliding state. This is proven by calculating correlation functions and Lyapunov spectra. An effective temperature is attributed to the fluid-sliding state. Even though the velocity fluctuations are Gaussian distributed, the fluid-sliding state is clearly not in equilibrium because the equipartition theorem is violated. We also study the transition between frozen states (stationary solutions) and=7F molten states (fluid-sliding states). The transition is similar to a first-order phase transition, and it shows hysteresis. The depinning-pinning transition (freezing) is a nucleation process. The frozen state contains usually two domains of different particle densities. The pinning-depinning transition (melting) is caused by saddle-node bifurcations of the stationary states. It depends on the history. Melting is accompanied by precursors, called micro-slips, which reconfigurate the chain locally. Even though we investigate the dynamics at zero temperature, the behavior of the Frenkel-Kontorova model is qualitatively similar to the behavior of similar models at nonzero temperature.Comment: Written in RevTeX, 13 figures in PostScript, appears in PR

On the driven Frenkel-Kontorova model: I. Uniform sliding states and dynamical domains of different particle densities

The dynamical behavior of a harmonic chain in a spatially periodic potential (Frenkel-Kontorova model, discrete sine-Gordon equation) under the influence of an external force and a velocity proportional damping is investigated. We do this at zero temperature for long chains in a regime where inertia and damping as well as the nearest-neighbor interaction and the potential are of the same order. There are two types of regular sliding states: Uniform sliding states, which are periodic solutions where all particles perform the same motion shifted in time, and nonuniform sliding states, which are quasi-periodic solutions where the system forms patterns of domains of different uniform sliding states. We discuss the properties of this kind of pattern formation and derive equations of motion for the slowly varying average particle density and velocity. To observe these dynamical domains we suggest experiments with a discrete ring of at least fifty Josephson junctions.Comment: Written in RevTeX, 9 figures in PostScrip