Covalent effects in molecule-surface charge exchange: oxygen on silver(111)

The formation of neg. charged ions (O2-, O-) in the scattering of oxygen from silver surfaces is difficult to explain with the Brako-Newns scheme. Extended-Hueckel tight-binding calcns. are performed on model Ag(111) slabs, placing O2 adsorbates at various sites and mol. orientations. As the survival probability of a neg. charged ion leaving the surface is detd. by the back-tunneling frequency, the mol. ion survival probably should increase as the O2-Ag interaction decreases, and vice versa. On the other hand, the dissocn. probability will increase with increasing interaction strength. The strength of the O2-Ag interaction can be gaged by the dispersion found in the projected d. of states of the O2 affinity level 1pg. The second moment serves as a measure of relative dispersion, and in addn., can be related to the tunneling frequency. The bridging and hollow sites have the strongest O2-Ag interaction, thus the lowest survival probability of neg. ions and the highest degree of dissocn. The interaction at top sites is much less. These results can be used to explain the higher O-/(O- + O2-) ratio obsd. at more grazing incident beam angle

Covalent effects in molecule-surface charge exchange: oxygen on silver(111)

The formation of neg. charged ions (O2-, O-) in the scattering of oxygen from silver surfaces is difficult to explain with the Brako-Newns scheme. Extended-Hueckel tight-binding calcns. are performed on model Ag(111) slabs, placing O2 adsorbates at various sites and mol. orientations. As the survival probability of a neg. charged ion leaving the surface is detd. by the back-tunneling frequency, the mol. ion survival probably should increase as the O2-Ag interaction decreases, and vice versa. On the other hand, the dissocn. probability will increase with increasing interaction strength. The strength of the O2-Ag interaction can be gaged by the dispersion found in the projected d. of states of the O2 affinity level 1pg. The second moment serves as a measure of relative dispersion, and in addn., can be related to the tunneling frequency. The bridging and hollow sites have the strongest O2-Ag interaction, thus the lowest survival probability of neg. ions and the highest degree of dissocn. The interaction at top sites is much less. These results can be used to explain the higher O-/(O- + O2-) ratio obsd. at more grazing incident beam angle

The STACEE-32 Ground Based Gamma-ray Detector

We describe the design and performance of the Solar Tower Atmospheric Cherenkov Effect Experiment detector in its initial configuration (STACEE-32). STACEE is a new ground-based gamma ray detector using the atmospheric Cherenkov technique. In STACEE, the heliostats of a solar energy research array are used to collect and focus the Cherenkov photons produced in gamma-ray induced air showers. The large Cherenkov photon collection area of STACEE results in a gamma-ray energy threshold below that of previous detectors.Comment: 45 pages, 25 figures, Accepted for publication in Nuclear Instruments and Methods

Thin-shell wormholes with a generalized Chaplygin gas in Einstein-Born-Infeld theory

We construct spherically symmetric thin-shell wormholes supported by a generalized Chaplygin gas in Born-Infeld electrodynamics coupled to Einstein gravity, and we analyze their stability under radial perturbations. For different values of the Born-Infeld parameter and the charge, we compare the results with those obtained in a previous work for Maxwell electrodynamics. The stability region in the parameter space reduces and then disappears as the value of the Born-Infeld parameter is modified in the sense of a larger departure from Maxwell theory.Comment: 9 pages, 6 figures; v2: improved versio

What is a sustainable healthy diet? A discussion paper

The food system today is destroying the environment upon which future food production depends. While the food system generates enough food energy for our population of over 7 billion it does not deliver adequate and affordable nutrition for all. About half the global population is inadequately or inappropriately nourished. Without action, these problems are set to become acute. As our global population grows, urbanises and becomes wealthier, it is demanding more resource intensive, energy rich foods. What, and how much we eat directly affects what, and how much is produced. We therefore need to consume more „sustainable diets‟ – diets that have lower environmental impacts, and are healthier. But what does such a diet look like? Can health, environmental sustainability, and all the other goals we have for our food system really be reconciled, or will there be trade offs

NMR and NQR Fluctuation Effects in Layered Superconductors

We study the effect of thermal fluctuations of the s-wave order parameter of a quasi two dimensional superconductor on the nuclear spin relaxation rate near the transition temperature Tc. We consider both the effects of the amplitude fluctuations and the Berezinskii-Kosterlitz-Thouless (BKT) phase fluctuations in weakly coupled layered superconductors. In the treatment of the amplitude fluctuations we employ the Gaussian approximation and evaluate the longitudinal relaxation rate 1/T1 for a clean s-wave superconductor, with and without pair breaking effects, using the static pair fluctuation propagator D. The increase in 1/T1 due to pair breaking in D is overcompensated by the decrease arising from the single particle Green's functions. The result is a strong effect on 1/T1 for even a small amount of pair breaking. The phase fluctuations are described in terms of dynamical BKT excitations in the form of pancake vortex-antivortex (VA) pairs. We calculate the effect of the magnetic field fluctuations caused by the translational motion of VA excitations on 1/T1 and on the transverse relaxation rate 1/T2 on both sides of the BKT transitation temperature T(BKT)<Tc. The results for the NQR relaxation rates depend strongly on the diffusion constant that governs the motion of free and bound vortices as well as the annihilation of VA pairs. We discuss the relaxation rates for real multilayer systems where the diffusion constant can be small and thus increase the lifetime of a VA pair, leading to an enhancement of the rates. We also discuss in some detail the experimental feasibility of observing the effects of amplitude fluctuations in layered s-wave superconductors such as the dichalcogenides and the effects of phase fluctuations in s- or d-wave superconductors such as the layered cuprates.Comment: 38 pages, 12 figure