Quantized enveloping algebras for Borcherds superalgebras

We construct quantum deformations of enveloping algebras of Borcherds superalgebras, their Verma modules, and their irreducible highest weight modules.

     

High-Resolution Laser Spectroscopy of YbCl: The A(2)Pi-X(2)Sigma(+) Transition

The A(2)Pi-X(2)Sigma(+) transition of (174)Yb(35)Cl and (172)Yb(35)Cl has been rotationally analyzed for the first time. Doppler-limited laser excitation spectroscopy with selective detection of fluorescence was used to obtain spectra of the 0-0 and 1-0 bands with a measurement accuracy of approximately 0.0035 cm(-1). Resolved fluorescence was used to record the 0-1, 0-2, and 0-3 bands and to unequivocally assign the rotational numbering, N, to the laser excitation spectra. In total, over 1300 line positions have been measured and assigned for each of the two isotopomers and employed in least-squares fits of molecular parameters. The principal results for the A(2)Pi state are A(e) = 1491.494(2) cm(-1) and R(e) = 2.4433(1) ?, and for the X(2)Sigma(+) state, R(e) = 2.4883(2) ? and gamma(e) = 4.59(2) x 10(-3) cm(-1). The interaction between the X(2)Sigma(+) and A(2)Pi states has been investigated and is shown to be the main contributor to the spin-rotation splitting in the ground state. Copyright 2000 Academic Press.     

Hydrophobic silica aerogels prepared via rapid supercritical extraction

Hydrophobic silica aerogels have been prepared using the rapid supercritical extraction (RSCE) technique. The RSCE technique is a one-step methanol supercritical extraction method for producing aerogel monoliths in 3 to 8 h. Standard aerogels were prepared from a tetramethoxysilane (TMOS) recipe with a molar ratio of TMOS:MeOH:H₂O:NH₄OH of 1.0:12.0:4.0:7.4 × 10⁻³. Hydrophobic aerogels were prepared using the same recipe except the TMOS was replaced with a mixture of TMOS and one of the following organosilane co-precursors: methytrimethoxysilane (MTMS), ethyltrimethoxysilane (ETMS), or propyltrimeth-oxysilane (PTMS). Results show that, by increasing the amount of catalyst and increasing gelation time, monolithic aerogels can be prepared out of volume mixtures including up to 75% MTMS, 50% ETMS or 50% PTMS in 7.5–15 h. As the amount of co-precursor is increased the aerogels become more hydrophobic (sessile tests with water droplets yield contact angles up to 155°) and less transparent (transmission through a 12.2-mm thick sample decreases from 83 to 50% at 800 nm). The skeletal and bulk density decrease and the surface area increases (550–760 m²/g) when TMOS is substituted with increasing amounts of MTMS. The amount of co-precursor does not affect the thermal conductivity. SEM imaging shows significant differences in the nanostructure for the most hydrophobic surfaces.     

Two-dimensional Monte Carlo studies of lipid molecules in a bilayer membrane

We present a new model to study in-plane liquid properties of lipid membranes. The different conformations of lipids are represented by a seven-state system of hard triatomic particles, or triples, of varying lengths which correspond to the different cross-sectional areas of the lipids in the plane of the membrane. Two-dimensional Monte Carlo simulations are performed in both the constant NVT and NPT ensembles. The distribution of states has a strong density dependence and a small temperature dependence over the biologically relevant range. There is no long range orientational order in the systems before freezing. The short range orientational order increases with density. Widom's particle insertion method is used to obtain the excess chemical potential of the system for the seven states. These values, along with the pressure, are in excellent agreement with estimates from scaled particle theory.     

A.C. loss and related effects in type II superconductors

This review gathers together information important to the understanding of superconducting materials under a.c. conditions, and points out some features which are at present not too well understood. The idea is to try to build a consistent picture of the performance of superconductors rather than to present a historical account of the subject.

The basic behaviour of fluxons in the bulk of type II superconductors can be explained by the critical state model, but some of the details of flux pinning and the method of flux motion are in doubt. Flux instabilities under d.c. conditions have received a lot of attention and so have the methods of stabilization, but instabilities under a.c. conditions have not been studied in so much detail, and the problem of stabilization is more difficult than for f.c.

Generally surface effects are less well understood than bulk effects, partly because there are a number of possible phenomena to contend with. A.C. loss in the Meissner state can be explained by field enhancement and flux penetration at peaks in the surface. For fields between H _c3 and H _c2 present solutions of the Ginzburg-Landau equations for the currents in the surface sheath do not give sufficient agreement with experiment and some alternative explanation, such as flux pinning, may be necessary. There are a number of effects between H _c2 and H _c1, but the most important are Meissner currents and flux pinning.