Hole pairing induced by antiferromagnetic spin fluctuations

The effective interaction induced by antiferromagnetic spin fluctuations is considered in the random phase approximation in the context of the recently discovered highT _c oxide superconductors. This effective attraction favours a triplet pairing of holes. The implications of such pairing mechanism are discussed in connection with the current experimental observations.     

Reconstruction, disordering and roughening of metal surfaces

Until recently, surface reconstruction and roughening have been studied as separate surface phenomena and have been described theoretically by means of separate models. This approach is reasonable when the deconstruction transition has a displacive character as occurs for example on Mo(001) and W(001) surfaces. In these cases only in-plane degrees of freedom are involved in the reconstruction process. Conversely, a unified approach to reconstruction and roughening seems more appropriate when the reconstructed phase is produced by the low-temperature ordering of a large concentration of point defects (vacancies, adatoms) or extended defects (steps). The reconstruction in these cases involves off-plane degrees of freedom. Since roughening derives from the proliferation of thermally excited steps, the interplay between steps and deconstructive defects may provide the connection between the two transitions. A notable example of these latter systems is provided by the (110) surface of noble and near-noble metals; the (110) surface of the heavier metals (Au, Pt) reconstructs at low temperature in the 2 x 1 missing-row structure. In the present article we review some statistical mechanics models able to display both deconstruction and roughening of the fcc(110) missing-row phase, chosen as a paradigmatic example of the interplay between the two transitions. The properties of the phase between deconstruction and roughening, produced by the different models are analysed, and compared with the results of molecular-dynamics simulations with continous potentials. The fingerprint of the different phases in scattering measurements are discussed and compared with the experimental data available.     

Self-trapping vs. non-trapping of electrons and holes in organic insulators: polyethylene

We show, by electronic structure based molecular dynamics simulations, that an extra electron injected in crystalline polyethylene should fall spontaneously into a self-trapped state, a shallow donor with a large novel distortion pattern involving a pair of trans-gauche defects. Parallel calculations show instead that a hole will remain free and delocalized. We trace the difference of behavior to the intrachain nature of the hole, as opposed to the interchain one of the electron, and argue that applicability of this concept could be more general. Thus electrons (but not holes) should tend to self-trap in saturated organic insulators, but not for example in aromatic insulators, where both carriers are intrachain.     

Expanding the enantioselectivity of the gas-chromatographic chiral stationary phase chirasil-val-C11 by doping it with octakis(3-O-butanoyl-2,6-di-O-n-pentyl)-gamma-cyclodextrin

Combination of the enantioselective properties of the two versatile gas-chromatographic chiral stationary phases (CSPs) octakis(3-O-butanoyl-2,6-di-O-n-pentyl)-gamma-CD (Lipodex E) 1 and L-valine-diamide-based CSP Chirasil-Val-C11 2 has been realized by doping the chiral polymer 2 with the nonpolymeric selector 1. The resulting mixed-mode CSP Chirasil-Val(gamma-Dex) 3 was found to have a greatly improved enantioselectivity toward proline and aspartic acid (as N-trifluoroacetyl ethyl or methyl esters) in comparison to the single-mode CSP 2. The presence of the CD selector in 3 extended the scope of gas-chromatographic enantioseparations achievable on 2 to underivatized alcohols, terpenes, and other chiral compounds that are exclusively enantioseparated on 1.     

Multi-molecule reaction of serum albumin can occur through thiol-yne coupling

The free-radical hydrothiolation of alkynes (thiol-yne coupling, TYC) unites two thiol fragments across the carbon-carbon triple bond to give a dithioether derivative with exclusive 1,2-addition; this reaction can be used for modification of peptides and proteins allowing glycoconjugation and fluorescent labeling. These results have implications not only as a flexible strategy for attaching two modifications at a single site in proteins but also for unanticipated side-reactions of reagents (such as cycloalkynes) used in other protein coupling reactions.     

Construction dʼopérades ensemblistes à partir de monoïdes

We study a functorial construction from the category of monoids to the category of set-operads and we give some combinatorial examples of applications.     

Atomic spin-sensitive dissipation on magnetic surfaces

We identify the mechanism of energy dissipation relevant to spin-sensitive nanomechanics including the recently introduced magnetic exchange force microscopy, where oscillating magnetic tips approach surface atomic spins. The tip-surface exchange couples spin and atom coordinates, leading to a spin-phonon problem with Caldeira-Leggett-type dissipation. In the overdamped regime, that can lead to a hysteretic flip of the local spin with a large spin-dependent dissipation, even down to the very low experimental tip oscillation frequencies, describing recent observations for Fe tips on NiO. A phase transition to an underdamped regime with dramatic drop of magnetic tip dissipation should in principle be possible by tuning tip-surface distance.