"Pair" Fermi contour and repulsion-induced superconductivity in cuprates

The pairing of charge carriers with large pair momentum is considered in connection with high-temperature superconductivity of cuprate compounds. The possibility of pairing arises due to some essential features of quasi-two-dimensional electronic structure of cuprates: (i) The Fermi contour with strong nesting features; (ii) The presence of extended saddle point near the Fermi level; (iii) The existence of some ordered state (for example, antiferromagnetic) close to the superconducting one as a reason for an appearing of "pair" Fermi contour resulting from carrier redistribution in momentum space. In an extended vicinity of the saddle point, momentum space has hyperbolic (pseudoeuclidean) metrics, therefore, the principal values of two-dimensional reciprocal reduced effective mass tensor have unlike signs. Rearrangement of holes in momentum space results in a rise of "pair" Fermi contour which may be defined as zero-energy line for relative motion of the pair. The superconducting gap arises just on this line. Pair Fermi contour formation inside the region of momentum space with hyperbolic metrics results in not only superconducting pairing but in a rise of quasi-stationary state in the relative motion of the pair. Such a state has rather small decay and may be related to the pseudogap regime of underdoped cuprates. It is concluded that the pairing in cuprates may be due to screened Coulomb repulsion. In this case, the superconducting energy gap in hole-doped cuprates exists in the region of hole concentration which is bounded both above and below. The superconducting state with positive condensation energy exists in more narrow range of doping level inside this region. Such hole concentration dependence correlates with typical phase diagram of cuprates.Comment: 23 pages, 11 figures. Submitted to Phys. Rev.

Green function techniques in the treatment of quantum transport at the molecular scale

The theoretical investigation of charge (and spin) transport at nanometer length scales requires the use of advanced and powerful techniques able to deal with the dynamical properties of the relevant physical systems, to explicitly include out-of-equilibrium situations typical for electrical/heat transport as well as to take into account interaction effects in a systematic way. Equilibrium Green function techniques and their extension to non-equilibrium situations via the Keldysh formalism build one of the pillars of current state-of-the-art approaches to quantum transport which have been implemented in both model Hamiltonian formulations and first-principle methodologies. We offer a tutorial overview of the applications of Green functions to deal with some fundamental aspects of charge transport at the nanoscale, mainly focusing on applications to model Hamiltonian formulations.Comment: Tutorial review, LaTeX, 129 pages, 41 figures, 300 references, submitted to Springer series "Lecture Notes in Physics

Approche de la génétique de populations parasites du genre Pediculus par l'analyse isoenzymatique

International audienc

International guidelines for effective control of head louse infestations

Head louse infestations are increasing or remain high in most countries. In order to reduce the proportion of children infested with head lice and slow down the emergence of strains of lice resistant to pediculicides, more active involvement of health and educational authorities, as well as parents, is of paramount importance. We suggest that health authorities should introduce more efficient methods for evaluating pediculicides and more stringent regulations for adoption of new anti-louse products. Baseline studies are also essential for new pediculicides. Children should be properly screened, especially in problematic areas. The media should be used to educate parents on louse control. Health providers need to be aware of which anti-louse remedies are demonstrably effective and be capable of assisting families with louse control. Academic institutions should conduct baseline and efficacy studies on pediculicides and other treatment modalities, as well as research on the biology and epidemiology of lice. Parents should regularly inspect their children, treat as necessary, and try to avoid creating stigmas and emotional problems for the child. The pharmaceutical industry should aim to introduce pediculicides based on new chemical compounds, especially natural products. Companies should develop effective and safe repellents and nit removal remedies. General recommendations are given on how to diagnose and treat louse infestations with chemicals, biological agents, and louse combs and how to protect children from infestations. The no-nit policy, based on the persistence of empty egg cases, is not justified and does more harm than good; therefore, we recommend that it be immediately halted