The Parallel Magnetoconductance of Interacting Electrons in a Two Dimensional Disordered System

The transport properties of interacting electrons for which the spin degree of freedom is taken into account are numerically studied for small two dimensional diffusive clusters. On-site electron-electron interactions tend to delocalize the electrons, while long-range interactions enhance localization. On careful examination of the transport properties, we reach the conclusion that it does not show a two dimensional metal insulator transition driven by interactions. A parallel magnetic field leads to enhanced resistivity, which saturates once the electrons become fully spin polarized. The strength of the magnetic field for which the resistivity saturates decreases as electron density goes down. Thus, the numerical calculations capture some of the features seen in recent experimental measurements of parallel magnetoconductance.Comment: 10 pages, 6 figure

Identify of a tilapia pheromone released by dominant males that primes females for reproduction

Knowledge of the chemical identity and role of urinary pheromones in fish is scarce, yet necessary to understand the integration of multiple senses in adaptive responses and the evolution of chemical communication. In nature, Mozambique tilapia (Oreochromis mossambicus) males form hierarchies and females mate preferentially with dominant territorial males which they visit in aggregations or leks

Antiferromagnetism and single-particle properties in the two-dimensional half-filled Hubbard model: a non-linear sigma model approach

We describe a low-temperature approach to the two-dimensional half-filled Hubbard model which allows us to study both antiferromagnetism and single-particle properties. This approach ignores amplitude fluctuations of the antiferromagnetic (AF) order parameter and is valid below a crossover temperature $T_X$ which marks the onset of AF short-range order. Directional fluctuations (spin waves) are described by a non-linear sigma model (NL$\sigma$M) that we derive from the Hubbard model. At zero temperature and weak coupling, our results are typical of a Slater antiferromagnet. The AF gap is exponentially small; there are well-defined Bogoliubov quasi-particles (QP's) (carrying most of the spectral weight) coexisting with a high-energy incoherent excitation background. As $U$ increases, the Slater antiferromagnet progressively becomes a Mott-Heisenberg antiferromagnet. The Bogoliubov bands evolve into Mott-Hubbard bands separated by a large AF gap. A significant fraction of spectral weight is transferred from the Bogoliubov QP's to incoherent excitations. At finite temperature, there is a metal-insulator transition between a pseudogap phase at weak coupling and a Mott-Hubbard insulator at strong coupling. Finally, we point out that our results straightforwardly translate to the half-filled attractive Hubbard model, where the ${\bf q}=(\pi,\pi)$ charge and ${\bf q}=0$ pairing fluctuations combine to form an order parameter with SO(3) symmetry.Comment: Revtex4, 19 pages, 14 figures; (v2) final version as publishe

Tilapia male urinary pheromone stimulates female reproductive axis

Mozambique tilapia males congregate in leks where they establish dominance hierarchies and attract females to spawn in sandy pits. Dominant males store more urine than subordinates and the pattern of urination and the high sensitivity of females to male urine suggest chemical signalling via the urine. Here we show that pre-ovulated and post-spawn females when exposed to dominant male urine increased significantly, in less than 1 h, the release rate of the maturation-inducing steroid 17,20bdihydroxypregn- 4-en-3-one which is maintained elevated for at least 6 h. This indicates a pheromonal role for male urine in the synchronisation of spawning. Furthermore, we show that the lack of affinity of 17,20bP to sex steroid binding globulin explains, at least partly, its rapid release and lack of detection in the blood. Thus tilapia urine involvement in several communication processes confirms that cichlids have evolved a sophisticated chemical signalling system together with their complex visual, acoustic and behavioural displays

Two-time Green function method in quantum electrodynamics of high-Z few-electron atoms

The two-time Green function method in quantum electrodynamics of high-Z few-electron atoms is described in detail. This method provides a simple procedure for deriving formulas for the energy shift of a single level and for the energies and wave functions of degenerate and quasi-degenerate states. It also allows one to derive formulas for the transition and scattering amplitudes. Application of the method to resonance scattering processes yields a systematic theory for the spectral line shape. The practical ability of the method is demonstrated by deriving formulas for the QED and interelectronic-interaction corrections to energy levels and transition and scattering amplitudes in one-, two-, and three-electron atoms. Numerical calculations of the Lamb shift, the hyperfine splitting, the bound-electron g factor, and the radiative recombination cross section in heavy ions are also reviewed.Comment: 92 pages, 39 figures, 7 table

Designing animal welfare policies and monitoring progress

A single solution to promote higher animal welfare across the whole EU is unlikely due to significant regional differences and because what is most appropriate for each region depends on many factors. Based on analyses of eight member and candidate EU countries, this paper provides a conceptual framework, an 'animal welfare roadmap', which can be used to assess the stage of maturity of a country in farm animal welfare policy development and identify appropriate policy instruments and indicators to monitor progress towards higher animal welfare. The 'roadmap' consists of five sequential stages: increasing compliance with legislation; raising awareness; product development; mainstreaming; and integration of animal welfare with other issues. For each stage, specific policy instruments are identified alongside the category of stakeholders most likely to be influential in the implementation of each instrument. The policy instruments used to achieve these stages are those used by government departments/agencies, private enterprises, academic bodies or non-governmental organisations who formulate standards for animal welfare. These are supported by indicators best suited to document their effectiveness. Although we have emphasised how different situations and contexts within the EU mean that there is no single optimal policy instrument for the EU as a whole, but rather appropriate policy instruments should be selected according to the stage of development of a country or sector, we do propose a harmonised choice of indicators to allow benchmarking of changes at the EU level with regard to progress towards animal welfare

Olfactory Sensitivity to Amino Acids in the Blackspot Seabream (Pagellus Bogaraveo, Brünnich 1768): How Effective is the Electro-olfactogram in Seawater?

Intensive aquaculture is heavily dependent on high-quality fishmeal to meet their protein requirements. However, due to the increased demand, decreased availability, raising costs and the potential presence of organic and inorganic contaminants, it is of economical and environmental interest to replace fishmeal by a renewable protein sources. This is particularly challenging since replacing fishmeal may lack appropriate chemical stimulants for active consumption. Fish have a well-defined olfactory sensitivity to L-amino acids (Hara, 1994), and this phenomenon is generally held to be involved in the location and identification of food (Carr et al., 1996). Olfactory sensitivity in fish can be assessed by electro-olfactogram (EOG); a field potential recorded from just above the olfactory epithelium (Scott and Scott-Johnson, 2002). Due to the high electrical conductivity of seawater, EOG responses recorded in marine fish are, however, of lower amplitude than those recorded in freshwater fish (Silver et al., 1976). It is unclear, however, to what extent EOG recording in seawater may underestimate the olfactory sensitivity of a given species when compared to more invasive recording methods, such as multi-unit recording from the olfactory nerve. The blackspot seabream is a promising new species for the finfish aquaculture industry; however, little is known about its biology. The current study investigated its olfactory sensitivity to amino acids and compared the efficacy of two different experimental approaches: multi-unit recording from the olfactory nerve and the electro-olfactogram (EOG). Cited references: Carr W.E.S., J.C. Netherton, R.A. Gleeson and C.D. Derby. 1996. Stimulants of feeding behavior in fish: analyses of tissues of diverse marine organisms. Biological Bulletin 190: 149-160. Hara T.J. 1994. The diversity of chemical stimulation in fish olfaction and gustation. Reviews in Fish Biology and Fisheries 4: 1-35. Scott J.W. and P. E. Scott-Johnson. 2002. The electro-olfactogram: a review of its history and uses. Microscopy Research and Technique 58: 152-160. Silver W.L., J. Caprio, J.F. Blackwell and D. Tucker. 1976. The underwater electro-olfactogram: a tool for the study of the sense of smell of marine fishes. Experientia 32: 1216-1217