Resistivity and optical conductivity of cuprates within the t-J model

The optical conductivity $\sigma(\omega)$ and the d.c. resistivity $\rho(T)$ within the extended t-J model on a square lattice, as relevant to high-$T_c$ cuprates, are reinvestigated using the exact-diagonalization method for small systems, improved by performing a twisted boundary condition averaging. The influence of the next-nearest-neighbor hopping $t'$ is also considered. The behaviour of results at intermediate doping is consistent with a marginal-Fermi-liquid scenario and in the case of $t'=0$ for $\omega>T$ follows the power law $\sigma \propto \omega^{-\nu}$ with $\nu \sim 0.65$ consistent with experiments. At low doping $c_h<0.1$ for $T<J$ $\sigma(\omega)$ develops a shoulder at $\omega\sim \omega^*$, consistent with the observed mid-infrared peak in experiments, accompanied by a shallow dip for $\omega < \omega^*$. This region is characterized by the resistivity saturation, whereas a more coherent transport appears at $T < T^*$ producing a more pronounced decrease in $\rho(T)$. The behavior of the normalized resistivity $c_h \rho(T)$ is within a factor of 2 quantitatively consistent with experiments in cuprates.Comment: 8 pages, 10 figure

Edge Electron Gas

The uniform electron gas, the traditional starting point for density-based many-body theories of inhomogeneous systems, is inappropriate near electronic edges. In its place we put forward the appropriate concept of the edge electron gas.Comment: 4 pages RevTex with 7 ps-figures included. Minor changes in title,text and figure

Burial and seed survival in Brassica napus subsp. oleifera and Sinapis arvensis including a comparison of transgenic and non-transgenic lines of the crop

The creation of transgenic plants through genetic engineering has focused interest on how the fitness of a plant species may be altered by small changes in its genome. This study concentrates on a key component of fitness: persistence of seeds overwinter. Seeds of three lines of oilseed rape (Brassica napus subsp. oleifera DC Metzger) and of charlock (Sinapis arvensis L.) were buried in nylon mesh bags at two depths in four habitats in each of three geographically separated sites: Cornwall, Berkshire and Sutherland. Seeds were recovered after 12 and 24 months. Charlock exhibited much greater seed survival (average 60 per cent surviving the first year and 32.5 per cent surviving the second year) than oilseed rape (1.5 per cent surviving the first year and 0.2 per cent surviving the second) at all sites. Charlock showed higher survival at 15 cm burial than 2 cm burial at certain sites, but oilseed rape showed no depth effect. Different genetic lines of oilseed rape displayed different rates of seed survival; non-transgenic rape showed greater survival (2 per cent) than the two transgenic lines, one developed for tolerance to the antibiotic kanamycin (0.3 per cent) and one for tolerance to both kanamycin and the herbicide glufosinate (0.25 per cent). The absolute and relative performances of the different genetic lines of oilseed rape were context specific, illustrating the need to test hypotheses in a wide range of ecological settings

F as in Fat: How Obesity Threatens America's Future 2011

Outlines 2008-10 national and state obesity rates, health indicators, and policies to address the epidemic; regional, economic, and social barriers to healthy choices; impact of the 2010 healthcare reform and Let's Move initiative; and recommendations

The Decay Properties of the Finite Temperature Density Matrix in Metals

Using ordinary Fourier analysis, the asymptotic decay behavior of the density matrix F(r,r') is derived for the case of a metal at a finite electronic temperature. An oscillatory behavior which is damped exponentially with increasing distance between r and r' is found. The decay rate is not only determined by the electronic temperature, but also by the Fermi energy. The theoretical predictions are confirmed by numerical simulations

Generalization of the density-matrix method to a non-orthogonal basis

We present a generalization of the Li, Nunes and Vanderbilt density-matrix method to the case of a non-orthogonal set of basis functions. A representation of the real-space density matrix is chosen in such a way that only the overlap matrix, and not its inverse, appears in the energy functional. The generalized energy functional is shown to be variational with respect to the elements of the density matrix, which typically remains well localized.Comment: 11 pages + 2 postcript figures at the end (search for -cut here