Exact Casimir interaction between eccentric cylinders

The Casimir force is the ultimate background in ongoing searches of extra-gravitational forces in the micrometer range. Eccentric cylinders offer favorable experimental conditions for such measurements as spurious gravitational and electrostatic effects can be minimized. Here we report on the evaluation of the exact Casimir interaction between perfectly conducting eccentric cylinders using a mode summation technique, and study different limiting cases of relevance for Casimir force measurements, with potential implications for the understanding of mechanical properties of nanotubes.Comment: 5 pages, 4 figure

Water flows through mussel rafts and their relationship with wind speed in a coastal embayment (Ría de Ares-Betanzos, NW Spain)

14 páginas, 4 tablas, 8 figurasKnowledge of water flows through mussel rafts and their controlling factors is required for an ecosystem approach to the sustainable management of this culture in the Galician rías. With this aim, 4 acoustic 2D-ACM current meters were hung from the bow of 4 rafts located in the mussel cultivation areas of the Ría de Ares-Betanzos (NW Spain) during autumn 2007. Simultaneously, an Aanderaa DCM12 Doppler profiler was moored in an area free of rafts in the middle ría. There were differences in the subtidal and tidal dynamics of the middle channel and mussel farm areas. The tide explained 51.5% of the total variance of the surface current in the middle ría. The explained variance in the seed collection areas of Redes (inner ría) and Miranda (outer ría), where only 2–3 rafts are anchored, were 64.1% and 16.8%, respectively. In the cultivation areas of Arnela (inner ría) and Lorbé (middle ría), where 101 and 40 rafts are anchored, 14.3% and 53.4% of the total variance was explained by the tide. These disparities in the contribution of the tide are likely due to a combination of topographic and bathymetric differences among sites and distortions of the natural flow by the rafts and their hanging ropes. Furthermore, there was a marked influence of winds on the subtidal currents within the rafts; contrasting correlation coefficients and lag times between wind speed and currents were observed for the outer and inner sides of the embayment. The filtration rate of the growing mussels and the number of mussels per raft allow an efficient clearing of the particles transported across the hanging ropes by the measured subtidal currents of 2–3 cm s–1 characteristic of the cultivation areas of Arnela and LorbéPROINSA Mussel Farm, codes CSIC20061089 and 0704101100001Xunta de Galicia PGIDIT06RMA018E and PGIDIT09MMA038EESSMA project Spain–Canada Grant of MICINNPeer reviewe

Investigation on the role of red fox in tuberculosis maintenance community ¿ second opus: experimental infection with a virulent field Mycobacterium bovis strain

Trabajo presentado al: 69th Wildlife Disease Association and 14th European Wildlife Disease Association Conference. Cuenca, Spain. p. 135. 31 agosto-2 septiembre

Exact zero-point interaction energy between cylinders

We calculate the exact Casimir interaction energy between two perfectly conducting, very long, eccentric cylindrical shells using a mode summation technique. Several limiting cases of the exact formula for the Casimir energy corresponding to this configuration are studied both analytically and numerically. These include concentric cylinders, cylinder-plane, and eccentric cylinders, for small and large separations between the surfaces. For small separations we recover the proximity approximation, while for large separations we find a weak logarithmic decay of the Casimir interaction energy, typical of cylindrical geometries.Comment: 20 pages, 7 figure

Intermediate Valence Model for the Colossal Magnetoresistance in Tl_{2}Mn_{2}O_{7}

The colossal magnetoresistance exhibited by Tl_{2}Mn_{2}O_{7} is an interesting phenomenon, as it is very similar to that found in perovskite manganese oxides although the compound differs both in its crystalline structure and electronic properties from the manganites. At the same time, other pyrochlore compounds, though sharing the same structure with Tl_{2}Mn_{2}O_{7}, do not exhibit the strong coupling between magnetism and transport properties found in this material. Mostly due to the absence of evidence for significant doping into the Mn-O sublattice, and the tendency of Tl to form conduction bands, the traditional double exchange mechanism mentioned in connection with manganites does not seem suitable to explain the experimental results in this case. We propose a model for Tl_{2}Mn_{2}O_{7} consisting of a lattice of intermediate valence ions fluctuating between two magnetic configurations, representing Mn-3d orbitals, hybridized with a conduction band, which we associate with Tl. This model had been proposed originally for the analysis of intermediate valence Tm compounds. With a simplified treatment of the model we obtain the electronic structure and transport properties of Tl_{2}Mn_{2}O_{7}, with good qualitative agreement to experiments. The presence of a hybridization gap in the density of states seems important to understand the reported Hall data.Comment: 8 pages + 5 postscript fig

Dynamics of d-wave Vortices: Angle-Dependent Nonlinear Hall Effect

We study the dynamics of vortices in d-wave superconductors using a phenomenological time-dependent Ginzburg-Landau equation with mixing of s- and d-wave components. We present numerical simulations under an external driving current $J$ oriented with an angle $\phi$ with respect to the $b$ crystal axis, calculating the vortex motion and induced electric fields for $\kappa=\infty$. We find an intrinsic Hall effect for $\phi\neq 0$ which depends as $\sim\sin(4\phi)$, and increases non-linearly with $J$.Comment: 11 pages, 3 figures, submitted to Phys. Rev. Let

Phase separation and valence instabilities in cuprate superconductors. Effective one-band model approach

We study the Cu-O valence instability (VI) and the related phase separation (PS) driven by Cu-O nearest-neighbor repulsion $U_{pd}$, using an effective extended one-band Hubbard model ($H_{eff}$) obtained from the extended three-bandHubbard model, through an appropriate low-energy reduction. $H_{eff}$ is solved by exact diagonalization of a square cluster with 10 unit cells and also within a slave-boson mean-field theory. Its parameters depend on doping for $U_{pd}\neq 0$ or on-site O repulsion $U_p\neq 0$. The results using both techniques coincide in that there is neither VI nor PS for doping levels $x<0.5$ if $U_{pd}\lesssim 2$ eV. The PS region begins for $U_{pd}\gtrsim 2$ eV at large doping $x>0.6$ and increases with increasing $U_{pd}$. The PS also increases with increasing on-site Cu repulsion $U_d$.Comment: 16 pages and 10 figures in postscript format, compressed with uufile

Effect of disorder on the vortex-lattice melting transition

We use a three dimensional stacked triangular network of Josephson junctions as a model for the study of vortex structure in the mixed state of high Tc superconductors. We show that the addition of disorder destroys the first order melting transition occurring for clean samples. The melting transition splits in two different (continuous) transitions, ocurring at temperatures Ti and Tp (>Ti). At Ti the perpendicular-to-field superconductivity is lost, and at Tp the parallel-to-field superconductivity is lost. These results agree well with recent experiments in YBaCuO.Comment: 4 pages + 2 figure

Freezing transition of the vortex liquid in anisotropic superconductors

We study the solid-liquid transition of a model of pancake vortices in laminar superconductors using a density functional theory of freezing. The physical properties of the system along the melting line are discussed in detail. We show that there is a very good agreement with experimental data in the shape and position of the first order transition in the phase diagram and in the magnitude and temperature dependence of the magnetic induction jump at the transition. We analyze the validity of the Lindemann melting criterion and the Hansen-Verlet freezing criterion. Both criteria are shown to be good to predict the phase diagram in the region where a first order phase transition is experimentally observed.Comment: 9 pages, 10 figure