Casimir-like tunneling-induced electronic forces

We study the quantum forces that act between two nearby conductors due to electronic tunneling. We derive an expression for these forces by calculating the flux of momentum arising from the overlap of evanescent electronic fields. Our result is written in terms of the electronic reflection amplitudes of the conductors and it has the same structure as Lifshitz's formula for the electromagnetically mediated Casimir forces. We evaluate the tunneling force between two semiinfinite conductors and between two thin films separated by an insulating gap. We discuss some applications of our results.Comment: 8 pages, 3 figs, submitted to Proc. of QFEXT'05, to be published in J. Phys.

Brownian motion meets Riemann curvature

The general covariance of the diffusion equation is exploited in order to explore the curvature effects appearing on brownian motion over a d-dimensional curved manifold. We use the local frame defined by the so called Riemann normal coordinates to derive a general formula for the mean-square geodesic distance (MSD) at the short-time regime. This formula is written in terms of $O(d)$ invariants that depend on the Riemann curvature tensor. We study the n-dimensional sphere case to validate these results. We also show that the diffusion for positive constant curvature is slower than the diffusion in a plane space, while the diffusion for negative constant curvature turns out to be faster. Finally the two-dimensional case is emphasized, as it is relevant for the single particle diffusion on biomembranes.Comment: 16 pages and 3 figure

Neotropical tadpoles: spatial and temporal distribution and habitat use in a seasonal lake in Veracruz, México

We studied a tadpole assemblage in a seasonal neotropical lake where 14 species of anurans reproduce. Tadpoles were collected monthly at nine sampling stations at depth intervals of 1 m from the surface to the bottom (13 m). Sufficient numbers of tadpoles of three species were collected to compare habitat use. This three species assemblage breed in the following order (first to last): Smilisca baudinii, Gastrophryne usta, and Rana berlandieri. R. berlandieri had the greatest microhabitat breadth followed by S. baudinii. S. baudinii and G. usta had high microhabitat overlap, but significant differences in microhabitat use were found. S. baudinii tended to occur near the bottom, while G. usta was near the surface. This study shows that temporal and habitat partitioning both occur and depend on the species of tadpole. Dynamic interactions occur between habitat and temporal dimensions. Phenology and habitat selection depend both on the species and on abiotic and biotic factors

Critical materials supply and risk

Rare earth Elements (REEs) are a subset of Critical materials (CMs) and are found in numerous modern electronic products and advanced technologies where, due to their unique physical and chemical properties, they cannot be substituted. Disruption in their supply chain has the potential to cause devastating impacts on production and consumers. To help manage such disruptions, this research focusses on the areas of critical materials, supply chain resilience, and supply chain risk. Research has established the challenges that can occur from disruptions caused by up-stream suppliers, necessitating supply-side resilience strategies. Supply-side risk factors associated with CMSs differ from conventional material supply chain risk, and it is advocated that they cannot be effectively managed through traditional material supply risk management practices. This study investigates the research on CMs that appears around the periphery of the OM (operations management) field by adopting a systematic literature review. Findings show a dearth of research work directly focused on REEs and CMs from within the OM field. Importantly, a call is made for OM researchers to investigate REEs and CMs based on the imperative that geopolitics, resource scarcity and environmental issues present

Generally covariant state-dependent diffusion

Statistical invariance of Wiener increments under SO(n) rotations provides a notion of gauge transformation of state-dependent Brownian motion. We show that the stochastic dynamics of non gauge-invariant systems is not unambiguously defined. They typically do not relax to equilibrium steady states even in the absence of extenal forces. Assuming both coordinate covariance and gauge invariance, we derive a second-order Langevin equation with state-dependent diffusion matrix and vanishing environmental forces. It differs from previous proposals but nevertheless entails the Einstein relation, a Maxwellian conditional steady state for the velocities, and the equipartition theorem. The over-damping limit leads to a stochastic differential equation in state space that cannot be interpreted as a pure differential (Ito, Stratonovich or else). At odds with the latter interpretations, the corresponding Fokker-Planck equation admits an equilibrium steady state; a detailed comparison with other theories of state-dependent diffusion is carried out. We propose this as a theory of diffusion in a heat bath with varying temperature. Besides equilibrium, a crucial experimental signature is the non-uniform steady spatial distribution.Comment: 24 page

Casimir energy in spherical cavities

We calculate the Casimir energy at spherical cavities within a host made up of an arbitrary material described by a possibly dispersive and lossy dielectric response. To that end, we add to the coherent optical response a contribution that takes account of the incoherent radiation emitted by the host in order to guarantee the detailed balance required to keep the system at thermodynamic equilibrium in the presence of dissipation. The resulting boundary conditions allow a conventional quantum mechanical treatment of the radiation within the cavity from which we obtain the contribution of the cavity walls to the density of states, and from it, the thermodynamic properties of the system. The contribution of the cavity to the energy diverges as it incorporates the interaction energy between neighbor atoms in a continuum description. The change in the energy of an atom situated at the center of the cavity due to its interaction with the fluctuating cavity field is however finite. We evaluate the latter for a simple case.Comment: 9 pages, 4 figures, Proceedings of QFEXT07. To be published in J. Phys.

Strange matter in rotating compact stars

We have constructed equations of state involving various exotic forms of matter with large strangeness fraction such as hyperon matter, Bose-Einstein condensates of antikaons and strange quark matter. First order phase transitions from hadronic to antikaon condensed and quark matter are considered here. The hadronic phase is described by the relativistic field theoretical model. Later those equations of state are exploited to investigate models of uniformly rotating compact stars. The effect of rotation on the third family branch for the equation of state involving only antikaon condensates is investigated. We also discuss the back bending phenomenon due to a first order phase transition from $K^-$ condensed to quark matter.Comment: 8 pages, 4 figures; Plenary talk delivered at Strangeness in Quark Matter (SQM) 2004 held in Cape Town, South Africa from 15-20 September; Accepted for publication in the proceedings in Journal of Physics