Casimir effect across a layered medium

Using nonstandard recursion relations for Fresnel coefficients involving successive stacks of layers, we extend the Lifshitz formula to configurations with an inhomogeneous, n-layered, medium separating two planar objects. The force on each object is the sum of a Lifshitz like force and a force arising from the inhomogeneity of the medium. The theory correctly reproduces very recently obtained results for the Casimir force/energy in some simple systems of this kind. As a by product, we obtain a formula for the force on an (unspecified) stack of layers between two planar objects which generalizes our previous result for the force on a slab in a planar cavity.Comment: 5 pages, 1 figure, presented at QFEXT1

The Catastrophic Effects of Natural Disasters on Insurance Markets

Natural catastrophes often have catastrophic risks on insurance companies as well as on the insured. Using a very large dataset on homeowners%u2019 insurance coverage by state, by firm, and by year for the 1984 to 2004 period, this paper documents the positive effect on losses and loss ratios of both unexpected catastrophes as well as large events that the authors term %u201Cblockbuster catastrophes.%u201D Insurers adapt to these catastrophic risks by raising insurance rates, leading to lower loss ratios after the catastrophic event. There is a widespread event of unexpected catastrophes and blockbuster catastrophes that reduces total premiums earned in the state, reduces the total number writing insurance coverage in the state, and leads to the exit of firms from the state. Firms with low levels of homeowners%u2019 premiums are most adversely affected by the catastrophes.

Nonlinear Maxwell Equations

A new relativistic invariant version of nonlinear Maxwell equations is offerred. Some properties of these equations are considered.Comment: 6 pages, LaTe

Self-Interacting Electromagnetic Fields and a Classical Discussion on the Stability of the Electric Charge

The present work proposes a discussion on the self-energy of charged particles in the framework of nonlinear electrodynamics. We seek magnet- ically stable solutions generated by purely electric charges whose electric and magnetic fields are computed as solutions to the Born-Infeld equa- tions. The approach yields rich internal structures that can be described in terms of the physical fields with explicit analytic solutions. This suggests that the anomalous field probably originates from a magnetic excitation in the vacuum due to the presence of the very intense electric field. In addition, the magnetic contribution has been found to exert a negative pressure on the charge. This, in turn, balances the electric repulsion, in such a way that the self-interaction of the field appears as a simple and natural classical mechanism that is able to account for the stability of the electron charge.Comment: 8 pages, 1 figur

Selfduality of non-linear electrodynamics with derivative corrections

In this paper we investigate how electromagnetic duality survives derivative corrections to classical non-linear electrodynamics. In particular, we establish that electromagnetic selfduality is satisfied to all orders in $\alpha'$ for the four-point function sector of the four dimensional open string effective action.Comment: 8 page

Rapport ta' l-eżaminaturi fuq il-konkors ta’ essays dwar Malta u l-inkoronazzjoni tal-maestà tagħha r-Reġina Eliżabetta II

Dan hu rapport tal-eżaminaturi fuq il-konkors ta’ esejs dwar Malta u l-inkurunazzjoni tal-maestà tagħha r-Reġina Eliżabetta II.N/

Ab initio Wannier-function-based many-body approach to Born charge of crystalline insulators

In this paper we present an approach aimed at performing many-body calculations of Born-effective charges of crystalline insulators, by including the electron-correlation effects. The scheme is implemented entirely in the real space, using Wannier-functions as single-particle orbitals. Correlation effects are computed by including virtual excitations from the Hartree-Fock mean field, and the excitations are organized as per a Bethe-Goldstone-like many-body hierarchy. The results of our calculations suggest that the approach presented here is promising.Comment: 5 pages, to appear in Phys. Rev. B. (Rapid Comm., Dec 15, 2004

Singularity-Free Electrodynamics for Point Charges and Dipoles: Classical Model for Electron Self-Energy and Spin

It is shown how point charges and point dipoles with finite self-energies can be accomodated into classical electrodynamics. The key idea is the introduction of constitutive relations for the electromagnetic vacuum, which actually mirrors the physical reality of vacuum polarization. Our results reduce to conventional electrodynamics for scales large compared to the classical electron radius $r_0\approx 2.8\times10^{-13}$ cm. A classical simulation for a structureless electron is proposed, with the appropriate values of mass, spin and magnetic moment.Comment: 3 page

Nonperturbative calculation of Born-Infeld effects on the Schroedinger spectrum of the hydrogen atom

We present the first nonperturbative numerical calculations of the nonrelativistic hydrogen spectrum as predicted by first-quantized electrodynamics with nonlinear Maxwell-Born-Infeld field equations. We also show rigorous upper and lower bounds on the ground state. When judged against empirical data our results significantly restrict the range of viable values of the new electromagnetic constant which is introduced by the Born-Infeld theory. We assess Born's own proposal for the value of his constant.Comment: 4p., 2 figs, 1 table; submitted for publicatio

Dirac-Kronig-Penney model for strain-engineered graphene

Motivated by recent proposals on strain-engineering of graphene electronic circuits we calculate conductivity, shot-noise and the density of states in periodically deformed graphene. We provide the solution to the Dirac-Kronig-Penney model, which describes the phase-coherent transport in clean monolayer samples with an one-dimensional modulation of the strain and the electrostatic potentials. We compare the exact results to a qualitative band-structure analysis. We find that periodic strains induce large pseudo-gaps and suppress charge transport in the direction of strain modulation. The strain-induced minima in the gate-voltage dependence of the conductivity characterize the quality of graphene superstructures. The effect is especially strong if the variation of inter-atomic distance exceeds the value a^2/l, where a is the lattice spacing of free graphene and l is the period of the superlattice. A similar effect induced by a periodic electrostatic potential is weakened due to Klein tunnelling.Comment: 11 pages, 8 figure