Enhanced van der Waals interaction at interfaces

Using a recently obtained (general) formula for the interaction energy between an excited and a ground-state atom (Sherkunov Y 2007 Phys. Rev. A 75 012705), we consider the interaction energy between two such atoms near the interface between two media. We demonstrate that under the circumstances of the resonant coupling of the excited atom to the surface polariton mode of a vacuum-medium system the nonretarded atom*-atom interaction energy can be enhanced by (several) orders of magnitude in comparison with the van der Waals interaction energy of the two isolated atoms.Comment: 8 pages, 5 figures, local-field corrections included and improved presentatio

Spin Networks for Non-Compact Groups

Spin networks are natural generalization of Wilson loops functionals. They have been extensively studied in the case where the gauge group is compact and it has been shown that they naturally form a basis of gauge invariant observables. Physically the restriction to compact gauge group is enough for the study of Yang-mills theories, however it is well known that non-compact groups naturally arise as internal gauge groups for Lorentzian gravity models. In this context a proper construction of gauge invariant observables is needed. The purpose of this work is to define the notion of spin network states for non-compact groups. We first built, by a careful gauge fixing procedure, a natural measure and a Hilbert space structure on the space of gauge invariant graph connection. Spin networks are then defined as generalized eigenvectors of a complete set of hermitic commuting operators. We show how the delicate issue of taking the quotient of a space by non compact groups can be address in term of algebraic geometry. We finally construct the full Hilbert space containing all spin network states. Having in mind application to gravity we illustrate our results for the groups SL(2,R), SL(2,C).Comment: 43pages, many figures, some comments adde

A retarded coupling approach to intermolecular interactions

A wide range of physical phenomena such as optical binding and resonance energy transfer involve electronic coupling between adjacent molecules. A quantum electrodynamical description of these intermolecular interactions reveals the presence of retardation effects. The clarity of the procedure associated with the construction of the quantum amplitudes and the precision of the ensuing results for observable energies and rates are widely acknowledged. However, the length and complexity of the derivations involved in such quantum electrodynamical descriptions increase rapidly with the order of the process under study. Whether through the use of time-ordering approaches, or the more expedient state-sequence method, time-consuming calculations cannot usually be bypassed. A simple and succinct method is now presented, which provides for a direct and still entirely rigorous determination of the quantum electrodynamical amplitudes for processes of arbitrarily high order. Using the approach, new results for optical binding in two- and three-particle systems are secured and discussed

Parametric instabilities in magnetized multicomponent plasmas

This paper investigates the excitation of various natural modes in a magnetized bi-ion or dusty plasma. The excitation is provided by parametrically pumping the magnetic field. Here two ion-like species are allowed to be fully mobile. This generalizes our previous work where the second heavy species was taken to be stationary. Their collection of charge from the background neutral plasma modifies the dispersion properties of the pump and excited waves. The introduction of an extra mobile species adds extra modes to both these types of waves. We firstly investigate the pump wave in detail, in the case where the background magnetic field is perpendicular to the direction of propagation of the pump wave. Then we derive the dispersion equation relating the pump to the excited wave for modes propagating parallel to the background magnetic field. It is found that there are a total of twelve resonant interactions allowed, whose various growth rates are calculated and discussed.Comment: Published in May 2004; this is a late submission to the archive. 14 pages, 8 figure

Simulations of plasma confinement in an antihydrogen trap

The three-dimensional particle-in-cell (3-D PIC) simulation code WARP is used to study positron confinement in antihydrogen traps. The magnetic geometry is close to that of a UC Berkeley experiment conducted, with electrons, as part of the ALPHA collaboration (W. Bertsche et al., AIP Conf. Proc. 796, 301 (2005)). In order to trap antihydrogen atoms, multipole magnetic fields are added to a conventional Malmberg-Penning trap. These multipole fields must be strong enough to confine the antihydrogen, leading to multipole field strengths at the trap wall comparable to those of the axial magnetic field. Numerical simulations reported here confirm recent experimental measurements of reduced particle confinement when a quadrupole field is added to a Malmberg-Penning trap. It is shown that, for parameters relevant to various antihydrogen experiments, the use of an octupole field significantly reducesthe positron losses seen with a quadrupole field. A unique method for obtaining a 3-D equilibrium of the positrons in the trap with a collisionless PIC code was developed especially for the study of the antihydrogen trap; however, it is of practical use for other traps as well