125 research outputs found
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
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
An Approach to the Cosmological Constant Problem(s)
We propose an approach to explaining why naive large quantum fluctuations are
not the right estimate for the cosmological constant. We argue that the
universe is in a superposition of many vacua, in such a way that the resulting
fluctuations are suppressed by level repulsion to a very small value. The
approach combines several aspects of string theory and the early history of the
universe, and is only valid if several assumptions hold true. The approach may
also explain why the effective cosmological constant reamins small as the
universe evolves though several phase transitions. It provides a non-anthropic
mechansim leading to a small, non-zero cosmological constant.Comment: Talk given at Rencontres de Moriond, 2004 by G.L. Kan
On the conveyance of angular momentum in electronic energy transfer
When electronic excitation transfer occurs, it is of considerable interest to establish whether angular momentum can also be conveyed in the process. The question is prompted by a consideration that when the participating chromophores are atoms, ions, or molecular systems having high local symmetry, the electronic excited states that are involved are generally characterized not only by energy, but by angular momentum properties. Moreover, it is known that electron spin can be communicated between quantum dot exciton states. Resolving the general issue entails an electrodynamic representation exploiting irreducible tensor methods, the analysis being illustrated by application to energy transfer associated with a variety of multipolar transitions. The results exhibit novel connections between an angular momentum content of the electromagnetic coupling and a strongly varying distance dependence. It is concluded that the communication of angular momentum does not in general map unambiguously between a donor and energy acceptor
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