Plasmon Cohesive Energy of Voids and Void Lattices in Irradiated Metals

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Abstract
The contribution of the long-range electron-Coulomb correlation to the surface energy of an empty spherical cavity in a metal is calculated from the zero-point energy of surface plasmons characteristic of the spherical void boundary. At large void radii one recovers the result previously derived for a planar metal-vacuum interface. Next it is shown that, owing to the overlapping of surface-plasmon zero-point oscillations around two neighboring voids, an effective attraction exists between them which is analogous to the van der Waals (dispersion) forces between small metal spheres in vacuum. However, because of the monopole nature of the fundamental surface-plasmon mode of a void, the effective void-void interaction is much stronger and of much longer range than the attraction of spherical particles for which the lowest-order plasmon mode is of dipole type only. It is therefore proposed that plasmons may be one of the important physical processes responsible for void nucleation and growth and also for the observed occurrence of three-dimensional void arrays in some heavily irradiated transition metals. In a void lattice, the plasmon modes of an isolated void split and broaden into bands. The resulting plasmon cohesive energy per void is estimated to be of the order of -1 eV for the observed bcc void array in molybdenum and -2.5 eV for the fcc nickel array.