A ug 2 00 6 Collective dipole oscillations in hydrogenated metals

It is observed that low-lying transitions of an interstitial hydrogen adatom on a metallic lattice correspond classically to dipoles oscillating at frequencies where band electrons typically have a low electromagnetic absorption. Such transitions hence give rise to an essentially unscreened time-dependent perturbation of the lattice potential experienced by neighbouring hydrogen states. It is shown that above a certain hydrogen concentration, coherently phased oscillations at adjacent sites may result in a dipole-dipole interaction larger than the excitation energy. This would give rise to a new ground state for the ensemble of coupled oscillators and a reduction in lattice symmetry below some phase transition temperature. An explicit calculation is carried out for PdH and PdD, and indirect evidence for the existence of the postulated phase is presented.