Breakdown of the Quasi-Particle Approximation in Metals

R. W. Davies

Genrich L. Krasko

Erik Aldape, Tessa Cookmeyer, Aavishkar A. Patel +1 more

J-P Connerade

The terms "quasiatom" and "superatom" are introduced. A quasi-atom, for example contains two ions of opposite charge, which behave like an atom, forming Rydberg states. A superatom also contains many atoms, but behaves in a simple way, and can be modelled within the central field approximation. Both are examples of many-body systems which emulate quasi-particle properties. Their behaviour is the opposite of quasi-particle breakdown in isolated atoms. However, it also probes the boundary between simple and complex behaviour, for species larger than atoms. Examples are given, ranging from shallow donor impurities in semiconductors, through ion-pair molecules, confined and endohedral atoms, to metallic clusters.

J. Högaasen-Feldman

Dirk Witthaut, Eva-Maria Graefe, H. J. Korsch

We consider the Landau-Zener problem for a Bose-Einstein condensate in a linearly varying two-level system, for the full many-particle system as well as in the mean-field approximation. Novel nonlinear eigenstates emerge in the mean-field description, which leads to a breakdown of adiabaticity: The Landau-Zener transition probability does not vanish even in the adiabatic limit. It is shown that the emergence of nonlinear eigenstates and thus the breakdown of adiabaticity corresponds to quasi-degenerate avoided crossings of the many-particle levels. The many-particle problem can be solved approximately within an independent crossings approximation, which yields an explicit generalized Landau-Zener formula. A comparison to numerical results for the many-particle system and the mean-field approximation shows an excellent agreement.