Relativistic Effects in Positron Binding to Atoms.

Positron binding by atoms was a subject of intensive investigation in recent years. Although no such binding has been directly observed so far, there is strong theoretical evidence that positron-atom bound systems do exist. Most of the atoms studied so far were low-Z atoms where relativistic effects were negligible. To extend the study to heavy atoms these effects need to be included.

The role of relativity in positron-atom binding was not properly investigated for a long time. Moreover, it was widely believed that relativistic corrections to the positron-atom binding energy are small. Indeed, due to strong Coulomb repulsion, a positron cannot penetrate to within short distances from the nucleus and remains nonrelativistic. However, as we demonstrated in our accurate many-body calculations, there is a large relativistic correction to the binding energy which comes from electrons. The positron binding is due to an interaction with electrons which have large relativistic corrections to their energies and wave functions. The binding energy is the difference between the energies of a neutral atom and an atom bound with a positron. This difference is usually small. On the other hand, relativistic contributions to the energies of both systems are large and there is no reason to expect they are the same and cancel each other.

We investigated positron binding to copper, silver, and gold atoms. All these systems (e+Cu, e+Ag, e+Au) are bound with about the same binding energy in the nonrelativistic limit. Relativistic corrections are small for e+Cu. However, they reduce the binding energy of e+Ag by 40%, while it turns out that e+Au is not bound at all when considered relativistically.

V. A. Dzuba, V. V. Flambaum, C. Harabati.