Low-energy positron physics

J. Berakdar

Positrons, the anti-particles of electrons, have long been used to explore the electronic and crystal structure of solids by angular correlation measurement, Doppler broadening and life-time techniques (of the positronium). The use of positron beams excludes the complication resulting from exchange interaction between the particles' beam and the sample's electrons. However, in case of a positron beam a new channel opens, namely that of positronium formation. The annihilation parameters of the positronium can be employed as a useful tool for an insight into the surrounding medium, e.g. lattice imperfections in crystals are efficiently imaged by this technique. Our research in this area are driven mainly by two aspects:

1. The dynamics of a many-body electronic system can be probed in a unique way when one electron is replaced by a positron. This can be seen already in the case of a three-body problem (two electrons and a nucleus). Thus, studying the interaction of electronic systems with low-energy positrons offers a useful tool to test our many-body theories.


2. For practical application we envisage the possibility to employ low-energy positrons to explore the magnetic properties of surfaces. Here we look at two different reactions:
   
   
   1. At a ferromagnetic surface triplet and singlet positroniums are formed with a ratio dependent on the structure of the occupied majority and minority bands of the surface. The triplet and the singlet positroniums can be distinguished experimentally using their different life times. We investigate theoretically the relation between these different life times and the spin-resolved electronic structure of the surface
   2. The inelastic scattering of positrons from a ferromagnet may produce polarized secondary electrons whose properties are related to those of the sample. Thus, the polarization of the secondary electrons is a measure of the spin polarization of the sample. We simulate theoretically the above reaction taking into account the correlation between the positron and the electron

Collaboration:
G. Larrichia (University College London)