If an epithermal positron from an external radioactive source such as
sodium-22 is injected into a, say, metallic sample, it is usually
thermalized within a very short time (a few picoseconds) compared to its
mean lifetime in such a material (typically of the order of 100 to 1000
picoseconds). As a particle in thermal equilibrium with its environment,
the positron "survives" a certain time until it annihilates with one of
the electrons of the sample under emission of (usually) two annihilation
photons. An analysis of this annihilation radiation gives, dependent on
the complexity of the experimental equipment, a one- or two-dimensional
projection of the electron-positron momentum density distribution in the
material investigated. One of the advantages of this method is a
remarkably high momentum resolution what is especially important if
momentum distributions are used to investigate the Fermi surface of
From the theoretical point of view, the greatest objection to the
positron annihilation technique is the fact that the positively charged
test particle polarizes the electron gas in the solid. The resulting
so-called electron-positron enhancement effect represents, since many
years, one of the most challenging problems of theoretical positron
physics, especially if the annihilating fermions are not simply treated
as free particles but as Bloch particles.
The development of a theory describing the momentum distribution of
annihilating pairs of electron and positron Bloch particles in spatially
inhomogeneous electron gases including this enhancement effect was and
is the main topic of my theoretical work since about 1990. These efforts
led to the so-called Bloch-modified ladder (BML) theory (1996), an
approach which is based on a perturbation expansion of the two-particle
electron-positron Green's function.
A selected list of publications to the topics BML theory and
calculations of electron-positron momentum densities in metals is given
in the following. Further relevant papers are in preparation.
Back to Heinz Sormann
 H. Sormann: "Influence of lattice effects on the electron-positron
interaction in metals", Phys. Rev. B 54 (1996) 2401.
 H. Sormann and A. Fenkart: "Lattice and relativistic effects on the
electron-positron momentum density of transition metals",
Materials Sci. Forum 255-257 (1997) 590.
 H. Sormann and M. Sob: "Sensitivity of electron and
electron-positron momentum densities to various electron and
positron crystal potentials", Phys. Rev. 64 (2001), 045102.
 H. Sormann, G. Kontrym-Sznajd, and R. N. West: "On the reliability
of various enhancement theories for a description of
electron-positron densities in metals", Materials Sci. Forum
363-365 (2001) 609.