Positron annihilation in iron and B2-ordered alloys FeAl and FeTi

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Abstract
A band structure (BS) calculation of both the momentum density of annihilation pairs (MDAP) and the electron momentum density (EMD) is performed for B2-ordered stoichiometric alloys FeAl and FeTi as well as for pure iron in the principal crystallographic directions (100), (110) and (111). The electron and positron wavefunctions are determined by the symmetrised APW method. The electron-positron interaction is considered by means of the Kahana-like enhancement factor expressed as a function of the annihilating electron energy. It is found that the contribution of d electrons is enhanced by about a factor of two less than that of the nearly free electrons. An analysis of the experimental results for iron suggests that the majority spin bands Sigma 1 are probably not intersected by the Fermi level and, therefore, the hole arms along the HN direction at the majority Fermi surface are not connected at N. This is at variance with the recent BS calculations for iron, but in agreement with the rigid-exchange-approximation calculation performed. A mutual comparison of the BS results leads to the conclusion that the low-energy parts of the electronic structures of B2-ordered transition metal alloys have approximately the same character.