Spin-polarized measurements of the two-dimensional angular correlation of annihilation radiation in NiMnSb are presented. By making use of the inherent partial polarization of the positron beam and the alignment of the magnetic domains with the aid of an external magnetic field, the sum and difference of the spin-dependent contributions to the angular correlation have been obtained for integration directions (100), (110),and (111).The results are compared with calculated distributions. A least-squares analysis of the data yields a value of ( -8.4+0. 1)X 10 ' for the three-photon difference effect in NiMnSb and establishes the half-metallic character of the band structure with an accuracy of -+0 02 electrons per formula unit.

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A novel effect of alloying on the electron momentum density in a disordered a-Cu9iGe9 alloy is reported. The effect, which is the appearance of a bump in the derivative spectra at low momenta, is predicted by theory employing the coherent-potential approximation and is observed in two-dimensional angular correction experiments on the alloy. It reflects the presence of an s-p impurity band far below the Fermi level. Accurate experimental values for the Fermi-surface radii /cioo and k no and for the associated disorder-induced smearings in CuGe are also reported for the first time.