Multibeam Borrmann modes of single crystals exist in which in addition to the electric field being zero, the magnetic field and/or higher derivatives of the field (the electric field gradient, the magnetic field gradient, ...) are also zero at the equilibrium position of each atom in the crystal. These properties of multibeam modes are of particular interest in the case of γ-ray optics, since Mössbauer transitions occur with well-defined multipolarity, E1, M1, E2 (and in a few cases, M1-E2), in contrast to the electronic response of atoms which occurs as a hierarchy of multipole contributions, E1≫E2≫M1, etc. As we show, this well-defined multipolarity of nuclear transitions leads to three particularly interesting effects which occur at multibeam points of good single crystals containing resonant Mössbauer nuclei. (1) Certain multibeam Borrmann modes exhibit suppressed coupling both to the electrons and to the nuclei and will be anomalously transmitted at resonance, even when the nuclear transition is of multipolarity M1 or higher and there is no Zeeman splitting. Such simultaneous suppression is not possible in the usual two-beam case. (2) Other multibeam Borrmann modes exhibit particularly strong coupling to the nuclei, so that nuclear absorption is greatly enhanced. (3) Excited Mössbauer nuclei located at lattice sites within a crystal will have enhanced emission into certain of these weakly attenuated multibeam Borrmann modes (‘‘anomalous emission’’). While these last two effects can occur in the two-beam case, greater enhancements are possible in multibeam modes, since the coupling is proportional to the number of beams in the mode.

• Received 19 October 1987

DOI:https://doi.org/10.1103/PhysRevA.37.4269