The principle of a double crystal electron interferometer.

Sinusoidal interference fringes have been observed recently using a double crystal interferometer and convergent beam electron diffraction techniques. However, the formation mechanism of the interference fringes is not understood completely. In this paper, we explain this effect using wave theory and discuss the coherence conditions necessary for this special interferometer. The excitation errors of partial exit waves leaving a crystal, which is used as a wave splitter, are found to be responsible for the formation of the interference fringes. The double crystal interferometer used in the electron diffraction manifests a new interference mechanism. Compared with two-beam interference in real space, we observe the interference effect in the reciprocal space, i.e. in the Fourier space. This effect may enable us to undertake holography in the Fraunhofer diffraction plane and measure both the phase and amplitude information of reflections. The phase information of reflections contains information on the position of atoms and the symmetry of a unit cell. Furthermore, such an interferometer has no rigorous coherence conditions.