Phase retrieval through focus variation for ultra-resolution in field-emission transmission electron microscopy

The use of a coherent field-emission electron source in transmission electron microscopy is combined with phase retrieval by digital processing of a focal image series. For the first time, a dramatic improvement of the high-resolution performance of the electron microscope beyond the usual ``point-to-point'' resolution has been realized: Experiments on a 200-kV microscope with a point resolution of 0.24 nm reveal reconstructed information down to 0.14 nm. Examples are shown in the field of high-Tc superconductors and ferroelectric oxides. The oxygen sublattice in these structures is revealed.