The Enhancement of Fermi Surface Images in Positron Acar Spectra.

A digital image enhancement technique, a frequency limited derivative (FLD) or band-pass filter, capable of dramatically increasing the ability to resolve Fermi surface discontinuities in 2D ACAR spectra is introduced and discussed at length. Simulations of data are created to test the limitations of the FLD filter, and the results of applying our filter to these simulations are presented. This filter, along with more traditional data processing methods, is also applied to real 2D ACAR spectra and the results of the various techniques are compared to demonstrate the utility of the filter. These real data spectra include studies of the electron momentum densities (including Fermi surface contributions) of the more familiar crystals such as binary copper alloys and yttrium, as well as studies of the far more complicated momentum densities of high critical temperature superconductors, an organic superconductor and a heavy Fermion system. The filter was found to enhance the Fermi surface discontinuities in nearly all cases. Those few cases where the filter produced results in disagreement with the predictions of theory are explained.