Final state effects in photoemission studies of Fermi surfaces

Abstract

Photoelectron spectroscopy is one of the most important methods for extractinginformation about the Fermi surface (FS) of materials. An electron photoexcited from theFS is emitted from the crystal conserving the parallel momentum, , while the perpendicular momentum is reduced due to the surface potential barrier. A simple interpretation of the process assumes thefinal state is free-electron-like allowing one to ‘map’ the detected photoelectron back to its initialk momentum. There are multiple final state effects that can complicate the interpretation ofphotoelectron data and these effects are reviewed here. These can involve both energy andk broadening, which can give rise to shadow or ghost FS contours, scattering and final statediffraction effects that modify intensities, and matrix element effects which reflect thesymmetries of the states involved and can be highly dependent on photon polarization. Thesematrix elements result in contours of photoelectron intensity that follow the dispersion ink-space of the initial state, the FS, and the final state. Locations where intensities go tozero due to matrix element and symmetry effects can result in gaps where FScontours ‘disappear’. Recognition that these effects can play a significant role indetermining the measured angular distributions is crucial in developing an informedmodel of where the FS contours actually lie in relation to measured intensitycontours.