Magnetic phase transitions and correlation effects in two-particle spectroscopies

Abstract

We discuss the manifestation of magnetic phase transitions of 3d-transition metal or 4f-rare earth materials in two-particle spectroscopies such as Auger-electron (AES) and appearance-potential spectroscopy (APS). Starting point is a generalized multiband model suitable for the description of systems, the physical properties of which are strongly influenced by correlation effects. It is shown that this model predicts para- to ferromagnetic phase transitions with Curie temperatures of realistic orders of magnitude. By use of a diagrammatic vertex- correction method (Matsubara formalism) the two-particle spectral density, which is the key-quantity for both spectroscopies, AES as well as APS, is expressed in terms of the one-particle spectral density. The latter is approximately derived from a self-consistent moment method for arbitrary temperatures and arbitrary electron concentrations. In the ferromagnetic phase the AE (AP) line shapes get a distinctive temperature dependence. Special attention is devoted to the question what is really measured by AES and APS. It is possible to derive concrete information about typical correlation effects? We show that integrated AE and AP spectra reflect important correlation functions being not accessible by any other measuring technique.