Investigations of the Fermisurface using Positrons

Almost all properties of a material are defined by its electronic structure. For a metal, the most important characteristic is the Fermisurface, defining the boundary between occupied and unoccupied states in reciprocal space. The traditional experimental method for studying the Fermi surface is the de Haas-van Alphen effect, which is limited to low temperatures, and therefore cannot be utilized for the study of paramagnetic Cr above the Néel temperature of Cr is 311 K. For this reason, we applied the 2D-ACAR (Angular Correlation of Annihilation Radiation) technique to investigate the electronic structure of both, the paramagnetic and the anti-ferromagnetic phase of Cr. At the moment of electron-positron-annihilation the center-of-mass momentum of the system is almost exclusively given by the electron momentum. By measuring the angular correlation of the annihilation radiation the transverse component of the electron momentum can be determined and therefore a two-dimensional projection of the electron momentum density can be obtained. Using tomographic methods the three-dimensional electron momentum density, i.e. the Fermisurface is determined. We present the key features of our 2D-ACAR spectrometer and discuss the measurements on the pure metal system Cr which are also compared with first principle band structure calculations. This project is funded by the Deutsche Forschungsgesellschaft (DFG) within the Transregional Collaborative Research Center TRR 80 “From electronic correlations to functionality”.