Fermi-surface topology of (Rb,Cs)Fe₂As₂

The unique Fermi-surface topology of many iron-pnictide superconductors stimulates a number of theories on the nature of the pairing interactions in these materials. In the case of (K,Rb,Cs)Fe₂As₂, there is an ongoing discussion whether a Lifshitz transition is connected to the superconducting properties of these 122 iron arsenides. Studies of the pressure dependence of H_C2 did not find evidence for that kind of transition. In contrast, high-resolution band structure calculations show a van-Hove singularity close to the Fermi energy, favouring a Lifshitz transition. In order to investigate the possible impact of this feature, a precise knowledge of the electronic structure is eminent. We, therefore, calculated the Fermi surface of (Rb,Cs)Fe₂As₂ using the full-potential-local-orbital scheme and started investigating the de Haas-van Alphen effect of (Rb,Cs)Fe₂As₂ in static fields up to 18 T. In our contribution, we show the first results of our ongoing torque-magnetometry measurements and compare them with the results of our band-structure calculations.