Fermi-surface topology of (Rb,Cs)Fe${}_2$As${}_2$

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)Fe${}_2$As${}_2$, 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, favoring 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 in (Rb,Cs)Fe${}_2$As${}_2$ and investigated the de Haas-van Alphen effect of (Rb,Cs)Fe${}_2$As${}_2$ in static fields up to 38 T. In our contribution, we show the results of our torque-magnetometry measurements and use them to refine band-structure calculations.