In high-temperature superconductors, whose critical temperature greatly surpasses predictions made by the Bardeen-Cooper-Schrieffer theory of superconductivity, we have very strong electron correlations. Their phase diagrams exhibit a dome shaped control parameter dependence of the superconducting phase, which directly borders an antiferromagnetic insulator phase. Similar properties are also exhibited by fulleride superonductors. This kind of behaviour, where superconductivity competes with an antiferromagnetic insulating phase, is determined by electron correlations and is still the subject of many experiments nowadays.\\
In this masters thesis we studied the behaviour of K$_2$CsC$_{60}$, which at higher temperatures stands on the transition between the insulating and conducting phase. In the normal phase its physical properties were determined by electron spin resonance spectroscopy (ESR), whereas nonresonant microwave absorption enabled us to determine some of the superconducting properties along with the pressure dependent critical temperature.\\
A careful examination of the normal phase spectra showed, that there are two absorption components with a Lorentzian lineshape, which can be distinguished by the linewidth. First component exhibits a Curie-Weiss temperature dependence of the intensity and an activating behaviour of the linewidth with the corresponding activation energy $E_a = 48,5$ meV. From this observation we conclude that there is still some fraction of the sample with electrons localized on the C$_{60}$ molecules. There the $t_\textrm{1u}$ molecular orbital energy levels are split by the Jahn-Teller effect by $E_a$. Second component, which we connected to a metallic phase, significantly increases under $T' = 217$ K. At lower temperatures it shows a Pauli temperature independent signal intensity and a linear temperature dependence of the linewidth. It is the observation of both of these two phases, that shows the importance of the electron correlations in K$_2$CsC$_{60}$.\\
From such a strongly correlated state, superconductivity emerges at $T_c = 23,1$ K under ambient pressure. By measuring the nonresonant microwave absorption, we observed an increasing pressure dependence of $T_c$, with a maximum of 32 K under 0,5 GPa, which is consistent with the familiar fullerene phase diagram, where electron-electron and electron-phonon interactions are already considered.
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