Coherent spin dynamics in fermionic quantum gases. From two-body to many-body physics

The spin plays a fundamental role for various many-body effects in nature, ranging from magnetism in solids to even more complex phenomena in high-spin systems such as the quark-gluon plasma. Quantum gases are ideally suited to study such systems in an exceptionally clean and controllable environment. For example, experiments with spinor Bose-Einstein condensates have revealed completely new high-spin phenomena in the last years. In this context, interaction-driven dynamics of the spin degree of freedom have been found. However, since the constituents of matter are fermions, there is a huge interest in current research activities to understand to which extent these effects also exist in fermionic systems. This thesis is devoted to the investigation of fermionic spin dynamics. For the first time, spin-changing collisions could be observed in fermionic quantum gases. This novel effect has been explored in various regimes, ranging from the microscopic two-body process to the many-body regime. For these investigations, new methods for the experimental preparation and control of fermionic high-spin systems have been developed. In a first approach, spin dynamics were investigated between two isolated fermions, realized in deep optical lattices. The results demonstrate novel coherent multi-flavor spin dynamics, involving up to ten different spin states. An excellent agreement with a two-particle scattering model allowed for the determination of fundamental scattering parameters and for the first demonstration of spin-changing collisions with high-collision quanta. Moreover, the experiments reveal a new instability of a band insulator in high-spin systems - in contrast to conventional solid-state systems - induced by spin-changing collisions. Inspired by these fundamental findings, the impact of spin-changing collisions on a fermionic many-body system has been investigated. In a Fermi sea, consisting of several 105 particles with a spatial extension of several hundred micrometers, a fully unexpected phenomenon could be observed: the Fermi sea exhibits giant and long-lived spin oscillations induced by microscopic collisions despite its multi-mode structure. A detailed experimental study demonstrated that the whole system can be well captured in a mean-field approach using a single-mode approximation. This novel collective behavior is one of the few collective effects known in fermionic many-body systems such as superfluidity and constitutes a central result of this work. In a further investigation, these collective spin dynamics have been employed to study relaxation effects, which are currently one of the most important topics of manybody physics. Doing so, studies demonstrated that collective spin dynamics are only stabilized due to Pauli blocking at ultralow temperatures. In addition, a new stabilization mechanism was discovered, which stabilizes magnetically excited spin mixtures governed by the interplay between different collision processes. In addition, spin-relaxation dynamics of fermionic atoms were observed for the first time, which lead to a thermalization of the fermionic many-body system on long time scales. The results presented in this thesis provide an important contribution to a deeper understanding of fermionic many-body systems. They pave the way towards exciting studies of novel high-spin quantum phases, which so far have remained widely unexplored.