Universität zu Köln

Lange, Florian (2020). Giant response to weak pumping in quantum systems with approximate symmetries. PhD thesis, Universität zu Köln.

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Abstract

Our environment and nature as a whole are fundamentally not in equilibrium. The existence of life on earth is only possible for this reason. Everyday examples of non-equilibrium processes are various weather phenomena driven by air and heat flows, traffic jams on motorways or the swarm behavior of animals in groups. These are just a few examples of many more phenomena and motivate a better understanding of non-equilibrium processes in classical as well as quantum systems. Non-equilibrium means that the known concepts, which are valid in equilibrium, are generally not or only approximately applicable. In equilibrium, all macroscopic currents add on average up to zero and the configuration of a system is solely determined by the fundamental principle of entropy maximization and the symmetries present in the system. According to the Noether theorem, which represents one of the most fundamental relations in physics, each continuous symmetry is associated with a conservation law. In this thesis, we consider quantum systems that have a set of conservation laws of which some are weakly violated by an external perturbation. We show that under these circumstances highly non-equilibrium states can be reached which are characterized by large currents. The phenomenon that a small perturbation can have a very large effect if it breaks a conservation law can, for example, be illustrated with a greenhouse. Inside the greenhouse the energy is approximately conserved due to the good insulation. As a consequence, the interior can be heated up to very high temperatures by even weak sunlight.