Nonequilibrium Terahertz Dynamics of Interacting Quantum Spins: from Novel Driven States towards Coherent Controls

ongoing

The study of non-equilibrium dynamics of magnetic degrees of freedom has shown remarkable progress over the past two decades. This is in particular true concerning the understanding of ultrafast magnetization dynamics in classical magnets. In contrast, the field of non-equilibrium spin dynamics of many-body quantum magnetic systems is still in its infancy. Despite a number of ground-breaking recent theoretical proposals, experimental studies in this direction are truly scarce and this research field is a largely unexplored territory. Here I propose to study non-equilibrium dynamics in a number of well-selected quantum spin systems, utilizing a novel and powerful experimental technique – time-resolved terahertz spectroscopy under extreme conditions. By carrying out the here proposed program of non-equilibrium and nonlinear studies on low-dimensional and/or frustrated quantum magnets, I aim to explore and reveal novel physics and the governing fundamental principles for the non-equilibrium quantum spin dynamics. Firstly, I aim to realize novel quantum phenomena and quantum effects, which are difficult to be detected in the equilibrium state, such as complex many-body bound states. Secondly, I will explore novel characteristics for exotic quantum states like quantum spin liquids and quantum critical phases in the nonlinear response regime, by driving the quantum disordered states far from equilibrium. Thirdly, I aim to tune and control the non-equilibrium and nonlinear response of the quantum spin states either by the terahertz electromagnetic fields directly or via coupling to other degrees of freedom, such as phonons. Gaining momentum from the on-going intensive theoretical studies and based on my previous work in the field of quantum spin systems, I anticipate a productive, impactful, and successful research project exploring the new physics offered by non-equilibrium quantum spin systems.