PIs: Joachim Deisenhofer, Marcus Kollar

In this project we will explore ultrastrong coupling (USC) effects in the polar magnets A2Mo3O8 with A = Fe, Co, Mn, Zn and in the rare-earth orthoferrites REFeO3 (RE = Tm, Er, Y) using THz-time-domain spectroscopy in external magnetic fields. We will identify further material classes to clarify the origin of the USC and the involved bosonic states. New non-equilibrium phenomena will be investigated using intense coherent THz fields. We will develop a theoretical description in terms of quantum Rabi-type models using input from the THz data. Their complexity will depend on the importance of counter-rotating terms and the presence of asymmetric terms due to broken inversion symmetry. Numerical and analytical studies of these models will help to clarify the origin of the USC.