Millimeter-Wave Resonances in Polar Vortex Thin Films

Thin films of PbTiO₃/SrTiO₃ superlattices form Polar Vortices under the right lattice periodicity conditions. Experiments at kHz and THz frequencies have already shown emergent properties like emergent ferroelectricity, switchable chirality and phonon-like collective dynamics. However, data in the GHz range are sparse, despite the potential of ferroelectrics for electronic applications at these frequencies. Here, we report on broadband anisotropic complex dielectric spectroscopy from 100 MHz to 110 GHz. These measurements revealed broad relaxations as anticipated for ferroelectrics in that frequency range, but also unexpected sharp, voltage tunable resonances. We discuss our investigation of their origin and what they tell about the peculiar physics of Polar Vortices.