Polar Vortices at mmWave Frequencies

PbTiO₃-SrTiO₃ superlattice films form Polar Vortices under the right lattice periodicity conditions. Experiments at kHz and THz frequencies have already shown emergent properties of these superlattice films like negative permittivity and collective lattice dynamics. However, data in the GHz range are sparse. Measurements in this frequency range are both interesting for the fundamental understanding of Polar Vortices and for potential electronic applications of the films. Here, we report on broadband measurements from 70 kHz to 110 GHz using Coplanar Waveguide devices. We extract the permittivity of a PbTiO₃-SrTiO₃ superlattice film on a DyScO₃ substrate and compare the results to those from co-fabricated PbTiO₃ and SrTiO₃ films. For all samples, we characterize the complex in-plane permittivity in two perpendicular orientations and as a function of bias voltage. Based on our results, we discuss the potential of Polar Vortex films for mmWave applications.