Polar Vortices in Microwave Electronics

Under the right lattice periodicity conditions, internal stress in PbTiO₃-SrTiO₃ superlattices forces the polarization into a vortex structure. This structure is a polar vortex. Polar vortices have remarkable properties such as negative permittivity. More recently, we observed magnetic-like behavior in polar vortex films, and we suspect could be a form of nanoscale structural magnetism. Structural magnetism can occur when a non-magnetic material exhibits magnetic behavior under an alternating electromagnetic field due to structure. While there are experimental demonstrations of structural magnetism in metamaterials, there are no materials with structural magnetism at the nanoscale. Here, we study complex permittivity of superlattice films versus periodicity as a function of frequency to understand the frequency dependent signatures of vortex formation and how they might be indicative of negative permittivity and structural magnetism. Our measurements show the complex permittivity to 325 GHz both parallel and perpendicular to the vortex core and as a function of bias voltage. If true, such remarkable properties could enable new microwave devices including parametric amplifiers and non-reciprocal devices like circulators.