Multimodal Nano-Imaging of Bi₂Sr₂CaCu₂O_8+x/Graphene Cavities

The highly anisotropic, high-temperature cuprate superconductors host hyperbolic Cooper-pair polaritons, which encode information about the superconducting state. Similarly, graphene hosts surface plasmon polaritons that encode information about the conducting electrons and holes. In this preliminary work, we investigate the coupling of these two polaritonic modes when the two host materials are brought into close proximity. We fabricate two-dimensional van-der walls heterostructures comprised of graphene and exfoliated Bi₂Sr₂CaCu₂O_8+x (Bi-2212) microcrystals separated by a thin hBN spacer layer, thus forming a layered plasmonic cavity. Using nano-THz scattering near-field optical microscopy techniques in a homebuilt cryogenic microscope, we measure the local THz electrodynamic response of the cavity heterostructure. Using magnetic force microscopy (MFM) in the same microscope, we probe the local Meissner force of these heterostructures. Our nano-THz measurements allow us to probe how these two distinct polaritons interact and how strongly they couple to one another when in the presence of an electromagnetic field. Complimentary local MFM measurements afford us insight into local superfluid density as a function of temperature of the Bi-2212 when incorporated into a plasmonic cavity.