Sensing quasi-local Coulomb screening in moiré materials with planar scanning probes

The extent to which Coulomb interactions contribute to novel emergent behavior, such as correlated insulating states, superconductivity, and linear-in-T resistivity, is a key question in studies of moiré flat band systems. Global electronic transport or compressibility measurements of mesoscopic moiré devices are highly impacted by variations in twist angle in different regions of the sample, impeding the systematic investigation of its effect on electronic behavior. Using a scanning probe terminated by a few-micron diameter metallic plane parallel to a moiré heterostructure, we can measure the electronic compressibility with scanning capacitance microscopy. Examining small regions of a large-area device, we can determine the local twist angle and compressibility in each region. As the probe is lowered toward the moiré heterostructure, image charges in the tip begin to screen Coulomb interactions in the device region below. Tuning the voltages on the tip, sample and a graphite gate beneath the device enables inquiry into how states in the moiré system are affected by changes in electron-electron interactions. We report advances in development of this novel experimental system and in our understanding of Coulomb screening in 2D heterostructures.