Dr. Petr Stepanov
University of Notre Dame
Title: Strong Electronic Correlations in Moiré Materials
Abstract: The unexpected discovery of superconductivity in magic angle twisted bilayer graphene (MATBG) immediately generated a wave of intense theoretical and experimental research attracted by its rich phase diagram, which seemingly resembles ones of copper-oxide high-temperature superconductors. Originated in low-energy ¨flat¨ electronic bands, MATBG hosts a collection of exotic phases including but not limited to superconductivity, correlated insulators, topological and magnetic orders. Compared to other strongly-correlated systems, graphene multilayers offer a unique opportunity to tune the charge carrier density in situ and adjust system properties in other ways (for example, by alternating the distance to the gate or varying the dielectric environment), thus offering a potentially faster progress in understanding the underlying microscopic mechanisms governing its strong correlations. In this talk, as an example of such tuneability, I will discuss how the dielectric environment engineering affects the strong correlations in MATBG. Under a close proximity to the graphite gate (i. e. strong Coulomb interaction screening), MATBG exhibits a quenching of correlated insulator phases, while the vacated phase space is taken over by the superconductor domes. This observation demonstrates that the correlated insulating phases in MATBG can be untied from the superconductors in contrast to the case of cuprates, where the pairing occurs in a heavily interacting environment that locally favors the insulating state. In the second part of my talk, I will present an ongoing work revealing local photovoltage generation in magic angle bilayer and trilayer graphene superlattices, studied by cryogenic near-field imaging (cryo-SNOM). Light-matter interactions probed at the nanoscale help us uncover important symmetry breaking patterns, investigate strongly-correlated phases at slightly elevated temperature above the Tc, where ¨strange¨ metal and nematic ordering have been observed, and finally reveal a complex domain structure explained by the strain and twist angle inhomogeneity inherent to the entire class of moiré materials.