Seminar By Petr Stepanov, University of Notre Dame

Title: Quantum Heavy Fermion Simulator in Moiré Materials

Host: Cyprian Lewandowski

Abstract: The unexpected discovery of superconductivity in magic angle twisted bilayer graphene 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 the low-energy narrow electronic bands, a family of magic angle graphene compounds hosts a collection of exotic phases including but not limited to superconductivity, correlated insulators, topological and magnetic orders. Compared to other strongly-correlated systems, 2D 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. While the seemingly disagreeing electronic transport and scanning tunneling microscopy experiments brought up a controversy about the locality of the Wannier orbitals in these materials, a definitive experimental evidence merging two patterns together has been much coveted. Here I discuss on the first local thermoelectric measurements in the flat electronic bands of the twisted symmetric trilayer graphene (TSTG). We use a cryogenic near-field optical microscope with an oscillating atomic force microscopy (AFM) tip irradiated by infrared photons to create a nanoscopic hot spot in the planar samples. We observe a breakdown of the non-interacting Mott formalism at low temperatures (~10 K) signaling an importance of the electronic interactions in PV generation. Explained by the interacting topological heavy-fermion model, our data suggest a spatial variation of the interaction strength dependent on the local twist angle. These experimental findings provide the first evidence of heavy fermion behavior in the topological flat bands of moiré graphene and epitomize an avenue to apply local thermoelectric measurements to other strongly correlated materials in the disorder-free limit.