Cryo-Near-Field Photovoltage Microscopy of Heavy-Fermion Twisted Symmetric Trilayer Graphene

The unexpected discovery of superconductivity in magic-angle twisted bilayer graphene sparked a wave of intense theoretical and experimental research, driven by its rich low-energy phase diagram. Originating from flattened electronic bands, the family of magic-angle graphene compounds hosts an array of exotic phases, including but not limited to superconductivity, correlated insulators, non-trivial topology, and magnetic orders. Compared to other strongly correlated systems, 2D multilayers offer unique opportunities to tune system properties. These include the ability to adjust charge carrier density in situ and modify other parameters, such as the distance to the gate or the dielectric environment, enabling potentially faster progress in understanding the microscopic mechanisms underlying their strong correlations. While seemingly conflicting results from electronic transport and scanning tunneling microscopy experiments have raised controversies regarding the locality of Wannier orbitals in these materials, a definitive experimental approach to reconcile these discrepancies has been highly sought after. In this talk, I will discuss local thermoelectric measurements in the flat electronic bands of twisted symmetric trilayer graphene (TSTG). Using a cryogenic near-field optical microscope with an oscillating atomic force microscopy (AFM) tip irradiated by infrared photons, we create a nanoscopic hot spot in planar samples. Driven by heat currents and sample inhomogeneities, we measure the local photovoltage and relate it to the local Seebeck coefficients. We observe a pronounced quasiparticle lifetime imbalance across the correlated minigaps, potentially linked to charge localization effects. The recently developed topological heavy fermion model applied to twisted trilayer graphene provides a natural framework for describing this complex system, enabling the use of well-established theoretical tools to study TSTG. Furthermore, we examine the statistical distribution of this phenomenon across samples with varying twist angles near the magic-angle condition. Our findings reveal a breakdown of the non-interacting Mott formalism at low temperatures (~10 K), underscoring the pivotal role of electronic interactions in photovoltage generation.