Anomalous transport in isolated flatbands with interactions

In flatband systems, the vanishing bandwidth amplifies even a small Coulomb repulsion, leading to observations of a multitude of unconventional phases. Motivated by strange metallic behavior in these systems [1], we study thermoelectric transport in the Lieb lattice, with a flatband arising from geometric frustration. Using Determinant Quantum Monte Carlo, we calculate the temperature-dependent compressibility which allows us to distinguish metallic vs insulating regimes. We find an anomalous sign change of the Seebeck coefficient near half filling across an interaction driven metal-insulator crossover, similar to previous results in a single band Hubbard model relevant for cuprates [2]. In addition, the optical conductivity shows the absence of a Drude peak in the metallic regime. We attribute the anomalous behavior to local moment formation in the flat bands interacting with nearby dispersive bands. The emergent Kondo-like behavior is reflected in the thermodynamics (entropy, specific heat, and magnetic susceptibility). Our results are important to understand the universal behavior of kagome metals and other flatband systems.