Interaction-limited conductivity of twisted bilayer graphene revealed by giant terahertz photoresistance.

Abstract

Identifying the microscopic processes limiting conductivity is essential for understanding correlated quantum materials. In twisted bilayer graphene, metallic resistivity follows ρ ~ T^α with widely varying α, fueling competing interpretations from phonon-limited transport and umklapp scattering to strange metallicity and heavy-fermion renormalization. Here, we use terahertz excitation to selectively heat electrons while keeping the lattice cold, separating electron-electron from electron-phonon contributions to resistivity. We observe a giant terahertz photoresistance - reaching several kΩ - showing that electron-electron scattering remains significant even in regimes previously attributed to phonons, including the linear-in-T resistivity near the magic angle. Away from it, photoresistance coexists with quadratic-in-T resistivity at low carrier densities where umklapp and Baber scattering are kinematically forbidden. We identify the breakdown of Galilean invariance in the Dirac dispersion, enabling inter-valley electron-electron collisions, as a possible origin. Our approach establishes terahertz-driven hot-electron transport as a framework for disentangling scattering mechanisms in low-density quantum materials.