The kinetic theory of ultra-subsonic fermion systems and applications to flat band magic angle twisted bilayer graphene

The only kinematically-allowed phonon-scattering events for bands of subsonic fermions (v_F < v_p) are interband transitions, leading to different low-T transport physics than in the typical supersonic case. We apply a kinetic theory of phonon-limited transport to a generic two-band system of subsonic fermions, deriving formulae for relaxation times and resistivity that are accurate in the limit of close bands and small v_F/v_p. We predict regimes of ρ∝ T, ρ∝ T^2, and perfect conductivity. Our theory predicts linear-in-T resistivity down to a crossover temperature that is suppressed from its supersonic analogue by a factor of v_F/v_p, offering a new explanation for low-T "strange metal" behavior observed in flat band systems.