Large power dissipation of hot Dirac fermions in twisted bilayer graphene

We have carried out a theoretical investigation of hot electron power loss P, involving electron-acoustic phonon interaction, as a function of twist angle θ, electron temperature T _e and electron density n _s in twisted bilayer graphene. It is found that as θ decreases closer to magic angle θ _m, P enhances strongly and θ acts as an important tunable parameter, apart from T _e and n _s. In the range of T _e = 1-50 K, this enhancement is ∼250-450 times the P in monolayer graphene (MLG), which is manifestation of the great suppression of Fermi velocity v _F ^* of electrons in moiré flat band. As θ increases away from θ _m, the impact of θ on P decreases, tending to that of MLG at θ ∼ 3°. In the Bloch-Grüneisen (BG) regime, P ∼ T _e ⁴, n _s ^-1/2 and v _F ^*-2. In the higher temperature region (∼10-50 K), P ∼ T _e ^δ , with δ ∼ 2.0, and the behavior is still super linear in T _e, unlike the phonon limited linear-in-T (lattice temperature) resistivity ρ _p. P is weakly, decreasing (increasing) with increasing n _s at lower (higher) T _e, as found in MLG. The energy relaxation time τ _e is also discussed as a function of θ and T _e. Expressing the power loss P = F _e(T _e) - F _e(T), in the BG regime, we have obtained a simple and useful relation F _e(T)μ _p(T) = (ev _s ²/2) i.e. F _e(T) = (n _s e ² v _s ²/2)ρ _p, where μ _p is the acoustic phonon limited mobility and v _s is the acoustic phonon velocity. The ρ _p estimated from this relation using our calculated F _e(T) is nearly agreeing with the ρ _p of Wu et al (2019 Phys. Rev. B 99 165112).