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).