Giant thermopower and power factor in magic angle twisted bilayer graphene at low temperature

Abstract

Abstract The in-plane phonon-drag thermopower S g, diffusion thermopower S d and the power factor PF are theoretically investigated in a twisted bilayer graphene (tBLG) as a function of twist angle θ, temperature T and electron density n s in the region of low T (1–20 K). As θ approaches magic angle θ m, the S g and S d are found to be strongly enhanced, which is manifestation of great suppression of effective Fermi velocity v F * of electrons in moiré flat band near θ m. This enhancement decreases with increasing θ and T. In the Bloch–Grüneisen (BG) regime, it is found that S g ∼ v F *−2, T 3 and n s −1/2. As T increases, the exponent δ in S g ∼ T δ , changes from 3 to nearly zero and a maximum S g value of ∼10 mV K−1 at ∼20 K is estimated. S g is found to be larger (smaller) for smaller n s in low (high) temperature region. On the other hand, S d, taken to be governed by Mott formula with S d ∼ v F *−1, T and n s −1/2 and S d ≪ S g for T > ∼2 K. The power factor PF is also shown to be strongly θ dependent and is very much enhanced. Consequently, possibility of a giant figure of merit is discussed. In tBLG, θ acts as a strong tuning parameter of both S g and S d and PF in addition to T and n s. Our results are qualitatively compared with the measured out-of-plane thermopower in tBLG.