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

Abstract

The phonon-drag thermopower S g , diffusion thermopower S d and the power factor P F are theoretically investigated in twisted bilayer graphene (tBLG) as a function of twist angle θ, temperature T and electron density ns. As θ approaches magic angle θm, the S g and S d are found to be highly enhanced, which is manifestation of great suppression of Fermi velocity νF * of electrons in moire flat band near θm. In the Bloch-Gruneisen (BG) regime, it is found that S g ∼ νF * −2 , T 3 and ns −1/2 . The T 3 and ns −1/2 dependencies are, respectively, signatures of 2D phonons and the Dirac fermions. An enhancement of S g up to ∼ 500 times that of monolayer graphene (MLG) is predicted at ∼ 1 K. This enhancement decreases with increasing θ and T . As T increases, the power δ in S g ∼ T δ , changes from 3 to nearly zero and a maximum S g value of the order of ∼ 10 mV/K at ∼ 20 K is estimated. Simple relations of 'Kohn anomaly temperature' TKA with θ and ns are obtained. S g is larger (smaller) for smaller ns in low (high) temperature region. On the other hand, S d , taken to be governed by Mott formula, ∼ νF * −1 , T and ns −1/2 and is much greater than that in MLG. S g dominates S d very significantly for T >∼ 2 K. In tBLG, θ acts as a strong tuning parameter of both S g and S d , in addition to T and ns. Moreover, in the BG regime Herring's law is found to be satisfied in tBLG. The power factor P F is also found to be strongly θ dependent and is very much larger than that in MLG. P F as a function of T is found to exhibit a broad maximum, which is, for example, ∼75 W/m-K 2 for θ = 1.2 • . Consequently, possibility of a giant figure of merit is discussed.I.