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

The in-plane phonon-drag thermopower, diffusion thermopowerand the power factor PF are theoretically investigated in a twisted bilayer graphene (tBLG) as a function of twist angle, temperatureand electron densityin the region of low(1-20 K). Asapproaches magic angle, theandare found to be strongly enhanced, which is manifestation of great suppression of effective Fermi velocityof electrons in moiré flat band near. This enhancement decreases with increasingand. In the Bloch-Grüneisen (BG) regime, it is found that∼,and. Asincreases, the exponentin∼, changes from 3 to nearly zero and a maximumvalue of ∼10 mV Kat ∼20 K is estimated.is found to be larger (smaller) for smallerin low (high) temperature region. On the other hand,, taken to be governed by Mott formula with∼,andand≪for> ∼2 K. The power factor PF is also shown to be stronglydependent and is very much enhanced. Consequently, possibility of a giant figure of merit is discussed. In tBLG,acts as a strong tuning parameter of bothandand PF in addition toand. Our results are qualitatively compared with the measured out-of-plane thermopower in tBLG.