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

S. S. Kubakaddi

doi:10.1088/1361-648X/abf0c2

paper

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

arXiv:2012.07103 · doi:10.1088/1361-648X/abf0c2

Abstract

The in-plane 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 $n_{s}$ 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 Fermi velocity $ν_{F}^{*}$ of electrons in moire flat band near $θ_{m}$ . This enhancement decreases with increasing $θ$ and $T$ . In the Bloch- Gruneisen (BG) regime, it is found that $S^{g} \sim ν_{F}^{*- 2}$ , $T^{3}$ and $n_{s}^{- 1/2}$ . As $T$ increases, the exponent $δ$ in $S^{g} \sim T^{δ}$ , changes from 3 to nearly zero and a maximum $S^{g}$ value of $\sim$ 10 mV/K at $\sim$ 20 K is estimated. $S^{g}$ is larger (smaller) for smaller $n_{s}$ in low (high) temperature region. On the other hand, $S^{d}$ , taken to be governed by Mott formula, $\sim ν_{F}^{*- 1}$ , $T$ and $n_{s}^{- 1/2}$ and $S^{d} << S^{g}$ for $T >\sim$ 2 K. The power factor $P F$ is also found to be strongly $θ$ dependent and 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 $P F$ in addition to $T$ and $n_{s}$ . Our results are qualitatively compared with the measured out-of-plane thermopower in tBLG.

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

Abstract

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