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

TL;DR

This paper theoretically investigates the thermopower and power factor in twisted bilayer graphene near the magic angle, revealing giant enhancements at low temperatures due to suppressed Fermi velocity, with potential implications for thermoelectric applications.

Contribution

It provides a detailed theoretical analysis of phonon-drag and diffusion thermopower in tBLG, highlighting the strong dependence on twist angle and temperature, and predicts giant thermopower near the magic angle.

Findings

Giant thermopower of ~10 mV/K at 20 K near the magic angle.

Strong enhancement of power factor and thermopower as twist angle approaches the magic angle.

Thermopower exhibits a transition from T^3 dependence to nearly zero exponent with increasing temperature.

Abstract

The in-plane phonon-drag thermopower $S^g$, diffusion thermopower $S^d$ and the power factor $PF$ are theoretically investigated in twisted bilayer graphene (tBLG) as a function of twist angle $\theta$, temperature $T$ and electron density $n_s$ in the region of low $T$ (1-20 K). As $\theta$ approaches magic angle $\theta_m$, the $S^g$ and $S^d$ are found to be strongly enhanced, which is manifestation of great suppression of Fermi velocity ${\nu_F}^*$ of electrons in moire flat band near $\theta_m$. This enhancement decreases with increasing $\theta$ and $T$. In the Bloch- Gruneisen (BG) regime, it is found that $S^g \sim {\nu_F}^{* -2}$, $T^3$ and ${n_s}^{-1/2}$. As $T$ increases, the exponent $\delta$ in $S^g \sim T^\delta$, 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, $\sim {\nu_F}^{* -1}$, $T$ and ${n_s}^{-1/2}$ and $S^d<<S^g$ for $T > \sim$ 2 K. The power factor $PF$ is also found to be strongly $\theta$ dependent and very much enhanced. Consequently, possibility of a giant figure of merit is discussed.In tBLG, $\theta$ 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.