Evidence of a compensated semimetal with electronic correlations at the CNP of twisted double bilayer graphene

TL;DR

This paper provides experimental evidence that twisted double bilayer graphene near the charge neutrality point behaves as a compensated semimetal with electronic correlations, showing enhanced thermopower, large magnetoresistance, and unusual temperature dependence of resistance.

Contribution

It demonstrates the emergence of a nearly compensated semimetal phase at the CNP of TDBLG, with signatures of electronic correlations and critical fluctuations, expanding understanding of moiré superlattice physics.

Findings

Enhanced thermopower by nearly two orders of magnitude in magnetic fields

Large magnetoresistance (~2500%) at CNP

Sublinear temperature dependence of resistance at low temperatures

Abstract

Recently, magic-angle twisted bilayer graphene (MATBLG) has shown the emergence of various interaction-driven novel quantum phases at the commensurate fillings of the moir'e superlattice, while the charge neutrality point (CNP) remains mostly a vanilla insulator. Here, we show an emerging phase of nearly compensated semimetallicity at the CNP of twisted double bilayer graphene (TDBLG), a close cousin of MATBLG, with signatures of electronic correlation. Using electrical and thermal transport, we find almost two orders of magnitude enhancement of the thermopower in magnetic fields much smaller than the extreme quantum limit, accompanied by a large magnetoresistance($\sim 2500\%$) at CNP. This provides indisputable experimental evidence that TDBLG near CNP is a compensated semimetal. Moreover, at low temperatures, we observe an unusual sublinear temperature dependence of resistance. A recent theory predicts the formation of an excitonic metal near CNP, where small electron and hole pockets coexist. We understand the sublinear temperature dependence in terms of critical fluctuations in this theory.