Interlayer electron-hole friction in tunable twisted bilayer graphene semimetal

TL;DR

This paper investigates how electron-hole interactions influence conduction in twisted bilayer graphene, demonstrating a tunable transition from a Dirac fluid to an electron-hole Fermi liquid with strong mutual friction.

Contribution

It introduces a dual-gated device to control electron-hole interactions in twisted bilayer graphene and develops a drag theory explaining the observed resistivity behavior.

Findings

Resistivity follows a T^2 dependence consistent with electron-hole drag.

Transition observed from Dirac fluid to electron-hole Fermi liquid.

Strong mutual friction between electrons and holes affects conduction.

Abstract

Charge-neutral conducting systems represent a class of materials with unusual properties governed by electron-hole (e-h) interactions. Depending on the quasiparticles' statistics, band structure, and device geometry these semimetallic phases of matter can feature unconventional responses to external fields that often defy simple interpretations in terms of single-particle physics. Here we show that small-angle twisted bilayer graphene (SA-TBG) offers a highly-tunable system in which to explore interactions-limited electron conduction. By employing a dual-gated device architecture we tune our devices from a non-degenerate charge-neutral Dirac fluid to a compensated two-component e-h Fermi liquid where spatially separated electrons and holes experience strong mutual friction. This friction is revealed through the T^2 resistivity that accurately follows the e-h drag theory we develop. Our results provide a textbook illustration of a smooth transition between different interaction-limited transport regimes and clarify the conduction mechanisms in charge-neutral SA-TBG.