Insulating state in tetralayers reveals an even-odd interaction effect in multilayer graphene

TL;DR

This study discovers an unexpected insulating state in Bernal-stacked tetralayer graphene caused by electron-electron interactions, challenging the assumption that thicker multilayers behave like graphite and revealing an even-odd interaction effect.

Contribution

It demonstrates the presence of a robust insulating state in tetralayer graphene, extending the understanding of interaction effects beyond bilayers and trilayers, and proposes a generalized interaction-driven state model.

Findings

Insulating state observed in tetralayer graphene around charge neutrality.

Insulating behavior persists at higher temperatures compared to bilayers.

Reveals an even-odd interaction effect in multilayer graphene systems.

Abstract

The absence of an energy gap separating valence and conduction bands makes the low-energy electronic properties of graphene and its multi-layers sensitive to electron-electron interactions. In bilayers, for instance, interactions are predicted to open a gap at charge neutrality, turning the system into an insulator, as observed experimentally. In mono and (Bernal-stacked) trilayers, interactions, although still important, do not have an equally drastic effect, and these systems remain conducting at low temperature. It may be expected that interaction effects become weaker for thicker multilayers, whose behavior should eventually converge to that of graphite. Here we show that this expectation does not correspond to reality by investigating the case of Bernal-stacked tetralayer graphene (4LG). We reveal the occurrence of a robust insulating state in a narrow range of carrier densities around charge neutrality, incompatible with the behavior expected from the single-particle band structure. The phenomenology resembles that observed in bilayers, but the stronger conductance suppression makes the insulating state in 4LG visible at higher temperature. To account for our findings, we suggest a natural generalization of the interaction-driven, symmetry-broken states proposed for bilayers. This generalization also explains the systematic even-odd effect of interactions in Bernal-stacked layers of different thickness that is emerging from experiments, and has implications for the multilayer-to-graphite crossover.