[Paper Review] Correlated Insulator and Chern Insulators in Pentalayer Rhombohedral Stacked Graphene

Tonghang Han, Zhengguang Lu+8 more|arXiv (Cornell University)|May 3, 2023

Graphene research and applications45 references11 citations

TL;DR

The paper reports a correlated insulating state at zero charge density and zero displacement field, and Chern insulating states with C = -5 and C = -3 at low magnetic field in rhombohedral-stacked pentalayer graphene, along with isospin-polarized metals at high fields.

ABSTRACT

Rhombohedral stacked multilayer graphene is an ideal platform to search for correlated electron phenomena, due to its pair of flat bands touching at zero energy and further tunability by an electric field. Furthermore, its valley-dependent Berry phase at zero energy points to possible topological states when the pseudospin symmetry is broken by electron correlation. However, experimental explorations of these opportunities are very limited so far, due to a lack of devices with optimized layer numbers and configurations. Here we present electron transport measurements of hBN-encapsulated pentalayer graphene at down to 100 milli-Kelvin. We observed a correlated insulating state with >MOhm resistance at zero charge density and zero displacement field, where the tight-binding calculation predicts a metallic ground state. By increasing the displacement field, we observed a Chern insulator state with C = -5 and two other states with C = -3 at a low magnetic field of ~1 Tesla. At high displacement fields and charge densities, we observed isospin-polarized quarter- and half-metals. Therefore, rhombohedral-stacked pentalayer graphene is the first graphene system to exhibit two different types of Fermi-surface instabilities: driven by a pair of flat bands touching at zero energy, and by the Stoner mechanism in a single flat band. Our results demonstrate a new direction to explore intertwined electron correlation and topology phenomena in natural graphitic materials without the need of moiré superlattice engineering.

Motivation & Objective

• Motivate rhombohedral-stacked multilayer graphene as a platform to explore correlated electron phenomena via flat bands and tunable displacement fields.
• Demonstrate experimentally that pentalayer rhombohedral graphene hosts a correlated insulator at zero density and zero displacement field, and topologyvia Chern insulators under finite displacement field and magnetic field.
• Show that Chern numbers C = -5 and C = -3 emerge at low fields, and identify isospin-polarized quarter- and half-metals at higher displacement fields and densities.

Proposed method

• Fabricate hBN-encapsulated pentalayer rhombohedral graphene devices with precise layer control.
• Perform electron transport measurements down to 100 milli-Kelvin to map resistance versus density and displacement field.
• Apply displacement field and low magnetic field to induce and identify topological (Chern) insulating states.
• Compare experimental observations with tight-binding calculations predicting a metallic ground state in certain regimes.
• Characterize high-field regimes to reveal isospin-polarized metallic phases and Stoner-type instabilities.

Experimental results

Research questions

• RQ1 Can rhombohedral-stacked pentalayer graphene realize correlated insulating states at zero density and zero displacement field?
• RQ2 Can a low-field Chern insulator with finite Chern numbers be realized in this system through displacement-field tuning?
• RQ3 What is the role of flat bands and electron correlations in driving topological and magnetic instabilities in this material?
• RQ4 How do high displacement fields and charge densities influence the emergence of quarter- and half-metal states?
• RQ5 Does this system represent a platform for intertwined correlation and topology without moiré engineering?

Key findings

• A correlated insulating state with resistance > MΩ at zero charge density and zero displacement field is observed, contrasting with metallic ground state predictions from tight-binding models.
• A Chern insulator state with C = -5 and additional C = -3 states appear at low magnetic fields (~1 Tesla) under finite displacement field.
• At higher displacement fields and charge densities, isospin-polarized quarter- and half-metal states are observed.
• Rhombrohased pentalayer graphene enables two distinct Fermi-surface instabilities: flat-band-driven and Stoner-type in a single flat band.
• This system demonstrates a new route to intertwine electron correlation and topology in graphene without moiré superlattice engineering.