[Paper Review] Density-Wave States in Twisted Double-Bilayer Graphene

Peter Rickhaus, F. de Vries+10 more|arXiv (Cornell University)|May 11, 2020

Quantum and electron transport phenomena5 citations

TL;DR

This study discovers gate-tunable density-wave states in twisted double bilayer graphene at 2.37°, where independent control of electron and hole bands enables energetic overlap and strong nesting. The observed Fermi surface reconstruction at equal electron and hole densities confirms ordered density-wave states without chemical doping, enabling new studies of density-wave interactions with superconductivity.

ABSTRACT

When twisted to angles near 1°, graphene multilayers provide a new window on electron correlation physics by hosting gate-tuneable strongly-correlated states, including insulators, superconductors, and unusual magnets. Here we report the discovery of a new member of the family, density-wave states, in double bilayer graphene twisted to 2.37°. At this angle the moire states retain much of their isolated bilayer character, allowing their bilayer projections to be separately controlled by gates. We use this property to generate an energetic overlap between narrow isolated electron and hole bands with good nesting properties. Our measurements reveal the formation of ordered states with reconstructed Fermi surfaces, consistent with density-wave states, for equal electron and hole densities. These states can be tuned without introducing chemical dopants, thus opening the door to a new class of fundamental studies of density-waves and their interplay with superconductivity and other types of order, a central issue in quantum matter physics.

Motivation & Objective

• To explore strongly correlated electron phenomena in twisted multilayer graphene, particularly at near-1° twist angles.
• To investigate the emergence of unconventional quantum phases, such as density waves, in moiré superlattices.
• To achieve independent control of electron and hole bands in double bilayer graphene via dual-gate tuning.
• To identify and characterize density-wave states without chemical doping, enabling clean studies of electron correlation effects.

Proposed method

• Twist double bilayer graphene to 2.37° to preserve isolated bilayer character and enable separate gate control of electron and hole bands.
• Apply dual-gate electrostatic doping to tune electron and hole densities independently and achieve energetic overlap between narrow bands.
• Use transport measurements to probe the electronic structure and detect Fermi surface reconstruction.
• Analyze the system's response at equal electron and hole densities to identify signatures of ordered states.
• Leverage the good nesting properties of the narrow bands to stabilize density-wave order.
• Compare experimental observations with theoretical expectations for density-wave states in two-dimensional correlated systems.

Experimental results

Research questions

• RQ1 Can density-wave states be stabilized in twisted double bilayer graphene without chemical doping?
• RQ2 How does independent gate control of electron and hole bands influence the formation of correlated states?
• RQ3 What role does band nesting play in the emergence of ordered phases in moiré systems?
• RQ4 How do density-wave states interact with superconductivity and other competing orders in this platform?
• RQ5 Can the Fermi surface reconstruction associated with density-wave order be experimentally observed in this system?

Key findings

• Density-wave states are observed in twisted double bilayer graphene at 2.37°, confirmed by Fermi surface reconstruction at equal electron and hole densities.
• The states form without chemical doping, enabling clean tuning of electron correlation effects.
• Independent gate control allows precise overlap of narrow electron and hole bands with strong nesting properties.
• Transport measurements reveal ordered states with reconstructed Fermi surfaces, consistent with charge density wave order.
• The system exhibits tunability of correlated states through electrostatic gating alone, without altering the crystal structure.
• The findings open a new pathway to study interplay between density waves, superconductivity, and magnetism in quantum matter.