Mapping the twist-angle disorder and Landau levels in magic-angle graphene

Area of Science:

• Condensed Matter Physics
• Materials Science
• Quantum Phenomena

Background:

• Magic-angle twisted bilayer graphene (MATBG) exhibits flat electronic bands, correlated phases, and superconductivity.
• These properties are highly sensitive to the precise interlayer twist angle (θ).
• While global twist angle control is established, local variations remain poorly understood.

Purpose of the Study:

• To investigate the distribution and impact of local twist angle variations in MATBG.
• To correlate twist angle disorder with the quality of electronic transport characteristics.
• To explore the role of twist-angle gradients in device physics and potential applications.

Main Methods:

• Utilized a nanoscale scanning superconducting quantum interference device (SQUID-on-tip) for high-resolution imaging.
• Obtained tomographic images of Landau levels in the quantum Hall state.
• Mapped local twist angle variations with sub-0.002-degree precision over a few moiré periods.

Main Results:

• Found a correlation between twist angle disorder and degraded transport characteristics in MATBG.
• Observed significant local twist angle variations (up to 0.1 degrees) even in state-of-the-art devices.
• Demonstrated that correlated states in MATBG are fragile to twist-angle disorder.
• Identified twist-angle gradients generating substantial, gate-tunable in-plane electric fields.

Conclusions:

• Twist angle disorder is a critical factor influencing MATBG properties, affecting correlated states and superconductivity.
• Twist-angle gradients can be leveraged for bandstructure engineering and creating built-in electric fields.
• Understanding and controlling local twist angle variations is essential for advancing MATBG-based device applications.