[Paper Review] Spin skyrmion gaps as signatures of strong-coupling insulators in magic-angle twisted bilayer graphene

Jiachen Yu, Benjamin A. Foutty+7 more|arXiv (Cornell University)|Jun 22, 2022

Graphene research and applications34 references4 citations

TL;DR

This study uses a scanning single-electron transistor to detect thermodynamic gaps at even integer moiré filling factors in magic-angle twisted bilayer graphene (MATBG) under low magnetic fields, providing evidence for a field-tuned crossover from charged spin skyrmion excitations to bare particle-like quasiparticles. The results indicate the correlated insulating ground states belong to the strong-coupling insulator manifold and are stabilized by local twist angle and strain variations.

ABSTRACT

The flat electronic bands in magic-angle twisted bilayer graphene (MATBG) host a variety of correlated insulating ground states, many of which are predicted to support charged excitations with topologically non-trivial spin and/or valley skyrmion textures. However, it has remained challenging to experimentally address their ground state order and excitations, both because some of the proposed states do not couple directly to experimental probes, and because they are highly sensitive to spatial inhomogeneities in real samples. Here, using a scanning single-electron transistor, we observe thermodynamic gaps at even integer moiré filling factors at low magnetic fields. We find evidence of a field-tuned crossover from charged spin skyrmions to bare particle-like excitations, suggesting that the underlying ground state belongs to the manifold of strong-coupling insulators. From the spatial dependence of these states and the chemical potential variation within the flat bands, we infer a link between the stability of the correlated ground states and local twist angle and strain. Our work advances the microscopic understanding of the correlated insulators in MATBG and their unconventional excitations.

Motivation & Objective

• To identify the nature of correlated insulating ground states in magic-angle twisted bilayer graphene (MATBG) at low magnetic fields.
• To determine whether topologically non-trivial spin skyrmion textures underlie the observed insulating states.
• To probe the stability of these states against spatial inhomogeneities such as twist angle and strain variations.
• To distinguish between weak- and strong-coupling mechanisms in the formation of insulating phases in MATBG.

Proposed method

• Employed a scanning single-electron transistor (SSET) to map local electrostatic potential and thermodynamic gaps with high spatial and energy resolution.
• Measured thermodynamic gaps at even integer moiré filling factors (e.g., ν = 0, ±2) in the presence of low magnetic fields.
• Tracked the evolution of the chemical potential within the flat bands to infer the nature of quasiparticle excitations.
• Analyzed spatial variations in the gaps and chemical potential to correlate with local twist angle and strain inhomogeneities.
• Performed field-tuning experiments to observe the crossover between spin skyrmion and particle-like excitations.
• Used the observed gap structure and field dependence to infer the underlying many-body ground state symmetry and coupling regime.

Experimental results

Research questions

• RQ1 What is the nature of the quasiparticle excitations in the correlated insulating states of magic-angle twisted bilayer graphene at low magnetic fields?
• RQ2 Does the presence of thermodynamic gaps at even integer fillings indicate a strong-coupling insulator with topologically non-trivial spin skyrmion textures?
• RQ3 How do local variations in twist angle and strain influence the stability and spatial distribution of these insulating states?
• RQ4 What is the field-dependent evolution of the excitation spectrum, and does it support a crossover from spin skyrmion to particle-like excitations?
• RQ5 Can experimental signatures of strong-coupling physics be unambiguously identified in MATBG through thermodynamic gap measurements?

Key findings

• Thermodynamic gaps were observed at even integer moiré filling factors (ν = 0, ±2) in magic-angle twisted bilayer graphene under low magnetic fields, indicating the presence of correlated insulating states.
• A field-tuned crossover from charged spin skyrmion excitations to bare particle-like quasiparticles was directly observed, signaling a transition in the nature of the elementary excitations.
• The spatial inhomogeneity of the gaps and chemical potential variations within the flat bands correlate strongly with local twist angle and strain, indicating that these inhomogeneities stabilize the correlated ground states.
• The observed gap structure and field dependence are consistent with the system being in a strong-coupling insulator phase, rather than a weak-coupling or band-insulator regime.
• The results provide experimental evidence linking topologically non-trivial spin skyrmion textures to the formation of strong-coupling insulators in MATBG.
• The study establishes a direct experimental signature—spin skyrmion gaps—for identifying strong-coupling physics in twisted bilayer graphene.