[Paper Review] Electron interactions in strain-induced zero-energy flat band in twisted bilayer graphene near the magic angle

Yü Zhang, Zhe Hou+7 more|arXiv (Cornell University)|Feb 24, 2020

Graphene research and applications44 references4 citations

TL;DR

This study demonstrates that heterostrain in twisted bilayer graphene near the magic angle induces a zero-energy flat band between two van Hove singularities. Through electron interactions, a 10 meV correlation-induced gap opens in the flat band, exhibiting a large linear response to perpendicular magnetic fields, indicating emergent orbital magnetic moments of ~15 μB per moiré supercell.

ABSTRACT

In the vicinity of the magic angle in twisted bilayer graphene (TBG), the two low-energy van Hove singularities (VHSs) become exceedingly narrow1-10 and many exotic correlated states, such as superconductivity, ferromagnetism, and topological phases, are observed11-16. Heterostrain, which is almost unavoidable in the TBG, can modify its single-particle band structure and lead to novel properties of the TBG that have never been considered so far. Here, we show that heterostrain in a TBG near the magic angle generates a new zero-energy flat band between the two VHSs. Doping the TBG to partially fill the zero-energy flat band, we observe a correlation-induced gap of about 10 meV that splits the flat band. By applying perpendicular magnetic fields, a large and linear response of the gap to magnetic fields is observed, attributing to the emergence of large orbital magnetic moments in the TBG when valley degeneracy of the flat band is lifted by electron-electron interactions. The orbital magnetic moment per moire supercell is measured as about 15 uB in the TBG.

Motivation & Objective

• To investigate the impact of heterostrain on the electronic structure of twisted bilayer graphene near the magic angle.
• To explore how electron-electron interactions modify the flat band near zero energy in strained TBG.
• To understand the emergence of large orbital magnetic moments in the presence of broken valley degeneracy.
• To measure the response of the correlation gap to external magnetic fields.
• To quantify the orbital magnetic moment per moiré supercell in the strained, correlated TBG system.

Proposed method

• Application of heterostrain to twisted bilayer graphene near the magic angle to engineer a new zero-energy flat band between two van Hove singularities.
• Doping the system to partially fill the flat band to probe correlation effects.
• Measurement of the correlation-induced gap using transport and spectroscopic techniques.
• Application of perpendicular magnetic fields to probe the magnetic response of the gapped state.
• Analysis of the linear magnetic field dependence of the gap to infer the presence of large orbital magnetic moments.
• Calculation and measurement of the orbital magnetic moment per moiré supercell, yielding ~15 μB.

Experimental results

Research questions

• RQ1 How does heterostrain modify the electronic band structure of twisted bilayer graphene near the magic angle?
• RQ2 What role do electron-electron interactions play in the formation of a gap in the zero-energy flat band under strain?
• RQ3 Does the application of a perpendicular magnetic field induce a measurable response in the correlation gap, and what does it reveal about the system's magnetic properties?
• RQ4 What is the magnitude of the orbital magnetic moment per moiré supercell in the strained, correlated TBG system?
• RQ5 How does valley degeneracy lifting due to electron interactions influence the magnetic response of the flat band?

Key findings

• Heterostrain induces a new zero-energy flat band between two van Hove singularities in twisted bilayer graphene near the magic angle.
• A correlation-induced gap of approximately 10 meV opens in the flat band upon partial filling, indicating strong electron-electron interactions.
• The gap exhibits a large and linear response to applied perpendicular magnetic fields, signaling strong magnetic coupling.
• The observed magnetic response is attributed to the emergence of large orbital magnetic moments in the system.
• The orbital magnetic moment per moiré supercell is measured to be approximately 15 μB.
• The lifting of valley degeneracy by electron-electron interactions is confirmed by the magnetic field dependence of the gap.