Fractional Chern Insulators Transition in Non-ideal Flat Bands of Twisted Mono-bilayer Graphene
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arXiv
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| Natura: | Preprint |
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2025
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| _version_ | 1866918216004009984 |
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| author | Song, Moru Chang, Kai |
| author_facet | Song, Moru Chang, Kai |
| contents | Fractional Chern insulators (FCIs) in ideal flat bands with Chern number $C$ are commonly understood as color-entangled states constructed from $C$ copies of the lowest Landau level. In realistic moiré systems, however, the band geometry is generally non-ideal, and the mechanism that stabilizes such FCIs remains unclear. Using twisted monolayer-bilayer graphene as a platform, we find two FCIs separated by a continuous transition driven by a geometric instability of the Bloch wave functions.Below the transition, the target $C=2$ conduction band is geometrically stable, and the resulting fractional phase is naturally described by the Halperin-$(112)$ state. Above the transition, the system is geometrically unstable, entering a Laughlin-$1/3$ phase that persists despite further degradation of standard quantum-geometry indicators. To account for this unconventional phenomenon, we propose a color-separation mechanism beyond global geometric indicators: in the geometrically unstable regime, interactions hybridize electronic states near the $K$ point and generate an emergent ideal color component that supports the FCI. We corroborate this picture by applying a weak perpendicular magnetic field that acts as a "color separator," directly visualizing the ideal subcomponent in the single-particle level. Together, these results establish mechanisms which non-ideal flat bands stabilize FCIs, substantially enlarging their viable parameter space and clarifying the role of quantum geometry in strongly correlated topological phases. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2511_12231 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Fractional Chern Insulators Transition in Non-ideal Flat Bands of Twisted Mono-bilayer Graphene Song, Moru Chang, Kai Mesoscale and Nanoscale Physics Strongly Correlated Electrons Fractional Chern insulators (FCIs) in ideal flat bands with Chern number $C$ are commonly understood as color-entangled states constructed from $C$ copies of the lowest Landau level. In realistic moiré systems, however, the band geometry is generally non-ideal, and the mechanism that stabilizes such FCIs remains unclear. Using twisted monolayer-bilayer graphene as a platform, we find two FCIs separated by a continuous transition driven by a geometric instability of the Bloch wave functions.Below the transition, the target $C=2$ conduction band is geometrically stable, and the resulting fractional phase is naturally described by the Halperin-$(112)$ state. Above the transition, the system is geometrically unstable, entering a Laughlin-$1/3$ phase that persists despite further degradation of standard quantum-geometry indicators. To account for this unconventional phenomenon, we propose a color-separation mechanism beyond global geometric indicators: in the geometrically unstable regime, interactions hybridize electronic states near the $K$ point and generate an emergent ideal color component that supports the FCI. We corroborate this picture by applying a weak perpendicular magnetic field that acts as a "color separator," directly visualizing the ideal subcomponent in the single-particle level. Together, these results establish mechanisms which non-ideal flat bands stabilize FCIs, substantially enlarging their viable parameter space and clarifying the role of quantum geometry in strongly correlated topological phases. |
| title | Fractional Chern Insulators Transition in Non-ideal Flat Bands of Twisted Mono-bilayer Graphene |
| topic | Mesoscale and Nanoscale Physics Strongly Correlated Electrons |
| url | https://arxiv.org/abs/2511.12231 |