Correlated States in Strained Twisted Bilayer Graphenes Away from the Magic Angle

Abstract

Graphene moiré superlattice formed by rotating two graphene sheets can host strongly correlated and topological states when flat bands form at so-called magic angles. Here, we report that, for a twisting angle far away from the magic angle, the heterostrain induced during stacking heterostructures can also create flat bands. Combining a direct visualization of strain effect in twisted bilayer graphene moiré superlattices and transport measurements, features of correlated states appear at “non-magic” angles in twisted bilayer graphene under the heterostrain. Observing correlated states in these “non-standard” conditions can enrich the understanding of the possible origins of the correlated states and widen the freedom in tuning the moiré heterostructures and the scope of exploring the correlated physics in moiré superlattices.

Moiré straintronics: a universal platform for reconfigurable quantum materials

“…Our results are experimentally feasible and can be realized with state of the art strain tuning setups [49][50][51]. Although heterostrain is beginning to be explored experimentally [17,52], most strain tuning experiments focus on homostrain. However, most existing homostrain se-tups can also achieve heterostrain by clamping one layer to the substrate while leaving the other one mechanically decoupled from the substrate [53,54].…”

“…Hence, the twist angle set during the fabrication can provide a rough alignment knob while heterostrain can be used to fine tune the moiré lattice. Further, heterostrain offers an additional mechansim to generate flat bands in graphene [52,55] or tune highly correlated moiré quantum materials around critical points in the phase diagram [28]. On the other hand, our mathematical framework presents an important starting point for exploring reconstructed moiré lattices [17][18][19][20][21][22].…”

“…Heterostrain can also be used to generate flat bands in twisted bilayer graphene at non magic angles [52,55]. Since strain is usually set by applying a voltage to a piezoelectric material strain can be tuned very precisely.…”

An Atomistic Insight into Moiré Reconstruction in Twisted Bilayer Graphene beyond the Magic Angle

“…Recent experimental studies have demonstrated the ability to control TBGs with and without strain and characterize moiré reconstruction for smaller θ systems. ,,− Imaging techniques such as scanning tunneling microscopy (STM) and transmission electron microscopy (TEM) become challenging when the feature size becomes comparable to its resolution. As the size of MP decreases with an increasing twist, imaging for θ > 2° systems becomes unfeasible. , Therefore, the current understanding of reconstruction through experimental visualization is limited to low angle twists and is primarily based on image analysis techniques rather than physically measurable quantities.…”

Phase transition from a nonmagnetic to a ferromagnetic state in a twisted bilayer graphene nanoflake: the role of electronic pressure on the magic-twist

“…The success in fine-tuning the rotational alignment between the layer in these structures during the synthesis to tune the IEC has made it possible to observe these exotic phases. For example, twisted bilayer graphene (BLG), a widely studied material, 1–48 has been reported to exhibit a variety of intriguing correlated states ranging from superconductivity 2–5,17,24,31,35,37,46 to ferromagnetism 13–15,30 at a small magic twist angle of ∼1°. The emergence of dispersionless energy minibands 7,8,18–23,26–29,33,34,47 in the vicinity of the Fermi energy at ∼1° in twisted BLG leads to complete suppression of the quasiparticle's kinetic energy in frontier states and enhancement of electron–electron interaction strength at the magic angle, which gives rise to these observed, strong IEC-driven correlated phases.…”