# Tunable crystal symmetry in graphene–boron nitride heterostructures with coexisting moiré superlattices

## Abstract

In van der Waals (vdW) heterostructures consisting of atomically thin crystals layered on top of one another, lattice mismatch and rotation between the layers can result in long-wavelength moiré superlattices. These moiré patterns can drive notable band structure reconstruction of the composite material, leading to a wide range of emergent phenomena including superconductivity1,2,3, magnetism4, fractional Chern insulating states5 and moiré excitons6,7,8,9. Here, we investigate devices consisting of monolayer graphene encapsulated between two crystals of boron nitride (BN), in which the rotational alignment of all three components is controlled. We find that bandgaps in the graphene arising from perfect rotational alignment with both BN layers can be modified considerably depending on whether the relative orientation of the two BN layers is 0° or 60°, suggesting a tunable transition between the absence or presence of inversion symmetry in the heterostructure. Small deviations (<1°) from perfect alignment of all three layers leads to coexisting long-wavelength moiré potentials, resulting in a highly reconstructed graphene band structure featuring multiple secondary Dirac points. Our results demonstrate that the interplay between multiple moiré patterns can be utilized to controllably modify the symmetry and electronic properties of the composite heterostructure.

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## Data availability

The datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request.

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## Acknowledgements

We thank R. Ribeiro-Palau, C. Zhang and S. Chen for technical support, as well as J. Jung, M. Koshino and C. Marianetti for helpful discussions. This work was primarily supported by the NSF MRSEC program through Columbia in the Center for Precision Assembly of Superstratic and Superatomic Solids (DMR-1420634). Sample device design and fabrication was partially supported by DoE Pro-QM EFRC (DE-SC0019443). N.R.F. acknowledges support from the Stewardship Science Graduate Fellowship program provided under cooperative agreement number DE-NA0002135. C.R.D. acknowledges the support of the David and Lucile Packard Foundation. K.W. and T.T. acknowledge support from the Elemental Strategy Initiative conducted by the MEXT, Japan and the CREST (JPMJCR15F3), JST.

## Author information

N.R.F. and L.M. fabricated the devices. N.R.F. and M.Y. performed the measurements and analysed the data. K.W. and T.T. grew the hBN crystals. C.R.D. and J.H. advised on the experiments. The manuscript was written with input from all authors.

Correspondence to Cory R. Dean or James Hone.

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### Competing interests

The authors declare no competing interests.

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## Supplementary information

### Supplementary Information

Supplementary Sections 1–8, Figs. 1–8, Table 1 and refs. 1–8.

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Finney, N.R., Yankowitz, M., Muraleetharan, L. et al. Tunable crystal symmetry in graphene–boron nitride heterostructures with coexisting moiré superlattices. Nat. Nanotechnol. 14, 1029–1034 (2019) doi:10.1038/s41565-019-0547-2