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Interaction-driven giant thermopower in magic-angle twisted bilayer graphene

Abstract

Magic-angle twisted bilayer graphene has proved to be a fascinating platform to realize and study emergent quantum phases arising from the strong correlations in its flat bands. Thermal transport phenomena, such as thermopower, are sensitive to the particle–hole asymmetry, making them a crucial tool to probe the underlying electronic structure of this material. Here we have carried out thermopower measurements of magic-angle twisted bilayer graphene as a function of carrier density, temperature and magnetic field. We report the observation of an unusually large thermopower reaching a value of the order of 100 μV K−1 at a low temperature of 1 K. The thermopower exhibits peak-like features that violate the Mott formula in close correspondence to the resistance peaks appearing around the integer filling of the moiré bands, including the Dirac point. We show that the large thermopower peaks and their associated behaviour arise from the emergent highly particle–hole-asymmetric electronic structure, due to the sequential filling of the moiré flat bands and the associated recovery of Dirac-like physics. Furthermore, the thermopower shows an anomalous peak around the superconducting transition, which points towards the possible role of superconducting fluctuations in magic-angle twisted bilayer graphene.

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Fig. 1: Thermopower for different band structures.
Fig. 2: Thermopower measurement set-up and device response.
Fig. 3: Thermopower response at integer moiré fillings.
Fig. 4: Thermopower across the superconducting transition.
Fig. 5: Cascade of Dirac revivals and thermopower peaks around integer fillings.

Data availability

Source data are provided with this paper. Additional information related to this work is available from the corresponding author upon reasonable request.

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Acknowledgements

A.D. thanks the Department of Science and Technology (DST) and SERB (DSTO-2051), India for financial support and the MHRD, Government of India under STARS research funding (STARS/APR2019/PS/156/FS), IRPHA(IPA/2020/000034) and also acknowledges the Swarnajayanti Fellowship of the DST/SJF/PSA-03/2018-19. K.W. and T.T. acknowledge support from the Elemental Strategy Initiative conducted by the MEXT, Japan and the CREST (JPMJCR15F3), JST.

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S.C., A.K.P. and U.R. contributed to device fabrication. A.G. and A.K.P. contributed to data acquisition and analysis. R.D. contributed to initial measurements. A.D. contributed to conceiving the idea and designing the experiment, data interpretation and analysis. K.W. and T.T. synthesized the hBN single crystals. A.P., A.A., S.M. and S.B. contributed to development of theory and data interpretation. All the authors contributed to writing the manuscript.

Corresponding author

Correspondence to Anindya Das.

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Nature Physics thanks Petr Stepanov, Yuanping Chen and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Paul, A.K., Ghosh, A., Chakraborty, S. et al. Interaction-driven giant thermopower in magic-angle twisted bilayer graphene. Nat. Phys. 18, 691–698 (2022). https://doi.org/10.1038/s41567-022-01574-3

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