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Excitons, the bosonic quasiparticle emerging from Coulomb interaction between electrons and holes, will undergo a Bose-Einstein condensa- tion(BEC) and transition into a superfluid state with global phase coherence at low tempera- tures. An important platform to study such excitonic physics is built on double-layer quan- tum wells or recent two-dimensional material het- erostructures, where two parallel planes of elec- trons and holes are separated by a thin insu- lating layer. Lowering this separation distance (d) enhances the interlayer Coulomb interaction thereby strengthens the exciton binding energy. However, an exceedingly small d will lead to the undesired interlayer tunneling, which results the annihilation of excitons. Here, we report the ob- servation of a sequences of robust exciton conden- sates(ECs) in double bilayer graphenes twisted to ∼10◦ with no insulating mid-layer. The large momentum mismatch between the two graphene layers well suppress the interlayer tunneling, al- lowing us to reach the separation lower limit ∼ 0.334 nm and investigate ECs in the extreme cou- pling regime. Carrying out transport measure- ments on the bulk and edge of the devices, we find incompressible states corresponding to ECs when both layers are half-filled in the N = 0 and N = 1 Landau levels (LLs). The comparison between these ECs and theoretical calculations suggest that the low-energy charged excitation of ECs can be meron-antimeron or particle-hole pair, which relies on both LL index and carrier type. Our results establish large-angle twisted bi- layers as an experimental platform with extreme coupling strength for studying quantum bosonic phase and its low-energy excitations.

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• Creative Commons BY Attribution 4.0 International
  

Keyword: Excitons, Bose-Einstein condensation, double bilayer graphene

Date published: 2024-06-13

Publisher: Springer Science and Business Media LLC

Journal:

• Nature Communications (ISSN: 20411723) vol. 15 issue. 1 5065

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Manuscript type: Publisher's version (Version of record)

MDR DOI:

First published URL: https://doi.org/10.1038/s41467-024-49406-7

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Updated at: 2025-02-07 16:30:21 +0900

Published on MDR: 2025-02-07 16:30:22 +0900