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Abstract The unusual properties of superconductivity in magic-angle twisted bilayer graphene (MATBG) have sparked considerable research interest1–13. However, despite the dedication of intensive experimental efforts and the proposal of several possible pairing mechanisms14–24, the origin of its superconductivity remains elusive. Here, by utilizing angle-resolved photoemission spectroscopy with micrometre spatial resolution, we reveal flat-band replicas in superconducting MATBG, where MATBG is unaligned with its hexagonal boron nitride substrate11. These replicas show uniform energy spacing, approximately 150 ± 15 meV apart, indicative of strong electron–boson coupling. Strikingly, these replicas are absent in non-superconducting twisted bilayer graphene (TBG) systems, either when MATBG is aligned to hexagonal boron nitride or when TBG deviates from the magic angle. Calculations suggest that the formation of these flat-band replicas in superconducting MATBG are attributed to the strong coupling between flat-band electrons and an optical phonon mode at the graphene K point, facilitated by intervalley scattering. These findings, although they do not necessarily put electron–phonon coupling as the main driving force for the superconductivity in MATBG, unravel the electronic structure inherent in superconducting MATBG, thereby providing crucial information for understanding the unusual electronic landscape from which its superconductivity is derived.
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Magic, symmetry, and twisted matter
The discovery in 2018 of superconductivity when two layers of graphene are stacked on top of each other at a “magic angle” has opened a new paradigm for studying electronic phenomena ( 1 ). Now, a pair of studies, one on page 1133 of this issue by Hao et al. ( 2 ) and the other by Park et al. ( 3 ), take the twisting magic trick one step further. More robust and tunable superconductivity was realized in three-layer stacks of graphene arranged at an alternating magic twist angle that is a factor of greater than the magic angle for bilayers. The authors also present evidence that superconductivity in twisted graphene is not caused by the conventional weak-coupling Bardeen-Cooper-Schrieffer (BCS) electron-pairing mechanism. The mechanism of pairing remains unknown, but the experiments suggest that the electrons form tightly bound pairs at temperatures above those at which superconductivity is macroscopically detected.
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- Award ID(s):
- 2011750
- PAR ID:
- 10324837
- Date Published:
- Journal Name:
- Science
- Volume:
- 371
- Issue:
- 6534
- ISSN:
- 0036-8075
- Page Range / eLocation ID:
- 1098 to 1099
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Magic-angle twisted bilayer graphene (MATBG) exhibits a panoply of many-body phenomena that are intimately tied to the appearance of narrow and well-isolated electronic bands. The microscopic ingredients that are responsible for the complex experimental phenomenology include electron–electron (phonon) interactions and nontrivial Bloch wavefunctions associated with the narrow bands. Inspired by recent experiments, we focus on two independent quantities that are considerably modified by Coulomb interaction-driven band renormalization, namely the density of states and the minimal spatial extent associated with the Wannier functions. First, we show that a filling-dependent enhancement of the density of states, caused by band flattening, in combination with phonon-mediated attraction due to electron-phonon umklapp processes, increases the tendency towards superconducting pairing in a range of angles around magic-angle. Second, we demonstrate that the minimal spatial extent associated with the Wannier functions, which contributes towards increasing the superconducting phase stiffness, also develops a nontrivial enhancement due to the interaction-induced renormalization of the Bloch wavefunctions. While our modeling of superconductivity (SC) assumes a weak electron-phonon coupling and does not consider many of the likely relevant correlation effects, it explains simply the experimentally observed robustness of SC in the wide range of angles that occurs in the relevant range of fillings.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1126/science.abg5641
