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Valley Jahn-Teller Effect in Twisted Bilayer Graphene

Angeli M.
•
Tosatti E.
•
Fabrizio M.
2019
  • journal article

Periodico
PHYSICAL REVIEW. X
Abstract
The surprising insulating and superconducting states of narrow-band graphene twisted bilayers have been mostly discussed so far in terms of strong electron correlation, with little or no attention to phonons and electron-phonon effects. We found that, among the 33 492 phonons of a fully relaxed θ=1.08° twisted bilayer, there are few special, hard, and nearly dispersionless modes that resemble global vibrations of the moiré supercell, as if it were a single, ultralarge molecule. One of them, doubly degenerate at Γ with symmetry A1+B1, couples very strongly with the valley degrees of freedom, also doubly degenerate, realizing a so-called EâŠ-e Jahn-Teller (JT) coupling. The JT coupling lifts very efficiently all degeneracies which arise from the valley symmetry, and may lead, for an average atomic displacement as small as 0.5 m Å, to an insulating state at charge neutrality. This insulator possesses a nontrivial topology testified by the odd winding of the Wilson loop. In addition, freezing the same phonon at a zone boundary point brings about insulating states at most integer occupancies of the four ultraflat electronic bands. Following that line, we further study the properties of the superconducting state that might be stabilized by these modes. Since the JT coupling modulates the hopping between AB and BA stacked regions, pairing occurs in the spin-singlet Cooper channel at the inter-(AB-BA) scale, which may condense a superconducting order parameter in the extended s-wave and/or d±id-wave symmetry.
DOI
10.1103/PhysRevX.9.041010
WOS
WOS:000489830000001
Archivio
http://hdl.handle.net/20.500.11767/108494
info:eu-repo/semantics/altIdentifier/scopus/2-s2.0-85075228708
http://harvest.aps.org/bagit/articles/10.1103/PhysRevX.9.041010/apsxml
Diritti
open access
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Scopus© citazioni
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Data di acquisizione
Jun 14, 2022
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Data di acquisizione
Mar 26, 2024
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Data di acquisizione
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