Effective model for superconductivity in magic-angle graphene

Research output: Contribution to journalResearch articleContributedpeer-review

Contributors

  • Disha Hou - , Beijing Normal University (Author)
  • Yuhai Liu - , Beijing University of Posts and Telecommunications (Author)
  • Toshihiro Sato - , University of Würzburg (Author)
  • Fakher F. Assaad - , University of Würzburg, Würzburg-Dresden Cluster of Excellence ct.qmat (Author)
  • Wenan Guo - , Beijing Normal University, China Academy of Engineering Physics (Author)
  • Zhenjiu Wang - , Ludwig Maximilian University of Munich, Max-Planck-Institute for the Physics of Complex Systems, Würzburg-Dresden Cluster of Excellence ct.qmat (Author)

Abstract

We carry out large-scale quantum Monte Carlo simulations of a candidate field theory for the onset of superconductivity in magic-angle twisted bilayer graphene. The correlated insulating state at charge neutrality spontaneously breaks U(1) Moiré valley symmetry. Owing to the topological nature of the bands, skyrmion defects of the order parameter carry charge 2e and condense upon doping. In our calculations we encode the U(1) symmetry by an internal degree of freedom such that it is not broken upon lattice regularization. Furthermore, the skyrmion carries the same charge. The nature of the doping-induced phase transitions depends on the strength of the easy-plane anisotropy that reduces the SU(2) valley symmetry to U(1)×Z2. For large anisotropy, we observe two distinct transitions separated by phase coexistence. While the insulator to superconducting transition is of mean-field character, the U(1) transition is consistent with three-dimensional XY criticality. Hence, the coupling between the gapless charge excitations of the superconducting phase and the XY order parameter is irrelevant. At small anisotropy, we observe a first-order transition characterized by phase separation.

Details

Original languageEnglish
Article number195122
JournalPhysical Review B
Volume109
Issue number19
Publication statusPublished - 15 May 2024
Peer-reviewedYes