Possible time-reversal-symmetry-breaking fermionic quadrupling condensate in twisted bilayer graphene



Ilaria Maccari, Johan Carlström, Egor Babaev


We study the effective model for superconducting magic-angle twisted bilayer graphene beyond mean-field approximation by using Monte Carlo simulations. We consider the parameter regime where the low-temperature phase is a superconductor that spontaneously breaks time-reversal symmetry. When fluctuations are taken into account, it is shown that a fluctuations-induced phase with a fermion quadrupling order appears, where a different condensate, formed by four electrons, breaks time-reversal symmetry.

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Thanks for the interesting talk and explaining complicated ideas about the multicomponent order parameter. I have a couple of questions though:
1. What is the relation between the generic two-order parameter Ginzburg-Landau action and magic bi-layer graphene? Is there an explicit relation between the parameters of the GL functional and the TBG?
2. What is the nature of time-reversal symmetry breaking in such theories: does it involve spin or it is similar to px+ipy type orbital symmetry breaking in superconductors or something different all-together? Is there some intuition?
Thank you!

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