Summarize the paper
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.
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?
Let me answer your questions:
1. The derivation of the GL model is discussed in this work [D. V. Chichinadze et al., Phys. Rev. B. 101, 224513 (2020)]. In a nutshell, the authors start from a 6-patch model for fermions near the VH points, solve the gap equations to find the pairing channels, and finally derive the Landau free energy.
Thank you again for your interest!