447 research outputs found
Superconductivity from Valley Fluctuations and Approximate SO(4) Symmetry in a Weak Coupling Theory of Twisted Bilayer Graphene
We develop a weak coupling approach to superconductivity in twisted bilayer
graphene, starting from the Fermi liquid regime. A key observation is that near
half filling, the fermiology consists of well nested Fermi pockets derived from
opposite valleys, leading to enhanced valley fluctuation, which in turn can
mediate superconductivity. This scenario is studied within the random phase
approximation. We find that inter-valley electron pairing with either chiral
( mixed with ) or helical form factor is the dominant
instability. An approximate SO(4) spin-valley symmetry implies a near
degeneracy of spin-singlet and triplet pairing. On increasing interactions,
commensurate inter-valley coherence wave (IVCW) order can arise, with
simultaneous condensation at the three M points in the Brillouin Zone, and a
pattern in real space. In simple treatments though, this leads to a
full gap at fillings , slightly away from half-filling. An SO(4)
symmetry breaking "anti-Hunds" coupling favors the spin-singlet order both for
the IVCW and the superconductor, consistent with observations. Mott insulators
derived from phase fluctuating superconductors are also discussed, which
exhibit both symmetry protected and intrinsic topological orders.Comment: 13 pages, 8 figures, 3 table
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