Slightly belated congratulations to Yu, Fangyuan, and collaborators for their lovely work published recently in Nature (some additional coverage here and here). This paper sorted out a puzzle: why, in twisted bilayer graphene does the system seem to “know” about spin, valley, and sublattice symmetry breaking at high temperature, but does not always break all the symmetries at low temperatures? It turns out that the entropy of those degrees of freedom is higher than expected, favoring a disordered ferromagnet at high temperatures even as the system is paramagnetic. The physics is similar to that of the Pomeranchuk effect in 3He, in which the liquid freezes as it is heated due to the increased entropy of its nuclear spins.
