The Pomeranchuk Effect in Magic Angle Graphene Revealed by Electronic Entropy Measurements

In the 1950's, Pomeranchuk predicted that, counterintuitively, liquid ³He may solidify upon heating, due to a high excess spin entropy in the solid phase. In this talk, I will review our electronic entropy and compressibility measurements, which demonstrate an analogous electronic effect in magic angle twisted bilayer graphene. Our experiments reveal a giant magnetic entropy (~1kB per moiré site) near a filling of one electron per moiré site. This entropy drives a Pomeranchuk-like transition from a rather convention metal to a correlated state with nearly-free magnetic moments. However, while in ³He it is easy to understand why the spins of localized atoms in the solid are practically free, it is very surprising to observe nearly-free moments in a metallic, compressible state in magic angle graphene. The nature of this newly observed correlated metallic state is thus still highly puzzling.

1. Rozen, A. et al. Entropic evidence for a Pomeranchuk effect in magic angle graphene. Nature 592, 214-219 (2021).