Isospin Pomeranchuk effect in twisted bilayer graphene.

In condensed-matter systems, higher temperatures typically disfavour ordered phases, leading to an upper critical temperature for magnetism, superconductivity and other phenomena. An exception is the Pomeranchuk effect in He, in which the liquid ground state freezes upon increasing the temperature, owing to the large entropy of the paramagnetic solid phase. Here we show that a similar mechanism describes the finite-temperature dynamics of spin and valley isospins in magic-angle twisted bilayer graphene. Notably, a resistivity peak appears at high temperatures near a superlattice filling factor of -1, despite no signs of a commensurate correlated phase appearing in the low-temperature limit. Tilted-field magnetotransport and thermodynamic measurements of the in-plane magnetic moment show that the resistivity peak is connected to a finite-field magnetic phase transition at which the system develops finite isospin polarization. These data are suggestive of a Pomeranchuk-type mechanism, in which the entropy of disordered isospin moments in the ferromagnetic phase stabilizes the phase relative to an isospin-unpolarized Fermi liquid phase at higher temperatures. We find the entropy, in units of Boltzmann's constant, to be of the order of unity per unit cell area, with a measurable fraction that is suppressed by an in-plane magnetic field consistent with a contribution from disordered spins. In contrast to He, however, no discontinuities are observed in the thermodynamic quantities across this transition. Our findings imply a small isospin stiffness, with implications for the nature of finite-temperature electron transport, as well as for the mechanisms underlying isospin ordering and superconductivity in twisted bilayer graphene and related systems.