Isospin analog of the Pomeranchuk effect in twisted bilayer graphene

In bilayer graphene rotationally faulted to 1.1 degrees, interlayer tunneling and rotational misalignment conspire to create a pair of low energy flat bands causing strongly correlated phenomena. An emerging question in twisted bilayer graphene is the role of isospin ferromagnetism. In this talk, I will describe experiments probing the finite-temperature phase diagram of isospin symmetry breaking in high quality twisted bilayer graphene using transport and thermodynamic measurements. We find that low-temperature transport at superlattice filling factor \nu=-1 shows no sign of a commensurate correlated phase, but a resistivity peak appears at a high temperature that resembles behavior observed near commensurate \nu where the low-temperature phase is a correlated insulator. Tilted field magnetotransport and direct measurements of the in-plane magnetic moment show that the resistivity peak near nu = -1 is adiabatically connected to a metamagnetic 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 it relative to the unpolarized Fermi liquid phase at elevated temperatures. Direct thermodynamic measurements of the entropy, S indeed find it to be large, S ~ 1k_B per moiré unit cell, for nu ~ ± 1, and we find that a fraction of S is suppressed by an in-plane magnetic field consistent with an isospin contribution. In contrast to Helium-3, no discontinuities are observed in the thermodynamic quantities across this transition, implying that the magnetic transitions are continuous in nature. Our findings imply a small isospin stiffness, with implications for the nature of finite temperature transport as well as the mechanisms underlying isospin ordering and superconductivity twisted bilayer graphene and related systems.