Flavour Hund's coupling, Chern gaps and charge diffusivity in moire graphene

Interaction-driven spontaneous symmetry breaking lies at the heart of many quantum phases of matter. In moire systems, broken spin/valley 'flavour' symmetry in flat bands underlies the parent state from which correlated and topological ground states ultimately emerge(1-10). However, the microscopic mechanism of such flavour symmetry breaking and its connection to the low-temperature phases are not yet understood. Here we investigate the broken-symmetry many-body ground state of magic-angle twisted bilayer graphene (MATBG) and its nontrivial topology using simultaneous thermodynamic and transport measurements. We directly observe flavour symmetry breaking as pinning of the chemical potential at all integer fillings of the moire superlattice, demonstrating the importance of flavour Hund's coupling in the many-body ground state. The topological nature of the underlying flat bands is manifested upon breaking time-reversal symmetry, where we measure energy gaps corresponding to Chern insulator states with Chern numbers 3, 2, 1 at filling factors 1, 2, 3, respectively, consistent with flavour symmetry breaking in the Hofstadter butterfly spectrum of MATBG. Moreover, concurrent measurements of resistivity and chemical potential provide the temperature-dependent charge diffusivity of MATBG in the strange-metal regime(11)-a quantity previously explored only in ultracold atoms(12). Our results bring us one step closer to a unified framework for understanding interactions in the topological bands of MATBG, with and without a magnetic field.

Automated summary

Interaction-driven spontaneous symmetry breaking plays a key role in the emergence of correlated and topological ground states in moire systems such as magic-angle twisted bilayer graphene (MATBG). Through thermodynamic and transport measurements, we have observed broken spin/valley 'flavour' symmetry in MATBG and its nontrivial topology. Furthermore, the topological nature of the flat bands is revealed by breaking time-reversal symmetry, leading to the observation of Chern insulator states with different Chern numbers at specific filling factors. Our findings shed light on the understanding of interactions in the topological bands of MATBG, both with and without a magnetic field.