Combinatorics

Physics

Topology (electrical circuits)

Charge (physics)

Mathematics

Ultracold atom

Spontaneous symmetry breaking

Symmetry breaking

Condensed matter physics

Quantum mechanics

Ground state

Quantum

Nature

Interaction-driven spontaneous symmetry breaking lies at the heart of many quantum phases of matter. In moiré systems, broken spin/valley ‘flavour’ symmetry in flat bands underlies the parent state from which correlated and topological ground states ultimately emerge1–10. 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 moiré 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 regime11—a quantity previously explored only in ultracold atoms12. 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. Chemical potential measurements in twisted bilayer graphene reveal the importance of Coulomb repulsion and exchange interactions in the symmetry-broken ground state, and provide the charge diffusivity in the strange-metal regime.

Flavour Hund’s coupling, Chern gaps and charge diffusivity in moiré graphene