Quantum critical behaviour in magic-angle twisted bilayer graphene

The flat bands(1) of magic-angle twisted bilayer graphene (MATBG) host strongly correlated electronic phases such as correlated insulators(2-6), superconductors(7-11) and a strange metal state(12). The strange metal state, believed to be key for understanding the electronic properties of MATBG, is obscured by various phase transitions and so it could not be unequivocally differentiated from a metal undergoing frequent electron-phonon collisions(13). Here we report transport measurements in superconducting MATBG in which the correlated insulator states are suppressed by screening. The uninterrupted metallic ground state shows resistivity that is linear in temperature over three orders of magnitude and spans a broad range of doping, including that where a correlation-driven Fermi surface reconstruction occurs. This strange metal behaviour is distinguished by Planckian scattering rates and a linear magnetoresistivity. By contrast, near charge neutrality or a fully filled flat band, as well as for devices twisted away from the magic angle, we observe the archetypal Fermi-liquid behaviour. Our measurements demonstrate the existence of a quantum-critical phase whose fluctuations dominate the metallic ground state throughout a continuum of doping. Further, we observe a transition to the strange metal upon suppression of the superconducting order, suggesting a relationship between quantum fluctuations and superconductivity in MATBG.