Quantum critical behaviour in magic-angle twisted bilayer graphene

The flat bands of magic-angle twisted bilayer graphene (MATBG) host strongly-correlated electronic phases such as correlated insulators, superconductors and a strange metal state. [1-4]. The latter state, believed to hold the key to a deeper understanding of the electronic properties of MATBG, is obscured by the abundance of phase transitions; so far, this state could not be unequivocally differentiated from a metal undergoing frequent electron-phonon collisions [5]. We report on transport measurements in superconducting (SC) MATBG in which the correlated insulator states were suppressed by screening [6]. The uninterrupted metallic ground state features a T-linear resistivity extending over three decades in temperature, from 40 mK to 20 K, spanning a broad range of dopings including those where a correlation-driven Fermi surface reconstruction occurs. This strange-metal behavior is distinguished by Planckian scattering rates and a linear magneto-resistivity ρ∝B. On the contrary, near charge neutrality or a fully-filled flat band, as well as for devices twisted away from the magic angle, the archetypal Fermi liquid behavior is recovered. Our measurements demonstrate the existence of a quantum-critical point whose fluctuations dominate the metallic ground state. Further, a transition to the strange metal is observed upon suppression of the SC order, which suggests an intimate relationship between quantum fluctuations and superconductivity in MATBG.

[1] Y. Cao, et al., Nature 556, 43-50 (2018).

[2] Y. Cao et al., Nature 556, 80-84 (2018).

[3] X. Lu et al., Nature 574, 653-657 (2019).

[4] Y. Cao et al., Physical Review Letters 124, 076801 (2020).

[5] H. Polshyn et al., Nature Physics 15, 1011-1016 (2019).

[6] P. Stepanov et al., Nature 583, 375-378 (2020).