Strange metal behavior of the Hall angle in twisted bilayer graphene*

Twisted bilayer graphene (TBG) has emerged as a tunable system exhibiting a number of correlated phases including Mott-like insulators, superconductivity, and magnetism. A linear in temperature normal state resistivity has been attributed to an exotic Planckian dissipation[1] mechanism but can be equally well explained in terms of conventional electron-phonon scattering [2, 3]. Here we discuss combined temperature-dependent transport measurements of both the longitudinal and Hall resistivities in close to magic-angle TBG. While the longitudinal resistivity is consistent with previous reports [1, 2], the Hall resistance shows an anomalous T dependence, providing new insight and a cotangent of the Hall angle cot(Θ_H)∝T². Boltzmann theory for quasiparticle transport predicts that both the resistivity and cot(Θ_H) should have the same T dependence, contradicting the observed behavior. This provides strong evidence that TBG is incompatible with ordinary quasiparticle transport, reminiscent of other correlated strange metals such as cuprates.
[1] Y. Cao, et al. PhysRevLett.124.076801(2020)
[2] H. Polshyn, et al. Nat. Phys.15, 1011-1016(2019)
[3] J. A. N. Bruin, et al. Science 339, 804-807(2013)