Strange metal behavior of the Hall angle in twisted bilayer graphene

Abstract

Twisted bilayer graphene (TBG) with interlayer twist angles near the magic angle \ensuremath{\approx}1.08\ifmmode^\circ\else\textdegree\fi{} hosts flat bands and exhibits correlated states including Mott-like insulators, superconductivity, and magnetism. A linear-in-temperature normal state resistivity in TBG has been attributed to an exotic Planckian dissipation mechanism but can be equally well explained in terms of conventional electron-phonon scattering. To address this issue, we perform combined temperature-dependent transport measurements of both the longitudinal and Hall resistivities in near-magic-angle TBG. While the observed longitudinal resistivity follows linear temperature T dependence consistent with previous reports, the Hall resistance shows an anomalous T dependence with the cotangent of the Hall angle $\mathrm{cot}\phantom{\rule{0.28em}{0ex}}{\mathrm{\ensuremath{\Theta}}}_{H}\phantom{\rule{0.28em}{0ex}}\ensuremath{\propto}\phantom{\rule{0.28em}{0ex}}{T}^{2}$. Boltzmann theory for quasiparticle transport predicts that both the resistivity and $\mathrm{cot}\phantom{\rule{0.28em}{0ex}}{\mathrm{\ensuremath{\Theta}}}_{H}$ should have the same T dependence, contradicting the observed behavior. This failure of quasiparticle-based theories is reminiscent of other correlated strange metals such as cuprates.