Temperature-linear resistivity in twisted double bilayer graphene

We report an experimental study of carrier density (n), displacement field, and twist angle (0) dependence of temperature (T)-linear resistivity in twisted double-bilayer graphene (TDBG). Fora large twist angle (0 > 1.5??) where correlated insulating states are absent, we observe a T-linear resistivity (with a slope on the order of - 10 O/K) over a wide range of carrier densities, and its slope decreases with increasing n, which is in agreement with the acoustic phonon scattering model semiquantitatively. The slope of T-linear resistivity is nonmonotonically dependent on the displacement field with a single peak structure. Fora device with 0 - 1.23?? at which correlated states emerge, the slope of T-linear resistivity is found to be a maximum (-100 O/K) at the boundary of the halo structure where phase transition occurs, with signatures of continuous phase transition, Planckian dissipation, and diverging effective mass. These observations are in line with quantum critical behaviors, which might be due to symmetry-breaking instability at the critical points. Our results shed new light on correlated physics in TDBG and other twisted moir?? systems.