Strange magnetotransport in high-mobility Dirac plasma

The most recognizable feature of graphene’s electronic spectrum is its Dirac point around which interesting phenomena tend to cluster. At low temperatures, the intrinsic behavior in this regime is often obscured by charge inhomogeneity but thermal excitations can overcome the disorder at elevated temperatures and create an electron-hole (e-h) plasma of Dirac fermions. The Dirac plasma has recently been found to exhibit unusual properties including quantum critical conductivity and hydrodynamic flow. However, little is known about its behavior in magnetic fields. Here we report magnetotransport in this quantum-critical regime. In low fields, the plasma exhibits giant parabolic magnetoresistivity reaching >100% in 0.1 T even at room temperature. This is orders of magnitude higher than magnetoresistivity found in any other system at such temperatures. We show that this behavior is unique to monolayer graphene, being underpinned by its massless spectrum and ultrahigh mobility, despite frequent (Planckian-limit) scattering. With the onset of Landau quantization in a few T, where the e-h plasma resides entirely on the zeroth Landau level, giant linear magnetoresistivity emerges. It is practically independent of temperature and can be suppressed by proximity screening, indicating a many-body origin. The linear behavior is qualitatively explained by diffusion of cyclotron orbits on the zeroth Landau level through a network of shallow e-h puddles. Clear parallels with magnetotransport in strange metals and so-called quantum linear magnetoresistance predicted for Weyl metals offer an interesting playground to further explore relevant physics using this well-defined quantum-critical 2D system.