  The magnetoresistance (MR) of a material is typically insensitive to
reversing the applied field direction and varies quadratically with magnetic
field in the low-field limit. Quantum effects [1], unusual topological band
structures [2], and inhomogeneities that lead to wandering current paths [3, 4]
can induce a crossover from quadratic to linear magnetoresistance with
increasing magnetic field. Here we explore a series of metallic charge- and
spin-density-wave systems that exhibit extremely large positive linear
magnetoresistance. By contrast to other linear MR mechanisms, this effect
remains robust down to miniscule magnetic fields of tens of Oersted at low
temperature. We frame an explanation of this phenomenon in a semi-classical
narrative for a broad category of materials with partially-gapped Fermi
surfaces due to density waves.
