Berry curvature and orbital magnetic moment effects on magnetoresistance in two-dimensional valley systems
The mechanisms of negative magnetoresistance in various topological materials have been the center of transport study from the rise of two-dimensional and three-dimensional topological semimetals and metals, especially when the chiral anomaly induced negative magnetoresistance was predicted and observed. The role played by the Berry curvature has been studied in many aspects, including chiral magnetic effect, chiral anomaly, and other intrinsic geometric effects. Here we revisit the effects of the Berry curvature on the negative longitudinal magnetoresistance in Weyl metals, and then extend our study to include the effect of orbital magnetic moment of Bloch electrons in semiclassical Boltzmann transport theory. By examining the two-dimensional multivalley model with the corrected Boltzmann equation, we find that the valley-contrasting orbital magnetic moment allows the lifting of valley degeneracy by an out-of-plane magnetic field. We demonstrate that this leads to negative magnetoresistance, utilizing a gapped graphene model as an example. An intuitive physical picture in terms of the increased carrier density from a magnetic gating effect is proposed for this negative transverse magnetoresistance. In particular, giant negative transverse magnetoresistance is achieved after one of the two valleys is depleted by the magnetic field. This mechanism of negative magnetoresistance is argued to be relevant in ionic-liquid gated gapped graphene with a small effective mass.