Charge carrier transport asymmetry in monolayer graphene

The conductivity and Hall effect were measured in CVD-grown monolayer graphene as a function of the gate voltage, Vg, at temperatures down to T=2 K and in magnetic fields up to B=8 T. The minimal conductivity was observed at positive Vg, which shows the position of the charge neutrality point, VNP. With decreasing T, VNP first decreases, but stops to decrease at low T. The hysteresis of conductivity shows a similar behavior: it decreases with decreasing T and disappears at low T. A significant asymmetry was observed at low density of charge carriers |n|=(n,p): the mobility of holes was less than the mobility of electrons. The asymmetry decreases with increasing |n|. It was observed that the value of |n| determined from the Hall effect is less than the full value induced by Vg. In strong perpendicular B, Shubnikov-de Haas (SdH) oscillations were observed in the longitudinal conductivity, σxx, together with half-integer quantum Hall plateaus. It was found that |n| determined from SdH oscillations is equal to the full value induced by Vg as opposed to the Hall effect. Explanatory models for all observed phenomena are discussed.