Charge carrier transport asymmetry in monolayer graphene

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The conductivity and Hall effect were measured in CVD-grown monolayer graphene as a function of the gate voltage, V-g, at temperatures down to T = 2 K and in magnetic fields up to B = 8 T. The minimal conductivity was observed at positive V-g, which shows the position of the charge neutrality point, V-NP. With decreasing T, V-NP 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 V-g. In strong perpendicular B, Shubnikov-de Haas (SdH) oscillations were observed in the longitudinal conductivity, sigma(xx), together with half-integer quantum Hall plateaus. It was found that |n| determined from SdH oscillations is equal to the full value induced by V-g as opposed to the Hall effect. Explanatory models for all observed phenomena are discussed.
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