Quantum Hall effect quantization tests in exfoliated bilayer and monolayer graphene

We report on quantization tests of the quantum Hall effect in monolayer and bilayer graphene based devices which are fabricated from natural graphite by micromechanical exfoliation. Measurements of the Hall resistance R_H with relative uncertainties in the range of 10^-7 performed using a cryogenic current comparator based resistance bridge have been combined with high-precision longitudinal resistance R_xx measurements to demonstrate that the quantization of R_H in such graphene based devices agrees with the theoretical prediction with a relative accuracy of about 2 parts in 10⁷. For a BL sample, the characterizations which have been carried out reveal the existence of a v= -4 Hall resistance plateau flat within 2 parts in 10⁶ over a finite range of electron density. At the center of the plateau, the Hall resistance agrees with R_K/4 within 1.5 part in 10⁷. Besides, a sample fabricated from a ML was also characterized: an agreement between the Hall resistance and R_K/2 on the v =2 plateau within 3 parts in 10⁷ has been demonstrated.