Figure 2: Results of the time-resolved transport measurement for the graphene device without a top gate.

(a) Current as a function of time around the positive charge transport for several magnetic fields between B=−12 (bottom) and 12 T (top). The path length between the injection gate and the detector Ohmic contact is 1.1 mm. The sign of B is defined as positive when B is applied from the back of the sample. Traces are vertically offset for clarity. (b,c), Examples for the data analysis. By subtracting the current trace at B=−12 T (black trace in the top panels) from that at each B, cross-talk can be eliminated (red and green traces in the bottom panels of b and c, respectively). The dashed traces are the results of fitting by the convolution of a Gaussian function and exp(−t/T_RC) with T_RC=3 ns. From the peak position of the Gaussian function (blue solid traces), the time of flight of the charge pulse is obtained. (d) We also obtained the time of flight at B=12 T in a sample with shorter path length of 320 μm. As expected, the time of flight is proportional to the path length. (e) Red circles are the velocity of the charge pulse determined from the time of flight. Blue line represents the velocity calculated using equation 1. For the calculation, we used σ_xy measured using the Hall bar sample. The size of the error bars originates from possible fluctuations in the cross-talk that may be induced by thermal drift or mechanical vibration of the high-frequency lines and/or magnetic field. We assumed that the amplitude of the cross-talk weakly fluctuates by 10%, which results in the uncertainty in the shape of the current traces to be fitted.