Investigation of local breakdown of the Quantum Hall effect in graphene probed with invasive metal contacts:

We investigate the local breakdown of the Quantum Hall Effect (QHE) effect in a graphene device caused by the constraints of the Hall bar geometry and finite size of the sample. First to account for principal features of the data we use a multi-carrier model that simulates the transverse and longitudinal resistance of doped samples in low magnetic fields. This model however can not reproduce the reduction of Hall voltage measured in the vicinity of the current leads. Therefore we introduce a second model which describes the effect of distribution of electric field in the device, giving a reasonable scenario that can explain our experimental data and deviations from the case of perfect geometry. We conclude that the existence of Hot Spots in two corners of the sample leads to the local break down of the QHE around this points.M.S.Includes bibliographical references (p. 65-68)by Fabian Duer