Quantum Hall Effect of Hybrid Monolayer-bilayer Graphene Structures: Observation of Broken electron-hole Symmetry

Quantum Hall Effect (QHE) in both monolayer (1L) and bilayer (2L) graphene has been well studied in the past few years. Little attention has been paid to the magneto-transport across the 1L $\sim $2L graphene interface. Here, we present the magnetotransport measurements of several exfoliated graphene quasi-Hall bar devices which consist of partly 1L and partly 2L graphene. We focused on the Hall resistance (R$_{xy})$ across the interface between 1L and 2L graphene when the carrier types and densities are changed using a back gate voltage. We observed that when the carrier type is p type (hole), R$_{xy}$ typically shows QHE of bilayer graphene with filling factor of 4N, N being integers. When the carrier type is changed into n type (electron), the corresponding R$_{xy}$ typically shows QHE of single layer graphene with filling factor of 4(N+1/2), N being integers. We discuss possible explanations for the observed broken electron-hole symmetry in such hybrid structures.