Scanning Probe microscopy of van der Waals heterostructures and non equilibrium magnetotransport in graphene

Abstract

Graphene is a two-dimensional semimetal that has linear dispersion in energy-momentum space. When graphene is subjected to a perpendicular magnetic field, the dispersion is no longer linear, resulting in discrete energy levels because of the formation of cyclotron orbits of different energies. This energy discretization leads to quantum oscillations in longitudinal magnetoresistance known as Shubhnikov de-Haas oscillations which provide a plethora of properties, including the effective mass of charge carriers and topological properties like Berry phase. Furthermore, the transverse resistance in the magnetic field is quantized, making it useful for resistance metrology. The quantization effects have been realized in graphene in the ohmic regime, i.e., with a small current density lt 0.01 A/m passing through the channel. Non-equilibrium magnetotransport studies in two-dimensional electron gas systems based on GaAs-AlGaAs quantum wells have been intensively investigated under high current densities, demonstrating the effect of carrier heating, magnetophonon-oscillations, and Hall field-induced magneto-oscillations in longitudinal resistance. However, the effect of high current densities on magnetotransport in graphene has not been thoroughly investigated. In this thesis, we have explored the magnetotransport in graphene Hall bar devices under non-equilibrium conditions by introducing a high current density (gt 1 A/m) through the channel, which produces a strong Hall field across the channel and results in tilting of the Landau levels. For the experiments aimed at realizing electron transitions between two cyclotron orbits in the presence of a magnetic field, the width of the channel becomes crucial. We have fabricated large-width Hall bar devices, which ensures the number of cyclotron orbits in the bulk is significant, and edge scattering will have less contribution, making it more sensitive to magnetotransport in the bulk of the channel. Making extra-large width devices becomes a significant step that requires a s...