Magnetophonon oscillations in graphene with a large applied DC bias voltage

Wengraf, Joshua and Ponomarenko, Leonid and Geim, Andre and Eaves, Laurence (2020) Magnetophonon oscillations in graphene with a large applied DC bias voltage. Masters thesis, Lancaster University.

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Abstract

Magnetophonon resonance (MPR) oscillations are a valuable spectroscopic tool for studying electron interactions in solids. This type of spectroscopy is performed by applying a magnetic field to the solid, to quantise the energy of the charge carriers, and inter-Landau level scattering transitions are observed as oscillations in magnetoresistance. Since the transition energy is known, these oscillations can be used to identify phonons responsible for the inter-Landau level scattering. These oscillations, which arise due to scattering of Dirac fermions by transverse and longitudinal acoustic phonons, appear only in large graphene Hall bars with dimensions in excess of 10 um (greater than the phonon scattering mean free path). Here we apply large current-induced bias voltages (up to 1 mA) through a large graphene Hall bar and study the effect on the MPR oscillations. We observe a splitting of these oscillations due to a spatial tilting of the Landau levels induced by a strong Hall electric field. At sufficiently large Hall fields we also observe `phonon-less' inter-Landau level scattering transitions which arise when the Landau level states become parallel in energy. Finally, we observe an additional scattering process when the drift velocity approaches the speed of the transverse acoustic phonon.