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Magneto-optics of carbon nano-allotropes

Final Report Summary - MOCNA (Magneto-optics of carbon nano-allotropes)

The work carried out within this project focussed on explorations of high field magneto-spectroscopy methods to investigate fundamental properties of nanostructures, mostly graphene-based systems but also individual carbon nanotubes. Additionally, single semiconductor quantum dots were investigated.

The observation of magneto-phonon resonances of Dirac fermions was the first relevant result of this work. Electron-phonon interaction was among the central themes in the studies of fundamental properties of graphene. One of the most spectacular manifestations of this interaction was the resonant coupling between the E2g-optical-phonon mode and selected, asymmetric, inter Landau level excitations. This effect was experimentally revealed within this project and the strength of the electron-phonon interaction in graphene was evaluated.

Magneto-Raman scattering studies of the so called two-dimensional phonon band of graphene were also carried out. The observed magnetic field evolution of this band, i.e. shift in frequency and broadening, was interpreted in terms of a curving of quasi-classical trajectories of photo-excited electrons and holes in a magnetic field. The phonon scattering efficiency that was derived from these experiments was found in fair agreement with theoretical expectations.

The observation of purely electronic response in the magneto-Raman scattering spectra was another significant result of this project. Raman scattering experiments on graphene materials were so fat limited to studies of phonon responses. The theoretical basis for Raman scattering responses due to electronic inter Landau level excitations was formulated, but signals were expected to be weak, implying the necessity to work with micro-probes and high magnetic fields. Those signals were successfully identified for the first time within the present project. The observation of electronic response in magneto-Raman scattering from graphene materials opened new possibilities to studying the properties of this new quantum Hall effect system.

Moreover, the aim of the pump-probe transmission studies that were carried out was to verify whether the application of the magnetic field slowed down the dynamics of photoexcited carries in graphene. The test was positive and the results were explained in terms of suppression of the electron-electron Auger scattering because of the non-equidistant Landau level spacing of the Dirac fermions in graphene. Auger scattering was long considered as the main obstacle for the fabrication of a tuneable far-infrared laser based on inter-Landau level emission. The obtained results pointed out that this obstacle could perhaps be overcome in case of graphene-based devices.

The magneto-transmission measurements in very high magnetic fields in the range of B = 20-60 T were also performed to probe the H-point and K-point Landau level transitions in natural graphite. At the H-point two series of transitions, whose energy evolved as a square root of the magnetic field, were observed. Polarisation-resolved measurements confirmed that the observed apparent splitting of the H-point transitions at high magnetic field could not be attributed to an asymmetry of the Dirac cone.

Investigations of individual carbon nano-tubes were also carried out, in collaboration with the group of R.J. Nicolas from the University of Oxford. Field induced brightening of 'dark excitons' was observed and the Aharonov-Bohm shifts were precisely investigated in magnetic fields that were as high as 28 T.

Vigorous research contacts with groups in Warsaw and in Ottawa resulted in some contributions to studies of semiconductor quantum dots, even though those were not initially planned within this project. Single quantum dot structures were studied. In this context, charge variation on the picoseconds scale, brightening of dark excitons in high magnetic fields and intershell exchange interaction were investigated.