Disorder and Interactions in Graphene and other Quantum Systems

TL;DR

This thesis explores disorder and electron interactions in quantum systems, including lattice transitions, exciton behavior under electric fields, and collective excitations in graphene, revealing new insights into phase diagrams, control mechanisms, and symmetry properties.

Contribution

It provides high-precision results on Anderson transitions in different lattices, analyzes electric field effects on excitons, and investigates collective excitations and symmetries in graphene under magnetic fields.

Findings

Critical disorder increases with lattice coordination number.

Electric field can invert Aharonov-Bohm oscillations in excitons.

SU(4) symmetry governs collective excitations in graphene.

Abstract

The thesis examines the topics of disorder and electron-electron interactions in three distinct quantum systems. Firstly, the Anderson transition is studied for the BCC and FCC lattices. We obtain high precision results for the critical disorder at the band centre and the critical exponent. Comparing the critical disorder between the SC, BCC and FCC lattices, an increase is observed as a function of the coordination number of the lattice. The critical exponent is found to be approximately 1.5 in agreement with the value for the SC lattice. Energy-disorder phase diagrams are plotted for both lattice types. Next, we consider the Aharonov-Bohm effect for an exciton in a 1D ring. The aim is to determine how the addition of a constant electric field in the plane of the ring affects the Aharonov-Bohm oscillations. We observe an inversion of the oscillations in the oscillator strength at a critical electric field, with the oscillation minimum reaching zero at half a magnetic flux quantum. This suggests a possible process for controlling the formation and recombination of excitons through tuning the applied fields. The final section is concerned with collective excitations of graphene in a strong perpendicular magnetic field. The oscillator strengths and energies of collective excitations are calculated and the good quantum numbers identified. In particular, we study those arising from the SU(4) symmetry, which is due to two spin and two valley pseudospin projections. This enables us to determine the multiplet structure of the states. In addition to neutral collective excitations or excitons, we investigate the possible formation of charged collective excitations or trions from nearly full or nearly empty Landau levels. The localisation of neutral collective excitations upon a single Coulomb or delta-function impurity is also examined.