In Part I, we propose a new method for dynamic nuclear polarisation in a quasi one-dimensional quantum wire utilising the spin-orbit interaction, the hyperfine interaction, and a finite source-drain potential difference. In contrast with previous methods, our scheme does not rely on external magnetic or optical sources which makes local control much more feasible. Using this method, a significant polarisation of a few per cent is possible in currently available InAs wires which may be detected by conductance measurements. This may prove useful for nuclear magnetic resonance studies in nanoscale systems as well as in spin-based devices where external magnetic and optical sources will not be suitable. In Part II, we study an electron-hole gas within a microcavity, which is a system that exhibits spontaneous quantum coherence. We consider a model of electrons and holes interacting with each other via Coulombic forces and coupled to light in the cavity. We propose a variational mean-field ansatz for the ground state of zero temperature that consists of a coherent photonic part and bound electron-hole pairs. By minimising the free energy, variational equations are derived and their solutions presented in the low and high excitonic density regimes, corresponding to analytical results. In the dilute limit, atomic excitons (bound electron-hole pairs) are Bose condensed; and at high densities, there is pairing in momentum space to give an excitonic insulator. The intermediate regime is calculated numerically. Finally, we discuss the phase diagram and make correspondence with finite temperatures.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:597586 |
| Date | January 2008 |
| Creators | Cheung, A. C. H. |
| Publisher | University of Cambridge |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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