In the first part of this thesis, a new method is proposed to circumvent the relaxation bottleneck that prevents high-temperature Bose-Einstein condensation of microcavity polaritons. This is achieved by relaxing polaritons with a coherent beam of phonons that is pumped in the growth direction of the quantum well. By tuning the frequency of these phonons to the energy difference between the bottleneck distribution and the ground state, resonant scattering relaxes the exciton reservoir very efficiently. Within a simple rate equation model, it is shown that above a threshold phonon intensity, an unstable and faster-than-exponential instability of the ground state population occurs, with macroscopic occupation possible on sub-picosecond timescales. Numerical results for GaAs and CdTe are presented, confirming that this method could be highly effective in alleviating the bottleneck effect. The second part addresses the problem of decoherence in the sub-Ohmic spin boson model. This previously unimportant model has recently attracted attention due to the discovery of several new physical realisations. Using Silbey and Harris’ variational method, a zero-temperature phase transition between coherent and incoherent ground states is found. The critical spin-bath coupling is extracted, and found to agree well with numerical calculations. Calculations at finite temperatures also yield transitions between coherent and incoherent spin dynamics at a critical temperature. Considering the dynamics of the variational Hamiltonian, it is shown that fluctuations of the bath lead to damping of the coherent spin-dynamics. Approaching the incoherence transition, these dynamics become highly non-Markovian. This chapter also contains a formal demonstration that an alternative variational state, the displaced-squeezed state (DSS), leads to incorrect results in the thermodynamic limit.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:597608 |
| Date | January 2008 |
| Creators | Chin, A. |
| Publisher | University of Cambridge |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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