Physics – Nuclear Physics – Nuclear Theory
Scientific paper
2004-11-16
Physics
Nuclear Physics
Nuclear Theory
117 pages, one figure. Thesis presented in partial fulfilment of the requirements for the degree of 'Master of Science' at the
Scientific paper
Generalised Dyson boson-fermion mappings are considered. These are techniques used in the analysis of the quantum many-body problem, and are instances of so-called boson expansion methods. A generalised Dyson boson-fermion mapping is a one-to-one linear but non-unitary operator that can be applied to vectors representing the states of a many-fermion system. A vector representing a fermion system maps onto a vector that represents a state of a many-body system that contains both bosons and fermions. The motivation for doing such a mapping is the hope that it will reveal some property of the system that simplifies its analysis and that was hidden in the original form. The aims of this text are to review the theory of generalized Dyson boson-fermion mappings and to find a useful application for a generalized Dyson boson-fermion mapping, by considering a non-trivial model, namely the Richardson model for superconductivity. It is the first time that a boson expansion technique is implemented for a system where the roles of both collective and non-collective fermion pairs are important. The Dyson mapping uncovers non-trivial properties of the system that aid the construction of time-independent as well as time-dependent perturbation expansions. The time-independent expansions agree with results that other authors have obtained through methods other than boson expansions. The time-dependent expansions might in future prove useful in understanding aspects of the dynamics of ultra-cold fermi gases, when time-dependent magnetic fields are used to vary the atom-atom interaction strenght.
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