Exchange calculations between donors in silicon and metal-phthalocyanine dimer

This thesis describes the calculation of exchange interactions for systems of electrons, and their potential uses for quantum information processing. The first part treats exchange between donor impurities (including especially deep donors) in silicon, while the second describes exchange in copper and manganese phthalocyanines. Part I uses the quantum defect method and a model central-cell correction to evaluate the exchange interaction between two deep donors by a Heitler-Londontype approach. This part also describes calculations of the exchange in a threedonor complex, where one donor plays the role of a ‘control atom ’ whose optical excitation switches the coupling strengths, and the other two are qubits embody­ ing the quantum information. Variational calculations are described which give the control electron freedom to reside on different parts of the complex; it is found th a t the exchange between two qubits is increased when the control electron be­ comes delocalized, and tha t it is possible for the exchange to become ferromagnetic. These observations are rationalised in terms of multi-centre exchange processes using Green’s function perturbation theory. In Part II, exchange interactions between copper(II) phthalocyanine (Cu(II)Pc) and manganese (II) phthalocyanine dimers (M n(II)Pc)are described; these systems have both long excited-state lifetimes and tunable magnetic properties. The the­ oretical studies described include both a model Green’s function calculation, to understand the roles of the competing exchange processes, and ab initio density functional theory calculations. The model calculations of Cu(II)Pc show tha t the dominant contribution comes from so-called indirect exchange, and depends strongly on the stacking angle. The magnitude of the exchange interaction from the ab initio calculations of Cu(II)Pc roughly agrees with the experiment.