Mean field theory of a dipolar excitonic insulator transition in matrix-bound systems

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Abstract
A mean field theory is developed for a dipolar excitonic insulator transition occurring in matrix-bound systems, paradigms of which are low temperature alkali-doped rare gas solids at low impurity concentration. The excitonic insulator transition is driven primarily by changes in the host matrix density, and is a transition from the normal insulating domain in which the impurity electronic ground state is spherically symmetric, to an excitonic phase in which the impurity atoms possess electric dipole moments. A description is given of the electrical and optical characteristics of the system in both the normal insulating phase and the dipolar excitonic state, and it is shown that a variety of properties characteristic of the excitonic state can be deduced from experiment. Comparison is made with experimental results, with which the theory is shown to be compatible, and it is suggested that the dipolar excitonic state may have been observed widely over many years.