Bose-Einstein condensation of a coupled two-component Bose gas

In experiments on excitons and spin-polarized atomic hydrogen, one deals with a gas involving two Bose branches (for example, paraexcitons and orthoexcitons). Based on a simple model for the coupling between these two components, we discuss how interconversion processes affect Bose condensation in such a system. Our analysis is based on an exact transformation to renormalized Bose particles which are uncoupled. We give results for the condensate fraction as a function of the temperature in the case when the two original Bose particles (a and b) are not in chemical equilibrium (i.e., the chemical potentials μₐ and μb are not equal) as well as in the case of chemical equilibrium (μₐ=μb). Our results are of interest in connection with current attempts to observe Bose condensation in atomiclike gases.