Theory of Biexcitons

The biexciton spectrum is analyzed taking into account the fact that the number of quasi-particles is not conserved. It is shown that in this case, apart from small corrections to the biexciton spectrum of the Heitler–London approximation, new biexciton levels appear. Besides, it is shown that, contrary to what is usually believed, in this case the approximate second quantization method does not give the correct zero-order spectrum of free excitons. The correct zero-order spectrum is also presented.