In close analogy to the hydrogen molecule, formed by the binding of two hydrogen atoms, two excitons of opposite spin can form a bound state, called a biexciton. It is clear that the reduction of the energy of two excitons by the additional biexciton binding energy E bi is yet smaller than the exciton binding energy E R itself. As a rule of thumb, Hayne’s rule  tells us that the biexciton binding energy in three dimensions is 10% of the exciton binding energy. In quantum wells, the experimental results indicate a value of 20%. For two excitons of equal spin, on the other hand, the interaction is effectively repulsive. This behaviour is obtained even within the Hartree-Fock approximation, where the effective exciton-exciton exchange interaction arising from the Pauli exclusion principle leads to a blueshift of the exciton as discussed in Sect. 6.3. The effective force remains repulsive if correlation effects are included.
KeywordsCircular Polarization Pump Pulse Nonlinear Absorption Interaction Contribution Exciton Density
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