Zeeman splitting of excitons and biexcitons in single<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">In</mml:mi></mml:mrow><mml:mrow><mml:mn>0.60</mml:mn></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Ga</mml:mi></mml:mrow><mml:mrow><mml:mn>0.40</mml:mn></mml:mrow></mml:msub></mml:mrow><mml:mi mathvariant="normal">A</mml:mi><mml:mi mathvariant="normal">s</mml:mi><mml:mo>/</mml:mo><mml:mi mathvariant="normal">G</mml:mi><mml:mi mathvariant="normal">a</mml:mi><mml:mi mathvariant="normal">A</mml:mi><mml:mi mathvariant="normal">s</mml:mi></mml:math>self-assembled quantum dots

Self-assembled ${\mathrm{In}}_{0.60}{\mathrm{Ga}}_{0.40}\mathrm{A}\mathrm{s}/\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}$ quantum dots have been studied by single dot photoluminescence spectroscopy at $T=1.5\mathrm{K}.$ Emission from the biexciton state is observed, for which we find a binding energy of 3.1 meV. In magnetic field we observe equal Zeeman splittings for the exciton and biexciton spectral lines. The splitting of the biexciton emission is given by the splitting of the exciton in the final state of the biexciton transition. The circular polarization of the quantum dot luminescence decreases with increasing magnetic field, although the spin splitting is clearly larger than the thermal energy. This indicates that the spin relaxation is strongly suppressed in these quantum dots.