X— Singlet and Triplet States at High Magnetic Fields

We present theoretical and experimental results for X— singlet and triplet states in a 300 Å GaAs quantum well with applied magnetic fields in the range 5 to 30 T. The calculations highlight the very different natures of the singlet and triplet states: the singlet binds by maximising the electron–hole attraction, the triplet by minimising the electron–electron repulsion. This difference is shown to be responsible for the fact that the singlet remains the ground state at high magnetic fields, and it leads to the prediction of a cross-over between singlet and triplet ground states in an applied electric field.