The blue shift of the optical absorption edge in α-MnS

The effect of antiferromagnetic ordering on the optical absorption edge in alpha -MnS is studied experimentally and theoretically. The absorption edge (3.33 eV) is conclusively determined from the dispersion of the interference fringe in a thin platelet crystal. The edge shows a blue shift of 0.22 eV on decreasing the temperature from room temperature to 4.2K. The electronic structures of MnS in both paramagnetic and antiferromagnetic phases are calculated by using the self-consistent-charge extended Huckel method in order to clarify the nature of the transition and the mechanism of the edge shift. The observed blue shift can be reasonably explained by a model which attributes the edge to a transition corresponding to the magnetic exciton constructed with a hole in the 3d orbitals and an electron in the 4p orbitals of Mn ions.