The shift of the optical absorption edge in an antiferromagnetic semiconductor

Thes-f model with spinS=7/2 has been used to study the temperature dependence of the optical absorption edge of an antiferromagnetic semiconductor. Two possible antiferromagnetic structures are considered: anAB two-sublattice model and an eight-subllattice model (MnO structure). For theAB two-sublattice model the density of states has been calculated as a function of temperature. A blue shift of the absorption edge of about 0.057 eV is obtained betweenT=T _N andT=0 K for this structure (using the parameters for EuTe). The dependence of the blue shift on the bandwidth and the band filling has been studied. For small values of the band filling the blue shift is practically unchanged.In the case of eight-sublattice structure the lowest single-particle energy (the bottom of the conduction band edge) has been observed as a function of temperature. Using the parameter set suitable for EuTe, it is concluded that the eight-sublattice structure does not show any shift of the optical absorption edge when the temperature is varied.