Band-edge polarized optical absorption in europium chalcogenides

The band-edge optical absorption of EuTe and EuSe, grown by molecular beam epitaxy, was measured, using circularly polarized light, in the Faraday configuration, at T=1.8 K. In thin layers (less than 0.4 µm ), a doublet of absorption lines could be observed, with a full width at half maximum of about 50 meV in EuTe and 80 meV in EuSe. The lines are separated by about 200 meV in EuTe and 300 meV in EuSe, and they show a strong magnetic circular dichroism. The oscillator strength for these lines increases by a factor of almost 20 when the antiferromagnetic order is changed to ferromagnetic by the application of an external magnetic field. Faraday rotation in the transparency gap shows a strongly nonlinear dependence on the intensity of the magnetic field. These results are interpreted in the framework of the model of electronic transitions between the localized ground |4f7(8S7/2)M=-7/2} and excited states comprised in |4f6(7F)JMJ;X} [where X represents the 5d(t2g) states], yielding good agreement with experiment. The strong enhancement of the oscillator strength with increasing magnetic field may be an indication of exciton formation due to the compression of the zero-field Bloch-like extended electronic wave function.