Band-edge polarized optical absorption in europium chalcogenides

Abstract

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 $T=1.8\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. In thin layers (less than $0.4\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$), a doublet of absorption lines could be observed, with a full width at half maximum of about $50\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ in EuTe and $80\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ in EuSe. The lines are separated by about $200\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ in EuTe and $300\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ 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 $\ensuremath{\mid}4{f}^{7}(^{8}S_{7∕2})M=\ensuremath{-}\frac{7}{2}⟩$ and excited states comprised in $\ensuremath{\mid}4{f\phantom{\rule{0.1em}{0ex}}}^{6}(^{7}F)J{M}_{J};X⟩$ [where $X$ represents the $5d({t}_{2g})$ 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.