Magnetic polaron induced near-band-gap luminescence in epitaxial EuTe

Abstract

Epitaxial EuTe layers exhibit narrow luminescence peaks with a large Stokes shift and a large magnetic-field dependence. At low temperatures, the band-gap energy as well as the energy of the narrow luminescence peaks decreases with increasing field. The Stokes shift, in contrast, increases with magnetic field from $340\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ up to $500\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$. From a comparison of the magnetic-field dependence of the band gap, the luminescence peaks, and the Stokes shift with density plots of the quasiparticle bandstructure of EuTe, it is concluded that the luminescence originates from a deeper conduction band than that causing the absorption edge. The existence of this deeper band is confirmed by an analysis of the dispersion of the refraction index and by a faint absorption onset. Time-resolved photoluminescence experiments and the continuous-wave photoluminescence intensity as a function of magnetic field and temperature indicate a change of the selection rule for the luminescence transition from optically allowed to optically forbidden, either when the magnetic field exceeds the critical field or when the temperature increases considerably above the phase transition temperature. This change is attributed to the presence of magnetic polarons, which breaks the selection rules at low temperatures and below the critical field and thus enables the luminescence. In the parameter range where no magnetic polarons exist, at high temperatures and above the critical field, almost no luminescence appears.