Origin of peculiar chalcogen and temperature dependence of the spectrum width for the luminescence inEuXc(Xc=S,Se, and Te)

Abstract

The origin of the peculiar chalcogen and temperature dependence of the spectrum width of the broad luminescence band in EuX c (X c =S, Se, and Te) at about 1.1-1.3 eV for EuS, 1.4-1.6 eV for EuSe, and 1.4-1.5 eV for EuTe [J. Phys. Soc. Jpn. 63, 4616 (1994)], is studied theoretically. The study confirms the validity of the photo-induced localized magnetic polaron model accompanied with large lattice relaxations around the 4f-hole produced in the absorption process [Phys. Rev. B 52, 8140 (1995)]. It is shown that the origin for the temperature variation of the spectrum width is attributed to the temperature variation of the lattice relaxation which is closely related with the degree in the screening of the localized 4f hole by the photoexcited electron. In other words, the temperature variation of the spectrum width is directly related to how the extent of the photoexcited electron varies with temperature by the magnetic polaron effect. The chalcogen dependence is also understood on this basis by considering the difference due to the magnetic ordering from EuS (ferromagnet) to EuTe (antiferromagnet). A brief comment concerning a narrow photoluminescence band recently observed at about 1.9 eV is also given.