RESUMO
Static and time-resolved mid-infrared spectroscopy of ferromagnetic single crystal Hg0.95Cd0.05Cr2Se4 was performed below the absorption edge, in order to reveal the origin of the electronic transitions contributing to the magneto-optical properties of this material. The mid-infrared spectroscopy reveals a strong absorption peak around 0.236 eV which formerly was assigned to a transition within the selenide-chromium complexes ([Formula: see text] Se-Cr2+). To reveal the sensitivity of the transition to the magnetic order, we performed the studies in a temperature range across the Curie temperature and magnetic fields across the value at which the saturation of ferromagnetic magnetization occurs. Despite the fact that the Curie temperature of this ferromagnetic semiconductor is around 107 K, the intensity of the mid-infrared transition reduces substantially increasing the temperature, so that already at 70 K the absorption peak is hardly visible. Such a dramatic decrease of the oscillator strength is observed simultaneously with the strong red-shift of the absorption edge in the magnetic semiconductor. Employing a time-resolved pump-and-probe technique enabled us to determine the lifetime of the electrons in the excited state of this optical transition. In the temperature range from 7 K to 80 K, the lifetime changes from 3 ps to 6 ps. This behavior agrees with the phenomenon of giant oscillator strength described earlier for weakly bound excitons in nonmagnetic semiconductors.
RESUMO
Optical characteristics of manganites with nanoscale electronic phase separation are simulated using an exact renormalization group transformation in Kirchhoff's equations. The local electric field is found to be highly inhomogeneous, exceeding the incident-wave field by orders of magnitude when the permittivities of the phases have opposite signs and plasmons are excited. The spatial scale of the field fluctuations suggests the collective character of the plasmon modes. The results of the simulation explain the optical anomalies of La(0.7)Ca(0.3)MnO(3) single crystals and films in the infrared frequency range in a natural way.
