ON THE PHYSICAL NATURE OF THE WIDE-BAND AT 2.8-EV IN THE VISIBLE LUMINESCENCE SPECTRUM OF HIGH-TC SUPERCONDUCTORS

Photoluminescence spectra of different Bi- and Y-based oxide high-T(c) superconductors are measured under laser excitation (E(exc) = 4.03 eV ). A wide luminescence band with a maximum near 2.8 eV is investigated and its temperature evolution (20-300 K) is followed. Comparison of t he spectra of various materials and their temperature evolution permit s us to draw conclusions about local luminescence centers in the bulk. The experimental data are used to propose a model of a charge-transfe r excimer complex involving the F-center and the negative molecular ox ygen ion. In spite of the low quantum yield, the luminescence of such centers can be observed due to absence of the radiation electron-hole recombination. Analysis of experimental data suggests the existence of a strong radiationless relaxation over discrete levels inside the opt ical pseudo-gap which competes with the radiation recombination.