On the physical nature of the wide band at 2.8 eV in the visible luminescence spectrum of high- Tc superconductors

Abstract

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 (20-300K) is followed. Comparison of the spectra of various materials and their temperature evolution permits us to draw conclusions about local luminescence centers in the bulk. The experimental data are used to propose a model of a charge-transfer excimer complex involving the F-center and the negative molecular oxygen 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 optical pseudo-gap which competes with the radiation recombination.