Various optical luminescence spectroscopic techniques were used to characterize the high T_c superconductor YBa_2Cu_3O_x.The emission bands reveal that Cu^(3+) coexist with Cu^(2+) and Cu^+ in this material.
This work was supported by National Science Fund of China.
It is suggested that the extremely large oxygen isotopic Tc shift recently reported in YBa2Cu3O7 may not be an intrinsic mass effect related to the mechanism but due to the small difference of the oxygen contents.