EFFECT OF SUBSTITUTIONAL DOPING ON THE 1.7 EV ELECTRONIC-TRANSITION OF YBA2(CU1-XMX)3O7-DELTA FOR M = CO, FE, NI AND ZN

This report is on the variation of the 1.7 eV electronic transition of YBa2(Cu1-xM(x)3O7-delta as a function of dopant concentration for M = Co, Fe, Ni and Zn. We have examined the spectroscopic ellipsometric measurement of Co and Fe doped samples from x = 0% to 20.0% and Ni and Zn doped samples from x = 0% to 9.0%. The 1.7 eV absorption peak is observed for trivalent dopants Fe (for x > 9.0%), Co (for x > 6.0%) and for an oxygen deficient sample. However, it is not observed for the undoped oxygen rich material and for the divalent dopants, Ni and Zn. The 1.7 eV transition appears only as the material becomes non-metallic and as the hole population in the CuO2 plane is decreased. Our results also show that Zn-doping hs the least effect on the metal-like dielectric behavior. Therefore, the rapid reduction of T(c) for Zn doping is not due to a decrease in free carriers. The dependencies of the 1.7 eV feature on oxygen coordination, dopant site occupancy and charge valency support the idea of a bound-excitonic charge-transfer transition from the oxygen to copper in the Cu-O planes.