LOCALIZATION OF CU2+ STATES BY SN4+ IONS IN THE YBASRCU3OY SYSTEM

The effects of Sn doping on the superconductivity and the electronic p roperties of the YBaSrCu3-xSnxOy system have been investigated by elec tronic transport and electron paramagnetic resonance (EPR) measurement s. The superconductivity of the system is gradually depressed by Sn do ping. Meanwhile, the carrier concentration is decreased and the EPR ab sorption is restored by doping Sn4+ at the Cu(1) site. Our results str ongly suggest the localisation of the Cu2+ states at Cu-O chains, whic h is responsible for the reduction of the carriers and the suppression of the superconductivity.