LOCAL CHARGE-DENSITY CHANGE AND SUPERCONDUCTIVITY - A POSITRON STUDY

The temperature dependence between 10 and 300 K of the positron lifetime was measured in the high-temperature superconductors YBA2(Cu1-xMx)3O6+where M=Zn and Ga with x=0.0 to 0.07 and >0.8. In the undoped and Ga-doped samples, the positron lifetime in the Bloch state, was observed to decrease below Tc. In the Zn-doped samples, a dramatic x-dependent temperature variation of was observed: from a decrease of Tb below Tc for x=0.01 to an increase of for x>0.02. These new experimental results are interpreted in terms of a change in the local charge density of high-Tc oxides associated with the superconducting transition. © 1990 The American Physical Society.