Local charge-density change and superconductivity: A positron study

Abstract

The temperature dependence between 10 and 300 K of the positron lifetime was measured in the high-temperature superconductors ${\mathrm{YBA}}_2$(${\mathrm{Cu}}_{1\mathrm{-}\mathit{x}}$ ${\mathit{M}}_{\mathit{x}}$ ${)}_3$ ${\mathrm{O}}_{6+\mathrm{\delta }}$, where M=Zn and Ga with x=0.0 to 0.07 and δ>0.8. In the undoped and Ga-doped samples, the positron lifetime in the Bloch state, ${\mathrm{\tau }}_{\mathit{b}}$, was observed to decrease below ${\mathit{T}}_{\mathit{c}}$. In the Zn-doped samples, a dramatic x-dependent temperature variation of ${\mathrm{\tau }}_{\mathit{b}}$ was observed: from a decrease of ${\mathit{T}}_{\mathit{b}}$ below ${\mathit{T}}_{\mathit{c}}$ for x=0.01 to an increase of ${\mathrm{\tau }}_{\mathit{b}}$ for x>0.02. These new experimental results are interpreted in terms of a change in the local charge density of high-${\mathit{T}}_{\mathit{c}}$ oxides associated with the superconducting transition.