Detection and imaging of the oxygen deficiency in single crystalline YBa$_{\text{2}}$Cu$_{\text{3}}$O$_{\text{7}-δ}$ thin films using a positron beam

Single crystalline YBa$_{\text{2}}$Cu$_{\text{3}}$O$_{\text{7}-δ}$ (YBCO) thin films were grown by pulsed laser deposition (PLD) in order to probe the oxygen deficiency $δ$ using a mono-energetic positron beam. The sample set covered a large range of $δ$ (0.191<$δ$<0.791) yielding a variation of the critical temperature $T_{\text{c}}$ between 25 and 90\,K. We found a linear correlation between the Doppler broadening of the positron electron annihilation line and $δ$ determined by X-ray diffraction (XRD). Both, the origin of the found correlation and the influence of metallic vacancies, were examined with the aid of ab-initio calculations that allowed us (i) to exclude the presence of Y vacancies and (ii) to ensure that positrons still probe $δ$ despite the potential presence of Ba or Cu vacancies. In addition, by scanning with the positron beam the spatial variation of $δ$ could be analyzed. It was found to fluctuate with a standard deviation of up to $0.079(5)$ within a single YBCO film.

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