Holes and vacancies across interfaces in superconducting Sr$_{2}$CuO$_{4}$-La$_{2}$CuO$_{4}$

Sr$_{2}$CuO$_{4}$ films with superconducting critical temperatures T$_{c}$ up to 90 K have a substantial number of apical oxygen vacancies. An increase in T$_{c}$ has been correlated in La$_{2}$CuO$_{4}$-La$_{2-x}$Sr$_{x}$CuO$_{4}$ bilayers with an increase in the apical oxygen -- Cu atom distance. This suggests that the apical oxygen in these structures is not an entirely passive participant in high-T$_{c}$ superconductivity. We studied Sr$_{2}$CuO$_{4}$-La$_{2}$CuO$_{4}$ superlattices with resonant soft x-ray scattering in order to quantify the apical oxygen scattering. At the O K edge we observed scattering resonances from mobile holes and apical oxygen vacancies. The apical vacancy resonant scattering factor was obtained. The distribution of apical vacancies measured at O K edge follows closely the structural imperfections measured with hard x-rays with a characteristic length 5.25 \underline {+} 0.75 A. The mobile holes diffuse with a characteristic length of $\lambda $=5.3 \underline {+} 1 A.