Current voltage measurements on magnetic ZnO /Y$_{1}Ba_{2}Cu_{3}O_{7-\delta}$ thin film heterostructures

We have grown ca. 200 nm magnetically doped ZnO films on Y$_{1}Ba_{2}Cu_{3}O_{7-\delta}$ on r-plane sapphire by pulsed laser deposition for future Andreev Reflection(AR) measurements to determine spin polarization. The critical current density, 5 x 10$^{6}$ A/cm$^{2}$, of the YBCO films at zero field and 77K [1]remains in same range after growth of granular doped ZnO and critical temperature is still above 77K. Four-point transport measurements across the ZnO/YBCO interfaces with junction area ( A $\geq$ 1 mm$^{2}$) resulted in nonlinear I-V characteristics. The observed conductivity fluctuations at zero bias in the low temperature range are masked by heavy noise due to the large A lying in the Maxwell range ($\it{d} $$\gg$$\it{l}$ , $\it{d}$ = contact radius, $\it{l}$ = mean free path). Fitting the conductivity data according to the Blonder-Tinkham-Klapwijk 0K theory shows that the superconducting gap ($\Delta$) of YBCO and ZnO/YBCO barrier strength (Z) amount to $\leq$ 20.4 ($\pm$1) meV and $\geq$0.74($\pm$ 0.23), respectively. Smaller junction area with reduced scattering AR measurements are under work. [1]M.Lorenz etal. IEEE Transactions on Applied Superconductivity 11(2001)3209.