Superconducting current and low-energy states in a mesa-heterostructure interlayered with a strontium iridate film with strong spin-orbit interaction

The superconducting current has been observed in mesa-heterostructures Nb/$\mathrm{S}{\mathrm{r}}₂\mathrm{Ir}{\mathrm{O}}₄$/$\mathrm{YB}{\mathrm{a}}₂\mathrm{C}{\mathrm{u}}₃{\mathrm{O}}_x$ with a $\mathrm{S}{\mathrm{r}}₂\mathrm{Ir}{\mathrm{O}}₄$ interlayer having strong spin-orbit interaction. The superconducting critical current density ${j}_C\ensuremath{\approx}0.3\phantom{\rule{0.16em}{0ex}}\mathrm{A}/\mathrm{c}{\mathrm{m}}²$ for $d=7$ nm was registered at $T=4.2\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Magnetic-field dependence of superconducting critical current ${I}_C(H)$ showed a sharp central peak and minor oscillating behavior for sidelobes indicated the absence of pinholes. Integer and fractional Shapiro steps were observed at voltages ${V}_m,n=(m/n)(h/2e){f}_e$ under microwave radiation at frequencies ${f}_e=38$ GHz and ${f}_e=50$ GHz. Fractional Shapiro steps ($m/n=1/2$, 3/2) point on the presence of the second harmonic in the superconducting current-phase relation. The zero-bias conductance peak was observed hinting on existence of low-energy states at $\mathrm{S}{\mathrm{r}}₂\mathrm{Ir}{\mathrm{O}}₄$/$\mathrm{YB}{\mathrm{a}}₂\mathrm{C}{\mathrm{u}}₃{\mathrm{O}}_x$ interface at temperatures higher than the superconducting critical temperature of Nb.