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

Abstract

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 $j_C\approx 0.3\mathrm{A}/\mathrm{c}{\mathrm{m}}^2$ for $d=7$ nm was registered at $T=4.2\mathrm{K}$. Magnetic-field dependence of superconducting critical current $I_C\left(H\right)$ 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 $V_{m,n}=\left(m/n\right)\left(h/2e\right)f_e$ 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.