Phase-Sensitive Evidence for the Sign-Reversal $s_{\pm }$ Symmetry of the Order Parameter in an Iron-Pnictide Superconductor Using $\mathrm{Nb}/{\mathrm{Ba}}_{1-x}{\mathrm{Na}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ Josephson Junctions

Josephson current provides a phase-sensitive tool for probing the pairing symmetry. Here we present an experimental study of high-quality Josephson junctions between a conventional $s$-wave superconductor Nb and a multiband iron-pnictide ${\mathrm{Ba}}_{1-x}{\mathrm{Na}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$. Junctions exhibit a large enough critical current density to preclude the $d$-wave symmetry of the order parameter in the pnictide. However, the $I_c{R}_n$ product is very small $\simeq 3\mu \mathrm{V}$, which is not consistent with the sign-preserving $s_{++}$ symmetry either. We argue that the small $I_c{R}_n$ value, along with its unusual temperature dependence, provides evidence for the sign-reversal $s_{\pm }$ symmetry of the order parameter in ${\mathrm{Ba}}_{1-x}{\mathrm{Na}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$. We conclude that it is the phase sensitivity of our junctions that leads to an almost complete (below a subpercent) cancellation of supercurrents from sign-reversal bands in the pnictide.

Figure 1

(a) A sketch of the Fermi surfaces: (blue) hole type, (red) electron type sheets. (b),(c) Scanning electron microscopy images of the studied sample (b) before and (c) after FIB trimming. (c) Temperature dependence of the junction resistance. Inset shows resistance below the $T_c$ of Nb. Blue curve at zero field. $R$ drops to zero due to appearance of the Josephson current. The red curve is measured at field 6 mT parallel to the junction plane, at which the critical current is fully suppressed. Therefore, it represents junction resistance $R_n(T)$. (e) Current-voltage characteristics of a small FIB trimmed junction at $T\simeq 0.4\mathrm{K}$ at zero field (blue) and at 15 mT in-plane field (red). (f) Critical current (top) and junction conductance (bottom) versus junction area for junctions at the same chip.

Figure 2

(a) Measured Fraunhofer modulation $I_c(H)$ for positive and negative currents. (b) Measured Fraunhofer modulation of ac resistance for two junctions with significantly different sizes $L=6.35\mu \mathrm{m}$ (blue line) and $2.52\mu \mathrm{m}$ (red line). (c) Scaling of the period of Fraunhofer modulation versus the inverse length of the junction for junctions at the same chip. (d) Current-voltage characteristics of the J2 junction at different $T$. Note the slight decrease of the junction resistance with increasing $T$. The black line represents a fit in the resistively shunted junction model. (e) Temperature dependencies of the positive and negative critical currents at zero field. (f) Temperature dependencies of the $I_c{R}_n$ product, normalized by the $T=0$ value, for three junctions. The dotted line and the dashed lines represent calculated dependencies for the sign-preserving $s/s_{++}$ and sign-reversal $s/s_{\pm }$ junctions, respectively (both from Ref. [28]). Inset shows $I_c{R}_n$ values for junctions at the same chip. It is seen that the $I_c{R}_n$ is very small, confirming phase-sensitive supercurrent cancellation from the sign-reversal $s_{\pm }$ bands.