Multi-component order parameter superconductivity of Sr$_2$RuO$_4$ revealed by topological junctions
Single crystals of the Sr$_2$RuO$_4$-Ru eutectic system are known to exhibit enhanced superconductivity at 3~K, in addition to the bulk superconductivity of Sr$_2$RuO$_4$ at 1.5~K. The 1.5-K phase is believed to be a spin-triplet, chiral $p$-wave state with the multi-component order parameter, giving rise to chiral domain structure. In contrast, the 3-K phase is attributable to enhanced superconductivity of Sr$_2$RuO$_4$ in the strained interface region between Ru inclusion of a few to tens of micrometers in size and the surrounding Sr$_2$RuO$_4$. We investigate the dynamic behavior of a topological junction, where a superconductor is surrounded by another superconductor. Specifically, we fabricated Nb/Ru/Sr$_2$RuO$_4$ topological superconducting junctions, in which the difference in phase winding between the $s$-wave superconductivity in Ru micro-islands induced from Nb and the superconductivity of Sr$_2$RuO$_4$ mainly governs the junction behavior. Comparative results of the asymmetry, hysteresis and noise in junctions with different sizes, shapes, and configurations of Ru inclusions are explained by the chiral domain-wall motion in these topological junctions. Furthermore, a striking difference between the 1.5-K and 3-K phases is clearly revealed: the large noise in the 1.5-K phase sharply disappears in the 3-K phase. These results confirm the multi-component order-parameter superconductivity of the bulk Sr$_2$RuO$_4$, consistent with the chiral $p$-wave state, and the proposed non-chiral single-component superconductivity of the 3-K phase.