Observation of superconducting gap spectra of long-range proximity effect in Au/SrTiO${}_3$/SrRuO${}_3$/Sr${}_2$RuO${}_4$ tunnel junctions

Abstract

We observe an unconventional superconducting minigap induced into a ferromagnet SrRuO${}_3$ from a spin-triplet superconductor Sr${}_2$RuO${}_4$ using a Au/SrTiO${}_3$/SrRuO${}_3$/Sr${}_2$RuO${}_4$ tunnel junction. Voltage bias differential conductance of the tunnel junctions exhibits V-shaped gap features around zero bias, corresponding to a decrease in the density-of-states with an opening of a superconducting minigap in SrRuO${}_3$. Observation of a minigap at a surface of a 15~nm thick SrRuO${}_3$ layers confirms the spin-triplet nature of induced superconductivity. The shape and temperature dependence of the gap features in the differential conductance indicate that the even-frequency $p$-wave correlations dominate, over odd-frequency $s$-wave correlations. Theoretical calculations support this $p$-wave scenario. Our work provides the density-of-states proof for $p$-wave Cooper pair penetration in a ferromagnet and significantly put forward our understanding of the $p$-wave spin-triplet proximity effect between spin-triplet superconductors and ferromagnets.