Observation of superconducting gap spectra of long-range proximity effect in Au/SrTiO3/SrRuO3/Sr2RuO4 tunnel junctions

Abstract

We observe an unconventional superconducting minigap induced into a ferromagnet SrRuO3 from a spin-triplet superconductor Sr2RuO4 using a Au/SrTiO3/SrRuO3/Sr2RuO4 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 SrRuO3. Observation of a minigap at a surface of a 15 nm thick SrRuO3 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.

A proposal for reconciling diverse experiments on the superconducting state in Sr2RuO4

“…A variety of other experimental observations need to be addressed. Evidence of triplet pairing has been adduced from experiments on 214/113 strontium ruthenate heterostructure tunneling 45 , of odd-parity pairing from phase-sensitive Josephson tunneling 20 , of horizontal line nodes from field-dependent specific heat measurements 46 , and of half quantum vortices from cantilever magnetometry 47 . All these observations pose some challenge to be reconciled with our ansatz and thus deserve further analysis and revisiting from both an experimental and a theoretical viewpoint.…”

A Review of Electronic Transport in Superconducting Sr2RuO4 Junctions

“…Interestingly, the same group reported an anisotropic response of the proximity effect with the direction of externally applied magnetic fields [83]. Moreover, they investigated the gap feature of induced superconductivity into SRO113 using Au/STO/SRO113/SRO214 tunnel junctions [84]. dI/dV versus bias voltage may indicate that the induced superconductivity in SRO113 has an anisotropic gap function such as p-wave or d-wave symmetry.…”

Anomalous anisotropic behaviour of spin-triplet proximity effect in Au/SrRuO3/Sr2RuO4 junctions

“…We comment that the formula with the same functional form can be used to approximate the temperature dependence of the superconducting gap, with a = 1.74 for the s-wave superconductivity. Interesting, a = 1.56 is indicates unconventional superconducting proximity effect and potentially consistent with p -wave superconductivity 41,45 . The monotonic suppression of V 3 under fields H out can be reasonably fitted with the expression .…”

“…These observations suggest that Δ R 1 and Δ R 2 correspond to resistance changes at the SRO113/SRO214 and Au/SRO113 interfaces, respectively. It means that dI / dV as a function of bias voltage should exhibit Andreev reflection (AR) features occurring at two distinct interfaces.Note that AR can occur for a bias voltage below superconducting gap (induced minigap in SRO113 41 ) at SRO113/SRO214 (Au/SRO113) interface. To study the AR, we obtained dI / dV ( V ) by taking first order numerical derivative of the measured current-voltage ( I - V ) curves shown in Fig.…”

“…Note that there are sharp dips associated with V 2 features (Fig. 2(a)), which may correspond to the destruction of superconductivity at SRO113/SRO214 interface with critical current 41 . However, the temperature dependence of dI / dV (Fig.…”