Long-ranged proximity effect and triplet superconductivity in hybrid superconductor-ferromagnet systems

I will present scanning tunneling spectroscopy measurements performed on half-metallic ferromagnetic La_2/3Ca_1/3MnO₃ (LCMO) films of variable thicknesses epitaxially grown on two types of high temperature superconductor films: the hole-doped YBa₂Cu₃O_7-d (YBCO) and the electron doped Pr_1.85Ce_­CuO₄ (PCCO). Surprisingly, our tunneling spectra reveal long-ranged penetration of superconductor order parameter into the LCMO layer, to distances as long as 30 nm, an order of magnitude larger than the expected coherence length associated with singlet-pairing superconductivity in LCMO. This anomalous proximity effect manifests itself in the tunneling spectra as gaps and zero-bias conductance peaks (ZBCPs), and in some cases split-ZBCPs. Our observations are accounted for by the emergence of parallel-spin triplet-pairing superconductivity at the bilayer’s interfaces, promoting the long-ranged proximity effect. The appearance of ZBCPs indicates that the orbital symmetry of the induced order parameter is anisotropic and sign-changing, of either d-wave of p-wave character, corresponding to an odd or even dependence on the Matsubara frequency, respectively, and possibly also of a complex symmetry, e.g., p_x+ip_y. The latter symmetry was also observed in our tunneling spectra measured on the topological insulator Bi₂Se₃ deposited on the s-wave superconductor NbN. Finally, I will discuss the effect of applied magnetic field on the proximity effect. Interestingly, a non-monotonic behavior was observed, where the proximity effect first increased and then suppressed with magnetic field.  The consequence of this behavior regarding the possible mechanisms yielding induced triplet superconductivity, and the corresponding role of magnetic inhomogeneity, will be discussed.