Long range second harmonic Josephson coupling across a half metallic ferromagnet*

The Josephson effect results from the coupling of two superconductors across a weak link or spacer to yield a phase coherent quantum state. In ferromagnets, singlet (opposite-spin) Cooper pairs decay over very short distances, and thus Josephson coupling requires a nanometric spacer. In systems with particular properties, however, equal spin triplet Cooper pairs can be generated, allowing the coupling of superconductors across magnetic barriers over much longer distances.  We demonstrate extremely long range high-temperature Josephson coupling across the half-metallic manganite La_0.7Sr_0.3MnO₃ combined with the superconducting cuprate YBa₂Cu₃O₇. This is shown in planar junctions which display the hallmarks of Josephson physics: critical current oscillations (Fraunhofer pattern) and quantum phase locking under microwave excitation (Shapiro steps). Interestingly, both in the Fraunhofer pattern and in the Shapiro steps, a dominating second harmonic can be observed, which further confirms the exotic superconducting state that can be present in this system. [1-3]

 

[1] D. Sanchez-Manzano et al. Nat. Mat. (in press)

[2] M.J.A. Stoutimore et al, Phys. Rev. Lett. 121, 177702 (2018)

[3] Y. Yao et al. Phys. Rev. B 104, 104414 (2021)