Scanning Tunneling Spectroscopy Study of Superconducting Proximity Effect in Cuprate/Perovskite Heterostructures

Recent studies have reported an anomalously long-range superconducting proximity effect (PE) in c-axis heterostructures of ferromagnetic La_2/3Ca_1/3MnO₃ (LCMO) and superconducting YBa₂Cu₃O_7-δ (YBCO) [1]. Although this long-range PE has been attributed to spin-triplet pairing with d-wave symmetry, it remains controversial since scanning tunneling spectroscopy (STS) has failed to observe any direct and microscopic evidence [2]. Our work extends prior STS studies to epitaxial {110}-oriented LCMO/YBCO bilayer films to take advantage of both the longer in-plane coherence length of YBCO and the presence of resonant d-wave Andreev bound states on {110} surfaces. We directly gauged the PE length scale, by probing these states and varying the LCMO layer thickness. To probe the effect of ferromagnetism on this d-wave PE, we substituted LCMO with LaNiO₃ as a paramagnetic control. Additionally, we substituted LCMO with Sr₂IrO₄ to explore the effect of strong spin-orbit coupling on this d-wave PE. Our results are discussed in terms of various mechanisms for singlet-to-triplet conversion.

[1] D. Sanchez-Manzano et al., Nat. Mater. 21 (2022)

[2] I. Fridman et al., Phys. Rev. B 84 (2011)