Scanning Tunneling Spectroscopy Study of the Superconducting Proximity Effect in (110)-oriented Manganite/Cuprate and Nickelate/Cuprate Thin Films

An anomalously long-ranged (~ 30 nm) superconducting proximity effect was previously reported in thin-film heterostructures of ferromagnetic La_2/3Ca_1/3MnO₃ (LCMO) and superconducting YBa₂Cu₃O_7−δ (YBCO), and attributed to the induction of spin-triplet pairing [1,2]. However, there remains debate over the existence of this long-ranged proximity effect, since scanning tunneling spectroscopy (STS) on c-axis LCMO/YBCO bilayers has failed to observe any direct and microscopic evidence of this effect [3]. In this work, we extend prior STS studies to (110)-oriented films, the orientation most likely to produce d-wave Andreev resonances that show up as zero-bias conductance peaks (ZBCP). We observe these ZBCPs ubiquitously on our unilayer YBCO films, but do not observe them on our bilayer LCMO/YBCO films down to an LCMO overlayer thickness of ~ 5 nm. The ZBCPs also do not appear on (110)-oriented LaNiO₃/YBCO bilayers, measured as a non-magnetic control. Our results are inconsistent with proximity-induced spin-triplet superconductivity in LCMO/YBCO structures.

[1] C. Visani et al. Nat. Phys. 8, 539 (2012)
[2] Y. Kalcheim et al., Phys. Rev. B 83, 064510 (2011)
[3] I. Fridman et al., Phys. Rev. B 84, 104522 (2011)