d-wave Andreev reflection study of the ab-plane proximity effect in cuprate/manganite and cuprate/nickelate thin films

An anomalously long-range proximity effect was previously reported in c-axis thin-film heterostructures of ferromagnetic La_2/3Ca_1/3MnO₃ (LCMO) and superconducting YBa₂Cu₃O_7−δ (YBCO), and attributed to spin-triplet d-wave pairing [1]. However, there is debate over the existence of such long-range proximity effect, given the short c-axis coherence length of YBCO, and since scanning tunneling spectroscopy (STS) on c-axis LCMO/YBCO bilayers has failed to observe any direct evidence [2]. For YBCO/LCMO bilayers oriented along the a-axis, similar STS measurements have reportedly observed a proximity length scale of ~ 30 nm [3]. In this work, we extend these prior STS studies to YBCO/LCMO bilayers oriented along the <110> axis, a geometry that is expected produce the most robust d-wave Andreev resonance states. We also measure <110>-oriented LaNiO₃/YBCO bilayers, as a non-magnetic control. Our data are analyzed in terms of the overlayer thickness, in an effort to determine the actual length scale of the manganite/cuprate proximity effect in the ab-plane.

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