Non-Magnetic Suppression of Superconductivity in Heteroepitaxial Perovskite/YBCO/Perovskite Thin Films

To distinguish between the effects of strain and magnetism on superconductivity in epitaxial thin-film heterostructures comprising YBa$_2$Cu$_3$O$_{7-\delta}$ (YBCO) and half-metallic manganites, we study perovskite/YBCO/perovskite trilayer films, using either ferromagnetic La$_{2/3}$Ca$_{1/3}$MnO$_3$ (LCMO) or paramagnetic LaNiO$_3$ (LNO) as the buffer and capping layers. For comparison with trilayers that are lattice-symmetry matched, orthorhombic PrBa$_2$Cu$_3$O$_{7-\delta}$ (PBCO) was also used instead of the perovskites. LCMO/YBCO/LCMO and LNO/YBCO/LNO trilayers show similar reduction in superconducting transition temperature ($T_c$) vs. decreasing YBCO thickness, from 50 and 12.5 nm, but PBCO/YBCO/PBCO trilayers show no such $T_c$ reduction. The tetragonal La$_2-x$Sr$_x$CuO$_4$ (LSCO) is also used in place of the orthorhombic YBCO, to further elucidate the effect of lattice-symmetry mismatch. Our results suggest that heteroepitaxial strain plays a stronger role than the proximity to ferromagnetism, for the suppression of superconductivity observed in manganite/YBCO heterostructures.