d-wave superconducting and odd frequency in the LSMO/YBCO superlattice system

When ferromagnet and superconductor are integrated into a superlattice, proximity effects from both orders coexist and coupled to each other at the interface, which can significantly influence the electronic and magnetic properties near the interface, potentially generating novel phenomena. In this study, we investigate the Mn-3d and O-2p states under such interactions using X-ray Magnetic Circular Dichroism (XMCD) in a La_0.67Sr_0.33MnO₃ (LSMO) / YBa₂Cu₃O₇₋δ (YBCO) superlattice. Our previous work showed that the superconducting critical current (Ic) was enhanced, peaking at ~0.6 T, under field-cooled conditions. This enhancement may be associated with the emergence of spin-triplet pairs in LSMO. In the present XMCD measurements, no significant signal was observed from O-2p across all temperatures, while Mn-3d exhibited a strong XMCD enhancement at low temperatures. This suggests that spin-triplet pairs may reside in the Mn-3d band. Since a Mn-O₆ octahedron shares the same  orbital 3d_x^2-y^2 with the Cu-O₅ pyramid along the Cu-O plane, it is reasonable to assume that orbital wave function of the spin-triplet pairs in the LSMO layer is a d-wave. Such behavior implies a breaking of time-reversal symmetry, consistent with the presence of odd-frequency spin-triplet superconductivity.