Superconducting Gap Opening in LSMO layer by the LSMO/YBCO Interfaces

The superconducting proximity effect arises in a non-superconducting layer adjacent to a superconductor, where the wavefunction of Cooper pairs penetrates across the interface, decaying exponentially with distance into the non-superconducting region. This Cooper pair incorporation modifies the electronic band structure of the non-superconducting layer, creating an energy gap in the proximity region to accommodate these pairs. Additionally, the intrinsic band structure of the non-superconducting layer controls the symmetry type of the proximally induced Cooper pairs: for example, an s-wave pairing symmetry is observed in a gold film deposited on the ab-plane of a YBa₂Cu₃O_7-δ (YBCO) film, whereas a p-wave or chiral d-wave symmetry is induced in single-layer graphene on a cuprate superconductor. When a strongly ferromagnetic manganite, La_0.67Sr_0.33MnO₃ (LSMO) film is grown on a YBCO film, the spin-triplet superconducting pairs emerges in the LSMO layer due to a complex coupling proximally between the superconductivity and ferromagnetic interactions at the LSMO/YBCO interface. The state of these induced spin-triplet pairs can be controlled by manipulating the orientation of the induced Cu moment at the interface by a specific magnetic field cooling procedure. In the case where the spin-triplet Cooper pairs exhibit a parallel spin alignment (even-spin state), we observed an energy gap of 38 ± 5 meV, compatible to the superconducting gap of YBCO in the b direction, opening in the odd symmetrical O-2p orbit of the LSMO layer. This preserves the total wavefunction of the triplet superconducting pairs in the odd symmetry.