Magnetotransport Signatures of Spin–Orbit Coupling in High‐Temperature Cuprate Superconductors

Spin transport in superconductors offers a compelling platform to merge the dissipationless nature of superconductivity with the functional promise of spin‐based electronics. A significant challenge in achieving spin polarization in conventional superconductors stems from the singlet state of Cooper pairs, which exhibit no net spin. The generation of spin‐polarized carriers, quasiparticles, or triplet pairs in superconductors has predominantly been realized in hybrid superconductor/ferromagnet systems through proximity‐induced spin polarization. Historically, cuprate superconductors have been characterized by strong electronic correlations but negligible spin–orbit coupling. In this study, we observe a large in‐plane angle‐dependent magnetoresistance and a pronounced planar Hall effect arising near the superconducting phase transition in the prototypical high‐temperature cuprate superconductor without using a proximity ferromagnet. These effects ‐ unusual in centrosymmetric cuprates ‐ may arise from spin‐polarized quasiparticle transport potentially mediated by strong spin–orbit coupling. By systematically tuning magnetic field strength, orientation, temperature, and doping, we identify transport signatures that are consistent with spin–orbit‐driven phenomena. Our findings suggest the presence of a previously underappreciated spin–orbit landscape in cuprates, which may provide the basis for exploring spintronic functionalities in high‐temperature superconductors.