Oral: Optical detection of Spin Torques in a superconductor -- BaPb_1-xBi_xO_3-δ Epitaxial Thin Films

We report measurements of current-induced spin-orbit torque generated by the superconductor BaPb_1-xBi_xO_3-δ (BPBO) above and below its transition temperature (Tc). Above Tc, this measurement can be performed using conventional electrical methods, which find that BPBO can generate a large (order 1) spin Hall efficiency acting on an adjacent ferromagnetic layer [1]. Below Tc, conventional electrical measurements are impossible, because the development of superconductivity shorts out the voltage signals. Instead, both above and below Tc we perform optical measurements of current-induced magnetic deflections using Sagnac interferometry, which provides the necessary sensitivity [2].

Our initial results indicate that the damping-like torque component vanishes at low temperature below a critical current and critical magnetic field. However, BPBO in proximity with a ferromagnet does not reach fully zero resistance. This obfuscates whether superconductivity itself quenches the torque. Measurements are underway with spacer layers designed to protect the superconductor while allowing spin currents to flow.

[1] Edgeton, A.L., Harris, I.A., Campbell, N.G. et al. Large spin–orbit torque in bismuthate-based heterostructures. Nat Electron 6, 973–980 (2023). https://doi.org/10.1038/s41928-023-01080-1

[2] Karimeddiny, S., Cham, T. M. J., Smedley, O., Ralph, D. C., & Luo, Y. K. (2023). Sagnac interferometry for high-sensitivity optical measurements of spin-orbit torque. Science Advances, 9(36), eadi9039.