Chirality-Induced Spin Selectivity (CISS) has recently attracted significant attention as a novel source of spin-polarized current [1]. Using a chiral molecular superconductor, k-(BEDT-TTF)₂Cu(NCS)₂, we have demonstrated that the CISS effect persists even in the superconducting state [2]. A notable feature of this system is the emergence of spin accumulation accompanied by time-reversal symmetry breaking (TRSB), as evidenced by antiparallel spin polarization at opposite edges of the crystal [2]. This observation is not consistent with the conventional picture of the CISS effect and calls for a deeper understanding of the underlying mechanism.

To address this issue, we have investigated other chirality-related effects expected in superconductors lacking spatial inversion symmetry. One such phenomenon is the superconducting diode effect [3], originally discovered in polar superconductors. We examined its chiral counterpart in an organic chiral superconductor and found a pronounced rectification effect comparable to that observed in inorganic systems.

In addition, the chiral superconductor exhibits electric magnetochiral anisotropy (eMChA), where a collinear electric current and magnetic field induce nonreciprocal transport. Analysis of the microscopic mechanism suggests parity mixing between s-wave and p-wave order parameters, as well as the presence of effective spin–orbit coupling enhancement [4]. The coexistence of a giant CISS effect with TRSB, strong eMChA, and a large superconducting diode effect highlights the fundamental importance of chirality in superconducting materials.

References

[1] K. Ray et al., Science 283, 814 (1999)

[2] R. Nakajima, HMY et al., Nature 613, 479 (2023)

[3] F. Ando et al., Nature 584, 373 (2020).

[3] T. Sato, HMY et al., Phys. Rev. Res. 7, 023056 (2025).