Speaker
Description
Long-range equal-spin triplet supercurrents induced by nontrivial spin textures are of fundamental importance both for the understanding and the applications of superconducting spintronics. Considering the interface between a conventional spin-singlet superconductor and a ferromagnetic material, the creation and control of equal-spin correlations is allowed due to two fundamental processes. First, the spin-mixing (or spin-dependent-phase-shift) effect due to the spin polarization of the interface converts spin-singlet pairs from the superconductor into mixed-spin triplet correlations in the ferromagnet. Second, if a noncollinear spin arrangement is present in the ferromagnet, the spin-rotation mechanism turns the short-range mixed-spin pairs into the long-range equal-spin pairs. Therefore, the noncollinearity of the spin texture is necessary for the existence of equal-spin triplet correlations. However, if the spin texture is not only noncollinear but in fact noncoplanar, new functionalities, such as an effective decoupling of the Josephson phases in the two spin bands, may appear. Consequently, junctions involving such materials can exhibit the spin-resolved Josephson diode effect. In this context, such an effect has been predicted in various setups, including strongly spin-polarized ferromagnetic trilayers with noncoplanar magnetization profiles, intrinsically noncoplanar magnetic materials such as conical magnets, and ferromagnetic trilayers involving altermagnets.
In this talk, we will present the necessary conditions for the appearance of the Josephson diode effect in junctions involving strongly spin-polarized magnetic materials without spin-orbit. As we will show, such an effect emerges if the Josephson current-phase relation (CPR) possesses no phase-inversion center, and, in what follows, we will examine the conditions under which this regime is realized. First, we will comment on the essential role of the noncoplanarity of the spin texture, which breaks the spatial inversion symmetry and gives rise to quantum geometric phases, Δ𝜑′ , that enter the Josephson CPR similarly to the superconducting phase difference, Δ𝜒. Second, we will show that both spin bands in themagnetic material have to contribute to transport, i.e., the effect is absent in half-metallic junctions. Third, different band-specific densities of states are required, and this condition is ensured by the strong spin polarization of the magnetic material. Finally, higher harmonics in
the Josephson CPR are necessary, i.e., the effect is absent in the tunneling limit. However, even in this case, the Josehson CPR must not have a phase-inversion center, which is ensured by the restriction of the quantum geometric phase to values Δ𝜑′ ≠ 𝑘𝜋/2, 𝑘 ∈ ℤ . Finally, we will illustrate our theory by formulating a simple phenomenological model that incorporates the abovementioned points and exhibits the spin-resolved Josephson diode effect.
[1] N. L. Schulz, D. Nikolić, and M. Eschrig, Phys. Rev. B 112 104514 (2025); Phys. Rev. B 112 104515 (2025) (2025)
[2] D. Nikolić, N. L. Schulz, A. I. Buzdin, and M. Eschrig, Phys. Rev. B 112, 224507 (2025)
[3] N. L. Schulz, D. Nikolić, and M. Eschrig, arXiv:2512.22017v1 (2025)