Abstract Body

The superconducting diode effect (SDE) has recently attracted considerable attention in superconductors under the simultaneous breaking of both time-reversal and space-inversion symmetries. In this effect, superconducting currents flow in one direction, while normal conducting currents flow in the opposite direction [1]. While it is known that SDEs can be induced in superconducting devices with asymmetric patterning of pinning centers [2] or superconductor/ferromagnetic hybrid devices [3,4], intrinsic SDEs due to band structure have received particular attention [5-7]. In systems coexisting ferromagnetism and superconductivity, precise control of exchange and spin-orbit interactions acting on Cooper pairs is essential to switch between superconducting and normal conducting states without a magnetic field [8].

One of the most accessible methods to simultaneously break time-reversal and space-inversion symmetries is to use an asymmetric stacking structure combining ferromagnets with superconductors. To further control the spin-orbit interaction in this system, it is effective to use heavy metals with large spin-orbit interactions in addition to ferromagnetic and superconducting materials. For instance, multi-element superlattices, where nanometer-scale layers of ferromagnetic, superconducting, and heavy metal materials are repeatedly stacked, offer a high degree of freedom in material design, including film thickness, number of layers, stacking order, and constituent elements.

Here, we report the recent results of asymmetric stacking structures combining ferromagnets and superconductors that have been investigated, particularly in the Co-inserted [5] and Fe/Pt-inserted superlattices [7]. We successfully controlled the magnetization of the non-reciprocal critical current without a magnetic field, realizing magnetization control of zero-field SDEs in the Fe/Pt-inserted superlattices [7]. The efficiency of ΔJ_c/J_c [9], defined by non-reciprocal critical current (ΔJ_c) relative to the average critical current density (J_c) at zero magnetic field, exceeded 40%, highlighting the significant potential of this material. This study contributes to advancing understanding of the SDE and presents new avenues for practical applications in superconducting devices, quantum computing, and energy-efficient electronics.

References

[1] F. Ando et al., Nature 584, 373–376 (2020).
[2] Y.-Y. Lyu et al., Nat. Commun. 12, 2703 (2021).
[3] A. Yu. Aladyshkin et al., Appl. Phys. Lett. 94, 222503 (2009).
[4] Y. Hou et al., Phys. Rev. Lett. 131, 027001 (2023).
[5] H. Narita et al., Nat. Nanotechnol. 17, 823–828 (2022).
[6] J. -X. Lin et al., Nat. Phys. 18, 1221–1227 (2022).
[7] H. Narita et al., Adv. Mater. 2304083 (2023).
[8] K. -R. Jeon et al., Nat. Mater. 21,1008–1013 (2022).
[9] A. Daido et al., Phys. Rev. Lett. 128, 037001 (2022).

Acknowledgment

This work was partly supported by JSPS KAKENHI (Grant Nos. 15H05702, 20H00337, 20H00349, 20H05665, 21K13883, and 21K18145); MEXT Initiative to Establish Next-generation Novel Integrated Circuits Centers (X-NICS) Grant Number JPJ011438; Cooperative Research Project Program of the Research Institute of Electrical Communication, Tohoku University; and Collaborative Research Program of the Institute for Chemical Research, Kyoto University. This study was also supported by the Futaba Foundation’s Futaba Research Grant Program, IketaniScience and Technology Foundation, Sumitomo Foundation, Tokuyama Science Foundation, and Iwatani Naoji Foundation.