Extrinsic Origin of the Josephson Diode Effect

Josephson junctions formed from conventional superconducting electrodes separated by several distinct families of metallic, non-superconducting spacer layers exhibit distinct critical supercurrent densities for current flowing in opposite directions. We present results on lateral junctions formed from the 2D van der Waals metals, NiTe₂ [1] and PtTe₂ [2], fabricated with sputter deposited Nb electrodes, and vertical junctions formed from WTe₂, with electrodes formed from exfoliated NbSe₂ flakes [3]. In each case a Josephson diode effect is observed, whose magnitude increases monotonically as the temperature is reduced below the temperature of the proximity induced superconductivity in the spacer layer. We note that this is in distinct contrast to the temperature dependence reported for the magnitude of the superconducting diode effect in several materials. We find that the asymmetry in the critical supercurrent densities can reach up to ~80 % at low temperatures. The asymmetry appears only in the presence of a small magnetic field that has a component perpendicular to the supercurrent direction. The detailed dependence of the strength of the JDE on the magnitude and orientation of the magnetic field, as well as temperature, can be well described by a finite momentum Cooper pairing model [1, 2]. By forming vertical Josephson junctions from exfoliated van der Waals flakes, an applied magnetic field can be applied that is always orthogonal to the supercurrent direction but whose direction can be rotated within the WTe₂ flake forming the junction. We show that the JDE strongly depends on the orientation of the field with nodes along a critical direction within the WTe₂ crystal structure. These results thereby demonstrate the origin of the JDE in these devices has an intrinsic origin. We propose that the JDE could have important applications as a novel magnetic field detector at cryogenic temperatures, for example, to “read” magnetic domain walls in a cryogenic racetrack memory^*.
^* Funded through an European Research Council Advanced Grant “SUPERMINT” (2022-2027).

[1] B. Pal et al., "Josephson diode effect from Cooper pair momentum in a topological semimetal," Nat. Phys., vol. 18, pp. 1228-1233, 2022.
[2] P. K. Sivakumar et al., "Long-range Phase Coherence and Second Order φ_0-Josephson Effect in a Dirac Semimetal 1T-PtTe₂ " Comm. Phys., vol. (accepted), 2024.
[3] J.-K. Kim et al., "Intrinsic supercurrent non-reciprocity coupled to the crystal structure of a van der Waals Josephson barrier," Nat. Commun., vol. 15, p. 1120, 2024.