0-π transition and 0-π Josephson junctions in van der Waals superconductor-ferromagnet heterostructures

 

Superconductor-ferromagnet heterostructures provide a promising platform to explore both unconventional superconductivity for fundamental physics and new superconducting device concepts for quantum technology. Examples include FFLO (Fulde–Ferrell–Larkin–Ovchinnikov) physics, spin-triplet and topological superconductivity, fractional fluxons and “quiet” phase qubits. The emergence of van der Waals (vdW) superconductors and magnets, which can form atomically sharp interfaces upon stacking, promises the studies of superconductor-ferromagnet heterostructures in new regimes. Here we report the fabrication of vdW Josephson junctions made of Cr₂Ge₂Te₆ (a ferromagnetic semiconductor) sandwiched between few-layer NbSe₂ (an Ising superconductor). A thickness-driven 0-π transition is observed upon increasing the layer number of Cr₂Ge₂Te₆. Very close to the critical thickness, where the critical current density nearly vanishes, we observe the formation of 0-π Josephson junctions with uniform Cr₂Ge₂Te₆ thickness. This is evidenced by the unusual supercurrent interference pattern that signifies regions of opposite-polarity supercurrents within the junction and the sensitivity of the interference pattern on the magnetic domain patterns. We attribute the formation of 0-π junctions to the nanoscale magnetic domain pattern in multilayer Cr₂Ge₂Te₆, which produces spatial modulations in the 0-π transition thickness. Our work highlights the potentials of vdW superconductor-ferromagnet heterostructures in the realization of unconventional superconductivity and superconducting devices.