van der Waals pi Josephson Junctions

Proximity-induced superconductivity in a ferromagnet can induce Cooper pairs with a finite center-of-mass momentum and stabilize Josephson junctions (JJs) with pi phase difference in superconductor-ferromagnet-superconductor heterostructures. The emergence of two-dimensional layered superconducting and magnetic materials promises a new platform for realizing pi JJs with atomically sharp interfaces. Here we demonstrate a thickness-driven 0-pi transition in JJs made of NbSe2 (an Ising superconductor) and Cr2Ge2Te6 (a ferromagnetic semiconductor). By systematically increasing the Cr2Ge2Te6 weak link thickness, we observe a vanishing supercurrent at a critical thickness of -8 nm, followed by a re-entrant supercurrent. Near the critical thickness, we further observe unusual supercurrent interference patterns with vanishing critical current around zero in -plane magnetic field. They signify the formation of 0-pi JJs (with both 0 and pi regions), induced the nanoscale domains in Cr2Ge2Te6.