High-field Josephson effect enabled by a moiré Hofstadter spectrum

Conventional Josephson junctions (JJs) are typically limited to low magnetic fields because the Lorentz force bends electron trajectories into closed cyclotron orbits, suppressing the phase-coherent Andreev transport required for supercurrent. This study investigates whether moiré superlattices in graphene/hBN heterostructures can overcome this constraint by engineering the electronic band structure to maintain phase coherence even in high-field regimes.

Researchers fabricated ballistic graphene/hBN Josephson junctions with MoRe superconducting leads. By aligning the graphene lattice to the hBN, they created a moiré superlattice that reconstructs the electronic spectrum into minibands. They measured the critical current ($I_c$) and normal-state resistance across a wide range of carrier densities and perpendicular magnetic fields (up to 6 T). They compared these experimental results with numerical calculations of the fractal Hofstadter-butterfly spectrum to determine how the moiré potential influences quasiparticle group velocity and Andreev bound state (ABS) transport.

In standard graphene JJs, superconductivity is suppressed when the cyclotron diameter becomes smaller than the junction length, as carriers are confined to localized Landau levels. In this study, however, the researchers observed robust supercurrents persisting up to 6 T—far beyond the typical 2.5 T limit. The data shows that the moiré potential transforms Landau levels with quenched group velocity into dispersive magnetic Bloch bands. These bands provide finite quasiparticle group velocity, which enables extended electron-hole Andreev trajectories across the junction. This mechanism effectively bypasses the standard quantum Hall regime constraints, allowing for high-field superconducting interferometry.

This work identifies a new strategy for stabilizing superconducting devices in high magnetic fields. By using moiré engineering to manipulate the Hofstadter spectrum, researchers can maintain phase-coherent transport in regimes previously considered inaccessible. This platform could enable new types of superconducting quantum devices and phase-sensitive probes of exotic quantum matter that only emerge under strong magnetic fields.