Experimental detection of vortices in magic-angle graphene

The tunability of superconducting magic-angle twisted-layer graphene films elevates this material system to a promising candidate for superconducting electronics. We implement a gate-tuned Josephson junction in a magic-angle twisted four-layer graphene film. Field-dependent measurements of the critical current show a Fraunhofer-like pattern that differs from the standard pattern with characteristics typical for a weak transverse screener. We observe sudden shifts associated with vortices jumping into and out of the leads. By tuning the leads to the edge of the superconducting dome, we observe fast switching between superconducting and normal states, an effect associated with vortex dynamics. Time-dependent measurements provide us with the vortex energy scale and an estimate for the London penetration depth, in agreement with recent kinetic inductance measurements on twisted graphene films. Our results prove the utility of our junction as a sensor for vortex detection, allowing us to extract fundamental properties of the 2D superconductor.

💡 Research Summary

In this work the authors fabricate a gate‑defined Josephson junction (JJ) in a magic‑angle twisted four‑layer graphene (MA‑T4G) heterostructure and use it as a highly sensitive probe of vortex dynamics in an atomically thin two‑dimensional superconductor. The device incorporates three independent electrostatic gates – a bottom graphite gate, a top metal gate, and a finger gate – which allow separate control of carrier density (n) and displacement field (D) in the superconducting leads and in the weak‑link region. By tuning the leads into a superconducting dome (hole‑doped ν≈−3.5…−2, Tc up to 2 K, or electron‑doped ν≈2…3.5, Tc≈1 K) and the junction region into a resistive state, a short JJ with width W≈1.1 µm and length Lj≈150 nm is realized. The graphene stack is only ~1 nm thick, far thinner than the bulk London penetration depth λL, placing the system in the “weak transverse screening” regime where the Pearl length Λ≫W and magnetic fields penetrate the entire film.

Magnetic interference measurements are performed by sweeping a perpendicular magnetic field B while recording the critical current I_cj(B). Unlike conventional JJs, where the Fraunhofer pattern has a period ΔB=Φ0/(2λL+Lj)W and a sinc‑shaped envelope decaying as 1/B, the observed pattern exhibits a period set by the flux through the junction area, ΔB≈Φ0/W²≈3 mT, and a slow envelope decaying as 1/√B. This behavior follows directly from the weak‑screening theory: the phase difference across the junction is Δγ(y)≈1.7 ΦW/Φ0 sin(πy/W) rather than a linear function of y, and the resulting critical current is proportional to the Bessel function J0