Proximity-induced superconductivity in the flat bands of Magic Angle Twisted Bilayer Graphene

The distinct electronic band structure of twisted bilayer graphene, marked by flat bands at specific twist angles, presents a unique platform for investigating the interplay between superconductivity and electron correlated states through the Josephson effect. This study investigates the proximity-induced superconductivity in a Josephson junction made from a magic-angle twisted bilayer graphene (MATBG) weak link coupled to superconducting electrodes.

While the Josephson effect is stronger when the junction is tuned into the high-energy dispersive bands, finite critical currents are still found at the flat bands despite their small Fermi velocity. In fact, calculations of the pairing amplitude of the injected Cooper pairs show good agreement with the experiment, but only when multiband processes and quantum geometry contributions of the flat bands are considered, revealing the importance of these mechanisms into the formation of superconducting phases in MATBG.

Finally, at the hole (electron) side of positive (negative) half-filling, we observe unconventional interference patterns with broken inversion and preserved time reversal symmetries, enabling us to engineer a reversible Josephson diode between opposite magnetic fields. Our experiment imposes constraints on the ground state symmetries of the flat bands at these fillings.