AC Josephson effect in Magic-Angle Twisted Bilayer Graphene

Since the discovery of superconductivity in magic-angle twisted bilayer graphene (MATBG) [1] many aspects of twisted Van der Waals materials are being explored. Here, we electrostatically define a Josephson junction in MATBG by selectively tuning the Fermi energy on different parts of our device [2]. We observe both the DC and AC Josephson effects. We tune the leads of the junction to the superconducting regime and explore different chemical potential configurations for the junction area. Hysteresis measurements of the IV traces give access to the quality factors of the different junctions we are able to form. We measure Shapiro steps, the most characteristic signature of the AC Josephson effect. By comparing the data to simulations obtained in the framework of the RCSJ model, we gain insight on how tuning the chemical potential of the junction area to the 2^nd and 3^rd moiré bands of MATBG affects the capacitance and quasiparticle resistance across the junction. Finally, the Shapiro step pattern shows no evidence of topological superconductivity.


[1] Cao, Y. et al. Unconventional superconductivity in magic-angle graphene superlattices. Nature 556, 43–50 (2018)
[2] de Vries, et al. Gate-Defined Josephson Junctions in Magic-Angle Twisted Bilayer Graphene. arXiv:2011.00011v2 (2020)