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Probing the Flat-Band Limit of the Superconducting Proximity Effect in Twisted Bilayer Graphene Josephson Junctions

A. Díez-Carlón1,2, J. Díez-Mérida1,2, P. Rout1,2, D. Sedov3, P. Virtanen4, S. Banerjee3, R. P. S. Penttilä5, P. Altpeter1, K. Watanabe6 et al.

T. Taniguchi7, S.-Y. Yang8, K. T. Law9, T. T. Heikkilä4, P. Törmä5, M. S. Scheurer3, and D. K. Efetov1,2,*

  • *Contact author: dmitri.efetov@lmu.de
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Phys. Rev. X 15, 041033 – Published 20 November, 2025

DOI: https://doi.org/10.1103/ccb4-tqxq

Abstract

While extensively studied in normal metals, semimetals, and semiconductors, the superconducting (SC) proximity effect remains elusive in the emerging field of flat-band systems. In this study, we probe proximity-induced superconductivity in Josephson junctions (JJs) formed between superconducting NbTiN electrodes and twisted bilayer graphene (TBG) weak links. Here, the TBG acts as a highly tunable topological flat-band system, which, due to its twist-angle-dependent bandwidth, allows us to probe the SC proximity effect at the crossover from the dispersive to the flat-band limit. Contrary to our original expectations, we find that the induced superconductivity remains strong even in the flat-band limit and gives rise to broad, dome-shaped SC regions, in the filling-dependent phase diagram. In addition, we find that, unlike in conventional JJs, the critical current Ic strongly deviates from a scaling with the normal state conductance GN. We attribute these findings to the onset of strong electron interactions, which can give rise to an excess critical current. By also studying the dependence of Ic on the filling and twist angle across multiple samples, we further uncover the importance of quantum geometric terms as well as multiband pairing mechanisms in describing the induced superconductivity in the TBG flat bands as their bandwidth decreases. To the best of our knowledge, our results present the first detailed study of the SC proximity effect in the flat-band limit and shed new light on the mechanisms that drive the formation of SC domes in flat-band systems.

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According to standard theory, a Josephson junction—a device that carries a supercurrent between two superconductors—should not conduct any current if the material in the middle has a flat electronic band, meaning that its electrons do not move or spread out in energy. However, recent theoretical advances suggest that the quantum geometry of such bands, describing how electron wave functions overlap, can sustain a finite supercurrent even without dispersion. To test this idea, we create Josephson junctions using twisted bilayer graphene (TBG), a material made by stacking two sheets of graphene at a slight angle. This twist produces flat bands whose degree of flatness can be adjusted by changing the angle or applying an electric voltage.

At very low temperatures, we measure how supercurrent flows through these TBG junctions. Surprisingly, we find that the current remains strong even when the electronic bands are extremely flat—just as strong as when the bands allow electrons to move more freely. This result cannot be explained by traditional ideas about how electrons travel through a material. Instead, it points to the importance of collective electronic behavior, quantum geometry, and the interplay among multiple electronic states in sustaining the current.

Our findings reveal that superconductivity can survive even when electrons are almost completely localized, challenging long-held assumptions about how supercurrents form. This discovery also opens new possibilities for exploring superconductivity in other materials with flat electronic structures, which may help in developing future quantum technologies, including more robust types of quantum computers.

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Phys. Rev. X 15, 041033– Published 20 November, 2025
  • Received 5 February 2025
  • Revised 15 May 2025
  • Accepted 1 October 2025
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