Probing the flat-band limit of the superconducting proximity effect in Twisted Bilayer Graphene Josephson junctions

TL;DR

This study investigates the superconducting proximity effect in twisted bilayer graphene Josephson junctions, revealing strong superconductivity even in the flat-band limit and highlighting the role of electron interactions and quantum geometry.

Contribution

First detailed experimental analysis of superconducting proximity effect in flat-band twisted bilayer graphene, exploring the crossover from dispersive to flat-band regimes.

Findings

Superconductivity remains strong in the flat-band limit.

Critical current deviates from normal conductance scaling.

Observation of broad, dome-shaped superconducting regions.

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 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 SC 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, and also work out the potential importance of quantum geometric terms as well as multiband pairing mechanisms. 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.