Point-Contact Measurements of the Phase Diagram of Magic-Angle Twisted Bilayer Graphene

Due to strong interactions and flat electronic bands, magic-angle twisted bilayer graphene (MATBG) hosts an incredibly complex phase diagram consisting of correlated metals, band insulators, correlated insulators, Chern and quantum Hall insulators, and superconductors. Disentangling this phase diagram though tunneling differential conductance is difficult because many phases (including insulators and superconductor) manifest as a gap in the quasiparticle energy spectrum. We use density-tuned point-contact spectroscopy (PCS) on a scanning tunneling microscope (STM) to unambiguously distinguish correlated insulators, which have strongly suppressed PCS conductance, from superconductors, which display enhanced PCS conductance around zero bias due to Andreev reflection. By comparing PCS with scanning tunneling spectroscopy (STS), we infer that the superconducting phase of MATBG has several unusual properties that are reminiscent of behavior in other unconventional superconductors [1]. Moreover, we remark that PCS is a relatively straightforward method for verifying the existence or absence of superconductivity in two-dimensional materials, as certain transport signatures can be occasionally misleading.