Correlations, Topology and Unconventional Superconductivity in Magic Angle Twisted Bilayer Graphene

We have explored magic angle twisted bilayer graphene’s (MATBG) electronic properties using variety of spectroscopic methods with the scanning tunneling microscope (STM). Beside unraveling signatures of strong correlations [1] driving a cascade of transitions [2] in the electronic properties of MATBG, we have shown that such correlations also drive formation of topological Chern insulators stabilized with weak magnetic fields[3]. Most recently, we have focused our attention on the nature of superconducting state that forms near half filling of MATBG’s valance flat band. [4] Our experiments show signatures of nodal superconductivity with tunneling gap to transition temperature ratio that far exceeds the BCS limit. Moreover, we find superconductivity emerges from a pseudogap phase, which might be indication of either preformed pairs or the formation of some ordered state that makes superconductivity possible. Remarkably, we find that samples in which MATBG is in perfect alignment with the hexagonal BN substrate, both the pesudogap phase and superconductivity are suppressed.<br/> <br/>[1] Y. Xie. et al. Nature 572, 101 (2019).<br/>[2] D. Wong, et al. Nature 582, 198 (2020).<br/>[3] K. Nuckolls et al. Nature 588, 610-615 (2020).<br/>[4] M. Oh et al. to appear in Nature (2021). https://doi.org/10.1038/s41586-021-04121-x