2018/03/31 by Yuan Cao, Valla Fatemi, Shiang Fang +4 · 11 citations
Physics and Astronomy · #cond-mat.mes-hall #cond-mat.str-el #cond-mat.supr-con
paper · pdf · doi:10.1038/nature26160
published as Nature 556, p. 43-50 (2018) · 18 pages, 9 figures (with Methods). A few typos corrected
arxiv created 2018/05/20 · arxiv updated 2018/05/22
The understanding of strongly-correlated materials, and in particular unconventional superconductors, has puzzled physicists for decades. Such difficulties have stimulated new research paradigms, such as ultra-cold atom lattices for simulating quantum materials. Here we report on the realization of intrinsic unconventional superconductivity in a 2D superlattice created by stacking two graphene sheets with a small twist angle. For angles near 1.1^∘, the first `magic' angle, twisted bilayer graphene (TBG) exhibits ultra-flat bands near charge neutrality, which lead to correlated insulating states at half-filling. Upon electrostatic doping away from these correlated insulating states, we observe tunable zero-resistance states with a critical temperature Tc up to 1.7 K. The temperature-density phase diagram shows similarities with that of the cuprates, including superconducting domes. Moreover, quantum oscillations indicate small Fermi surfaces near the correlated insulating phase, in analogy with under-doped cuprates. The relative high Tc, given such small Fermi surface (corresponding to a record-low 2D carrier density of 1011 \textrmcm-2 , renders TBG among the strongest coupling superconductors, in a regime close to the BCS-BEC crossover. These novel results establish TBG as the first purely carbon-based 2D superconductor and as a highly tunable platform to investigate strongly-correlated phenomena, which could lead to insights into the physics of high-Tc superconductors and quantum spin liquids.