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    Review of 'Magic-angle graphene superlattices: a new platform for unconventional superconductivity'

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    Magic-angle graphene superlattices: a new platform for unconventional superconductivityCrossref
    The article provides overview of the discovery of unconventional superconductivity in MA-TBG
    Average rating:
        Rated 4 of 5.
    Level of importance:
        Rated 4 of 5.
    Level of validity:
        Rated 4 of 5.
    Level of completeness:
        Rated 4 of 5.
    Level of comprehensibility:
        Rated 3 of 5.
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    Magic-angle graphene superlattices: a new platform for unconventional superconductivity

    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 \(1.1^\circ\), 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 \(T_c\) 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 \(T_c\), given such small Fermi surface (corresponding to a record-low 2D carrier density of \(10^{11} \textrm{cm}^{-2}\) , 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-\(T_c\) superconductors and quantum spin liquids.
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      Review text

      The article discusses the observation of intrinsic unconventional superconductivity in a 2D superlattice made of two graphene sheets stacked with a small twist angle. This system, known as Magic Angle Twisted Bilayer Graphene (MA-TBG), exhibits ultra-flat bands near charge neutrality, leading to correlated insulating states at half-filling. Upon electrostatic doping away from these states, tunable zero-resistance states with a critical temperature up to 1.7 K are observed. The temperature-density phase diagram shows similarities with that of cuprates, and quantum oscillations indicate small Fermi surfaces near the correlated insulating phase. The relatively high Tc for such small densities puts MA-TBG among the strongest coupling superconductors, making it a highly tunable platform to investigate strongly-correlated phenomena. The article also discusses the band structure of TBG and its evolution with twist angle.

      Overall, the article seems well-written and informative. Here are a few suggestions for improvement and clarification:

      It might be helpful to define some of the technical terms used in the section, such as "vortices," "dissipation," "Josephson junctions," "Ginzburg-Landau theory," and "BCS-BEC crossover." This would make the article more accessible to readers who are not experts in the field.

      In the paragraph on the phase diagram of MA-TBG, it would be helpful to provide a bit more context on what "half-filling density" means and why it is significant. Additionally, the sentence "A plausible explanation is that the many- body charge gap is closed by the Zeeman energy" could be clarified, perhaps by explaining what the "many-body charge gap" refers to and how the Zeeman effect might cause it to close.

      In the paragraph on quantum oscillations in the normal state, it might be helpful to provide a bit more background on what quantum oscillations are and why they are of interest in this context.

      The article might benefit from the inclusion of some figures or diagrams to help illustrate the concepts being discussed, particularly in the section on the phase diagram.

      What is the significance of the twist angle in the magic-angle twisted bilayer graphene (MA-TBG) system? What happens to the electronic band structure near this angle? What are the experimentally confirmed consequences of the flatness of the energy bands near charge neutrality?

      Overall, the article seems like it has a lot of valuable information, and these suggestions are relatively minor. 

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