Origin of Mott Insulating Behavior and Superconductivity in Twisted Bilayer Graphene

By H. Po, Liujun Zou, Ashvin Vishwanath, T. Senthil. Physical Review X, 2018

588 citations recorded by the source corpus.

A remarkable recent experiment has observed Mott insulator and proximate superconductor phases in twisted bilayer graphene when electrons partly fill a nearly flat mini-band that arises a `magic' twist angle. However, the nature of the Mott insulator, origin of superconductivity and an effective low energy model remain to be determined. We propose a Mott insulator with intervalley coherence that spontaneously breaks U(1) valley symmetry, and describe a mechanism that selects this order over the competing magnetically ordered states favored by the Hunds coupling. We also identify symmetry related features of the nearly flat band that are key to understanding the strong correlation physics and constrain any tight binding description. First, although the charge density is concentrated on the triangular lattice sites of the moir$\text{e }$ pattern, the Wannier states of the tight-binding model must be centered on different sites which form a honeycomb lattice. Next, spatially localizing electrons derived from the nearly flat band necessarily breaks valley and other symmetries within any mean-field treatment, which is suggestive of a valley-ordered Mott state, and also dictates that additional symmetry breaking is present to remove symmetry-enforced band contacts. Tight-binding models describing the nearly flat mini-band are derived, which highlight the importance of further neighbor hopping and interactions. We discuss consequences of this picture for superconducting states obtained on doping the valley ordered Mott insulator. We show how important features of the experimental phenomenology may be explained and suggest a number of further experiments for the future. We also describe a model for correlated states in trilayer graphene heterostructures and contrast it with the bilayer case.

Subfields: Atomic and Molecular Physics, and Optics, Materials Chemistry

View Paper: https://www.semanticscholar.org/paper/93802375ee9c996b7afb7d2cf9f0dbb565091110

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Citations in the corpus

Quantum critical behaviour in magic-angle twisted bilayer graphene (2021, result): “However, this scenario is inconsistent with our findings as we observe (1) a large interval of dopings where the low-T linear regime occurs, (2) the emergence of a T2-dependent resistivity of the same amplitude near the flat-band edges and (3) the evolution of the prefactor AT,γ as a function of dop”
Superconductivity, superfluidity and quantum geometry in twisted multilayer systems (2021, background): “…and theoretically, and the least theoretically controversial as extensive theoretical efforts have been aimed at their explanation [79–81 The strong coupling TBG Coulomb Hamiltonians of Kang and Vafek [79] and others [69, 102, 119, 120] (similarly to the negative U projected Hubbard…”
Phases of a phenomenological model of twisted bilayer graphene (2018, background): “As mentioned earlier, several papers [5, 6] have noted that there are significant theoretical barriers to a direct route from the nodal band structure of the individual graphene sheets to the effective two-orbital model we have studied.”
Origin of Magic Angles in Twisted Bilayer Graphene. (2018, methodology): “siders two layers of graphene described by an effective Dirac fields near K,K ′ points of the moire (mini) Brillouin Zone, each rotated by an angle ±θ/2, and coupled through a Moiré potential T (r) [3, 14, 35, 39]:”
Symmetry breaking in twisted double bilayer graphene (2020, background): “two-fold rotation and time reversal symmetry (C2T ) [21].”
Moiré heterostructures as a condensed-matter quantum simulator (2020, background): “honeycomb two-orbital extended Hubbard model [18] fragile topological insulator [19] twisted bilayer graphene (BN substrate, with/without twist) Mott insulation [8] superconductivity [7] correlated QAH insulator [20, 21]”