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Meizhen Huang et al.
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Summary: A topological Mott insulator with spontaneous time-reversal symmetry breaking and nonzero Chern number has been discovered in real-space effective model for twisted bilayer graphene at 3/4 filling in the strong coupling limit. The researchers obtained the finite-T phase diagram and dynamical properties of the model using thermal tensor network and perturbative field-theoretical approaches. They found that the transitions occur at significantly reduced temperature due to the proliferation of excitons, and explained the experimental signatures of the exciton phase.
PHYSICAL REVIEW LETTERS
(2022)
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Glenn Wagner et al.
Summary: We investigate the phase diagram of twisted bilayer graphene (TBG) in the normal state, considering doping and strain dependence. Our comprehensive calculations reveal the competition and coexistence of three intertwined orders: a fully symmetric phase, flavor-symmetry-breaking states, and an incommensurate Kekule spiral (IKS) order. The IKS order is found to be ubiquitous for noninteger doping as well, consistent with experimental observations of electronic compressibility and Fermi surface structure. This study suggests a unified picture of the phase diagram in terms of IKS order.
PHYSICAL REVIEW LETTERS
(2022)
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Johannes S. Hofmann et al.
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Summary: In this Letter, the authors theoretically investigate the correlated insulators and density wave states in magic-angle twisted bilayer graphene (TBG) and propose that nonlinear optical response can serve as a promising experimental probe for unveiling the nature of these states.
PHYSICAL REVIEW LETTERS
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Xu Zhang et al.
Summary: By considering the evidence of intervalley attraction-mediated by phonon or topological fluctuations, this study assumes the existence of intervalley attraction in moire flat bands and aims to identify universal properties that may emerge. The results show that the flat-band limit can be solved exactly by matching the interaction strength of intervalley attraction with intravalley repulsion. The study also demonstrates the presence of a boson fluid phase and a pseudogap phase at high temperatures.
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Summary: Recent research suggests that the topology of magic-angle twisted bilayer graphene could provide a novel mechanism for superconductivity, driven by the binding of electrons into charge-2e skyrmions.
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Xu Zhang et al.
Summary: Sign problems in fermion quantum Monte Carlo simulation have long been a challenging issue. Traditional beliefs suggest that the average sign in QMC simulation approaches zero exponentially fast with the space-time volume. However, we analytically show that this is not always true and manage to find physical bounds for the average sign. Our understanding is based on the connection between the sign bounds and the partition function of the reference system, which can determine whether the bounds exhibit exponential or algebraic scaling in the low-temperature limit.
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Frank Schindler et al.
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Gaopei Pan et al.
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Physics, Multidisciplinary
Yuan-Da Liao et al.
Summary: The review discusses analytical and numerical studies of correlated insulating states in twisted bilayer graphene, focusing on real-space lattice models constructions and their unbiased quantum many-body solutions. By constructing localized Wannier states for the narrow bands, the projected Coulomb interactions can be approximated by interactions of cluster charges with assisted nearest neighbor hopping terms. The Hamiltonian in the strong coupling limit is SU(4) symmetric, but perturbative kinetic terms break this symmetry and lead to the emergence of various insulating states.
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NATURE COMMUNICATIONS
(2021)
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Yuan Da Liao et al.
Summary: In this study, unbiased quantum Monte Carlo simulations were used to solve an effective interacting lattice model for twisted bilayer graphene (TBG) at charge neutrality. Various correlated insulating phases were discovered, including a quantum valley Hall state with topological edge states, an intervalley-coherent insulator, and a valence bond solid. These charge-neutrality correlated insulating phases provide important reference states for understanding insulating states at integer fillings and the proximate superconducting states of TBG.
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Andrew T. Pierce et al.
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Multidisciplinary Sciences
Asaf Rozen et al.
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Yu Saito et al.
Summary: The study explores the finite-temperature dynamics of spin and valley isospins in magic-angle twisted bilayer graphene, revealing a resistivity peak at high temperatures near a superlattice filling factor of -1, suggesting a Pomeranchuk-type mechanism. The data indicate the presence of a finite-field magnetic phase transition and a small isospin stiffness in the system.
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Multidisciplinary Sciences
Jeong Min Park et al.
Summary: Interaction-driven spontaneous symmetry breaking plays a key role in the emergence of correlated and topological ground states in moire systems such as magic-angle twisted bilayer graphene (MATBG). Through thermodynamic and transport measurements, we have observed broken spin/valley 'flavour' symmetry in MATBG and its nontrivial topology. Furthermore, the topological nature of the flat bands is revealed by breaking time-reversal symmetry, leading to the observation of Chern insulator states with different Chern numbers at specific filling factors. Our findings shed light on the understanding of interactions in the topological bands of MATBG, both with and without a magnetic field.
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Multidisciplinary Sciences
Eslam Khalaf et al.
Summary: Topological solitons play a crucial role in insulating and superconducting behavior in stacked and twisted graphene sheets. Symmetry breaking leads to an ordered insulator, while topological solitons result in a superconductor.
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Zheng Yan et al.
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Augusto Ghiotto et al.
Summary: Metal-to-insulator transitions in twisted WSe2 exhibit strange metal behavior at low temperatures and Fermi liquid behavior at intermediate temperatures, eventually reaching an anomalous saturated regime near room temperature. This new platform allows for the study of doping and bandwidth-controlled metal-insulator quantum phase transitions on the triangular lattice.
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Chengkang Zhou et al.
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PHYSICAL REVIEW LETTERS
(2021)
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Bin-Bin Chen et al.
Summary: This study investigates the correlated electron phenomena in magic-angle twisted bilayer graphene, proposing a Mott-Hubbard perspective and identifying a quantum phase transition in the lattice model. The results not only clarify the mechanism of the quantum anomalous Hall state, but also provide an example of the topological Mott insulator in the strong coupling limit.
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En Li et al.
Summary: The authors demonstrate the emergence of multiple ultra-flat electronic bands in twisted bilayer WSe2 using scanning tunneling microscopy and spectroscopy, indicating the potential for further study of exotic correlated phases in TB-TMDs.
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(2021)
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B. Andrei Bernevig et al.
Summary: We derive the explicit Hamiltonian of twisted bilayer graphene with Coulomb interaction, demonstrating positive semidefinite Hamiltonians when projected into flat bands. It is proved that the interacting TBG Hamiltonian exhibits an exact U(4) symmetry in the flat band limit. The existence of chiral limits with enlarged symmetry suggests a possible undiscovered duality of the model.
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Materials Science, Multidisciplinary
B. Andrei Bernevig et al.
Summary: In this study, exact analytic expressions for the energies and wave functions of charged and neutral excitations above the ground state in twisted bilayer graphene are determined, considering various forms of Coulomb interaction and tunneling. It was found that the Cooper pair binding energy is zero in all projected Coulomb models for actual TBG bands, indicating that superconductivity may require other factors such as phonons or nonzero kinetic energy. The study also suggests that if superconductivity is driven by kinetic terms, the highest critical temperature may not necessarily occur at the maximum density of states.
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Materials Science, Multidisciplinary
Zhi-Da Song et al.
Summary: The continuous model of twisted bilayer graphene (TBG) is shown to be anomalous and incompatible with lattice models, with numerical confirmation of an approximate particle-hole symmetry and stable topology. This stable topology implies the existence of 4l + 2 Dirac points, protected against arbitrary gap closings, and cannot be realized in lattice models that preserve both C2zT and P symmetries. Additionally, other topology related topics and their implications for interacting TBG problems are discussed.
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Materials Science, Multidisciplinary
Jianpeng Liu et al.
Summary: Using an all-band Hartree-Fock variational method, the mechanisms behind correlated insulating states and quantum anomalous Hall effects in twisted bilayer graphene at integer fillings of flat bands have been explained. Correlated insulating states exhibit valley polarization and moire orbital antiferromagnetic ordering, while the quantum anomalous Hall states show spin and orbital ferromagnetic characteristics.
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Physics, Multidisciplinary
Shang Liu et al.
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