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Article
Physics, Multidisciplinary
Gaopei Pan et al.
Summary: The dynamic and thermodynamic properties of correlated flat-band systems are investigated using momentum-space quantum Monte Carlo and exact diagonalization methods. It is found that the transition from the interaction-driven quantum anomalous Hall (QAH) insulator to the metallic state occurs at a much lower temperature compared to the zero-temperature single-particle gap generated by the long-range Coulomb interaction. This low transition temperature is attributed to the proliferation of excitonic particle-hole excitations, which restores the broken time-reversal symmetry and leads to an enhancement in charge compressibility. Future experiments are proposed to verify these generic thermodynamic characteristics.
PHYSICAL REVIEW LETTERS
(2023)
Article
Multidisciplinary Sciences
Meizhen Huang et al.
Summary: This study reports a significant breakthrough in the efficiency of nonlinear Hall generation in small-angle-twisted bilayer WSe2, reaching 1000 V-1, which is a hundred times higher than the previous records. The researchers explained this result through the correlation-induced continuous Mott transition effect.
NATIONAL SCIENCE REVIEW
(2023)
Article
Multidisciplinary Sciences
Jiang-Xiazi Lin et al.
Summary: This study investigates the combined influence of strong electron correlation and spin-orbit coupling on a two-dimensional electronic system at the interface between twisted bilayer graphene and tungsten diselenide crystal. The research shows that strong electron correlation stabilizes correlated insulating states at different fillings within the moire flatband and spin-orbit coupling transforms these insulators into ferromagnetic materials. The coupling between spin and valley degrees of freedom can be controlled with a magnetic or electric field.
Article
Multidisciplinary Sciences
Tianye Huang et al.
Summary: This study reports the direct observation of two new plasmon modes in macroscopic twisted bilayer graphene (tBLG) with a highly ordered moiré superlattice. These modes include chiral plasmons arising from the uncompensated Berry flux of the electron gas under optical pumping, and slow plasmonic modes resulting from interband transitions in AB-stacked domains. These findings are important for understanding the electromagnetic dynamics of small-angle tBLG and enabling strong light-matter interactions in the mid-wave infrared spectral window.
Article
Physics, Multidisciplinary
Saisab Bhowmik et al.
Summary: States near half-integer band fillings and a symmetry-broken Chern insulator have been observed in magic-angle twisted bilayer graphene (TBG) at near-zero magnetic field. The results suggest the presence of a spin or charge density wave ground state in TBG in the zero-magnetic-field limit.
Article
Physics, Multidisciplinary
Xiyue Lin et al.
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)
Article
Multidisciplinary Sciences
Weilun Jiang et al.
Summary: The origin of the pseudogap in high-Tc superconductors remains a puzzle. Using numerical simulations, the authors find that it arises from pairing fluctuations in a quantum-critical non-Fermi liquid, similar to the pseudogap phase observed in cuprate superconductors. This finding provides direct evidence for the formation of the pseudogap state.
NATURE COMMUNICATIONS
(2022)
Article
Physics, Multidisciplinary
Johannes S. Hofmann et al.
Summary: In this study, unbiased Monte Carlo simulations were used to investigate realistic models of magic-angle twisted bilayer graphene (TBG) at charge neutrality. The results show the competition between an insulating Kramers intervalley coherent ground state and a correlated semimetal phase in the absence of transport. The temperature evolution of order parameters and electronic spectral functions reveals a pseudogap regime, where gap features are established at a higher temperature than the onset of order. Furthermore, predictions for electronic tunneling spectra and their evolution with temperature are provided.
Article
Physics, Multidisciplinary
Shihao Zhang et al.
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
(2022)
Article
Physics, Multidisciplinary
Jonah Herzog-Arbeitman et al.
Summary: This Letter focuses on the study of magic angle twisted bilayer graphene (TBG) at 2 pi flux. By using a newly developed gauge-invariant formalism, the exact single-particle band structure and topology are determined. It is found that the characteristic TBG flat bands reemerge at 2 pi flux, but they split and acquire Chern number +1 due to the magnetic field breaking C2zT. In addition, reentrant correlated insulating states driven by the Coulomb interaction at integer fillings are discovered, and characteristic Landau fans from their excitation spectrum are predicted.
PHYSICAL REVIEW LETTERS
(2022)
Article
Physics, Multidisciplinary
Zhi-Da Song et al.
Summary: Magic-angle twisted bilayer graphene exhibits seemingly contradictory localization and delocalization behaviors. By constructing a model, we are able to capture both aspects and provide a new perspective on strong correlation physics.
PHYSICAL REVIEW LETTERS
(2022)
Article
Materials Science, Multidisciplinary
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.
Article
Physics, Multidisciplinary
Eric Brillaux et al.
Summary: This study investigates twisted bilayer graphene near charge neutrality using a generalized Bistritzer-MacDonald continuum model that takes into account corrugation effects. The findings show that the Fermi velocity vanishes for specific twist angles, accurately reproducing the physics of magic angles. By utilizing group representation theory, the research identifies all compatible contact interaction potentials based on the symmetries of the model. Furthermore, a renormalization group analysis is implemented to examine the impact of quantum fluctuations on the emergence of different states for twist angles approaching the first magic value. The combined group theory-renormalization study reveals the preference for a layer-polarized, gapped state with spatial modulation of interlayer correlations, referred to as a nematic insulator, near the first magic angle.
PHYSICAL REVIEW RESEARCH
(2022)
Article
Physics, Multidisciplinary
Chengkang Zhou et al.
Summary: This paper investigates the dynamical signature in the X-cube model in the presence of external Zeeman fields using large-scale quantum Monte Carlo simulation and stochastic analytic continuation. The study reveals the evolution of subdimensional excitations in fracton orders and their behavior under external fields.
PHYSICAL REVIEW RESEARCH
(2022)
Article
Materials Science, Multidisciplinary
Shubhayu Chatterjee et al.
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.
Article
Materials Science, Multidisciplinary
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.
Article
Materials Science, Multidisciplinary
Frank Schindler et al.
Summary: The strong-coupling phase diagram of magic-angle twisted bilayer graphene predicts exact one-particle charge +/- 1 gapped excitations above the ferromagnetic ground states. In this study, the trion bound state is identified as the lowest charge +1 overall excitation under certain conditions.
Article
Materials Science, Multidisciplinary
Gaopei Pan et al.
Summary: Using the momentum-space quantum Monte Carlo scheme, the dynamic response of single-particle and collective excitations in realistic continuum models of twisted bilayer graphene is studied. The findings reveal that repulsive interactions push the fermion spectral weight away from the Fermi energy, resulting in an insulating gap. The spectra of collective excitations suggest an approximate valley SU(2) symmetry, with long-lived valley waves observed at low energy.
Review
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.
Article
Physics, Multidisciplinary
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.
Article
Physics, Multidisciplinary
Andrew T. Pierce et al.
Summary: The discovery of unexpected incompressible states in MATBG, with Chern numbers incompatible with the simple sequential band filling assumption, can be understood as a consequence of broken translation symmetry that doubles the moire unit cell and splits each flavour band in two. These unusual incompressible phases expand the known phase diagram of MATBG and shed light on the close competition between different correlated phases in the system.
Article
Physics, Multidisciplinary
Jian Kang et al.
Summary: The research presents a framework for understanding the cascade transitions and Landau level degeneracies of twisted bilayer graphene, which sheds light on new insights. By varying the filling, different mass excitations can be generated, explaining the diversity of features observed.
PHYSICAL REVIEW LETTERS
(2021)
Article
Physics, Multidisciplinary
Petr Stepanov et al.
Summary: Research has shown that the odd integer filling factors in h-BN nonaligned devices correspond to symmetry broken Chern insulators with a Chern number of C = +1 and a relatively high Curie temperature. Under a perpendicular magnetic field, the Chern insulator at v = +1 transitions from C = +1 to C = 3, characterized by a quantized Hall plateau. Additionally, the device exhibits strong superconducting phases with critical temperatures up to Tc ≈ 3.5 K.
PHYSICAL REVIEW LETTERS
(2021)
Article
Multidisciplinary Sciences
Asaf Rozen et al.
Summary: The study reveals a transition from a low-entropy electronic liquid to a high-entropy correlated state in magic-angle twisted bilayer graphene under the influence of electron density, temperature, and magnetic field. The correlated state demonstrates a unique combination of properties associated with itinerant electrons and localized moments, with distinct energy scales for different characteristics. The hybrid nature of the correlated state and the separation of energy scales have significant implications for the thermodynamic and transport properties of twisted bilayer graphene.
Article
Multidisciplinary Sciences
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.
Article
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.
Article
Physics, Multidisciplinary
Yves H. Kwan et al.
Summary: The study reveals the topological features of neutral particle-hole pair excitations and their impact on the bound states in correlated QAH insulators. This results in the formation of topological exciton bands with robust features. The research also applies these ideas to broken-symmetry spontaneous QAH insulators in magic-angle twisted bilayer graphene with substrate alignment.
PHYSICAL REVIEW LETTERS
(2021)
Article
Multidisciplinary Sciences
Xiaomeng Liu et al.
Summary: This study demonstrates the tunability and electronic correlations of twisted double bilayer graphene (TDBG) through gate-tuned scanning tunneling spectroscopy, revealing the topological properties of its flat band. The application of a magnetic field leads to valley polarization and the splitting of Chern bands with a large effective g-factor in TDBG.
NATURE COMMUNICATIONS
(2021)
Article
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.
Article
Physics, Multidisciplinary
Chengkang Zhou et al.
Summary: The study focuses on the amplitude (Higgs) mode associated with longitudinal fluctuations of the order parameter at the continuous spontaneous symmetry breaking phase transition. By using quantum Monte Carlo simulations, stochastic analytic continuation, and a chain-mean field approach combined with a mapping to the field-theoretic sine-Gordon model, the amplitude mode is successfully observed in a weakly coupled spin chain system in a quasi-one-dimensional geometry. The study also compares the amplitude mode in different dimensions and finds that in contrast to higher-dimensional systems, the amplitude and bond order fluctuations carry significant spectral weight in the quasi-1D limit.
PHYSICAL REVIEW LETTERS
(2021)
Article
Multidisciplinary Sciences
Yan-Cheng Wang et al.
Summary: The experimental discovery of Anyons in two-dimensional electron gases has opened up new possibilities for studying quantum particles beyond bosons and fermions. Large-scale quantum Monte Carlo simulations have revealed unique conductivity properties near a phase transition, with implications for quantum materials research.
NATURE COMMUNICATIONS
(2021)
Article
Multidisciplinary Sciences
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.
NATURE COMMUNICATIONS
(2021)
Article
Materials Science, Multidisciplinary
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.
Article
Materials Science, Multidisciplinary
Yunqing Ouyang et al.
Summary: The paper introduces a projection approach for quantum Monte Carlo simulation of the infinite-U Hubbard model at some integer fillings, addressing sign problem and algebraic sign structure. It demonstrates the method on the infinite-U SU (2N) fermionic Hubbard model at half filling, suggesting possible correlated ground states. The scheme can be generalized to study extended Hubbard models and implement Gutzwiller projection to spin basis for studying SU (2N) quantum spin models and Kondo lattice models.
Article
Materials Science, Multidisciplinary
B. Andrei Bernevig et al.
Summary: This study examines the twisted bilayer graphene model proposed by Bistritzer and MacDonald, providing an analytical understanding of its energetics and wave functions for many-body calculations. Approximation schemes for wave functions are provided, revealing insights into the accuracy of the original calculations and the neglect of certain matrix elements in the Coulomb Hamiltonian. Analytical calculations are also conducted to understand band properties at magic angles and in isotropic limits, shedding light on the band structure and symmetry considerations in TBG.
Article
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.
Article
Materials Science, Multidisciplinary
Biao Lian et al.
Summary: This study investigates the exact insulator states and Chern insulators in different limits of the projected Hamiltonian for magic-angle twisted bilayer graphene flat bands with Coulomb interactions. The results show competitive low-energy states of TBG, with different Chern number states being degenerate in certain limits. Transition behaviors are also observed in the presence of magnetic fields. The TBG Hamiltonian converges into an extended Hubbard model in the stabilizer code limit.
Article
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.
Article
Physics, Multidisciplinary
Shang Liu et al.
Summary: We report a fully self-consistent Hartree-Fock calculation of the interaction effects on the moire flat bands of twisted bilayer graphene, assuming valley U(1) symmetry is respected. Three types of self-consistent solutions were found, including insulators breaking C2T symmetry, spin/valley-polarized insulators, and semimetals breaking rotation C-3 symmetry. The relative stability of these states can be tuned by weak strains that break C-3 rotation.
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