Designing Quantum Spin-Orbital Liquids in Artificial Mott Insulators

Dou, Xu; Kotov, Valeri N.; Uchoa, Bruno

doi:10.1038/srep31737

Cited by 7 publications

(6 citation statements)

References 49 publications

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“…Those flat bands have four-fold degeneracy, which is reminiscent of the valley and spin quantum numbers of the graphene sheets. In general, the confinement of interacting Dirac fermions in flat bands is expected to create an emergent SU(4) symmetry, as previously predicted in graphene heterostructures [15][16][17] and in graphene Landau levels [18][19][20][21][22][23][24]. Here, the Moire pattern forms a superlattice of quasi-localized states with the size of the unit cell set by the twist angle, as shown in Fig.…”

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confidence: 54%

Ferromagnetic Mott state in Twisted Graphene Bilayers at the Magic Angle

2019

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We address the effective tight-binding Hamiltonian that describes the insulating Mott state of twisted graphene bilayers at a magic angle. In that configuration, twisted bilayers form a honeycomb superlattice of localized states, characterized by the appearance of flat bands with four-fold degeneracy. After calculating the maximally localized superlattice Wannier wavefunctions, we derive the effective spin model that describes the Mott state. We suggest that the system is an exotic ferromagnetic Mott insulator, with well defined experimental signatures. arXiv:1812.02550v2 [cond-mat.str-el] 10 Dec 2018The most general exchange term between sites i, j that follows from (45) has the form

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confidence: 54%

Ferromagnetic Mott state in Twisted Graphene Bilayers at the Magic Angle

2019

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“…Kugel-Khomskii (KK) models [1] are effective Hamiltonians with couplings between spin and orbital degrees of freedom that describe various phenomena in transition metal oxides [2,3]. Recently, the applications of KK models have been extended to Mott insulators with strong spin-orbit coupling [4], iron-pnictide superconductors [5], Coulomb impurity lattices designed with scanning tunneling microscope [6], and cold atom systems [7]. In realistic KK models, the interplay between orbital configuration and lattice geometry generally constrains the virtual electron transfers and generates exchange frustration in the form of bond-dependent and anisotropic spin-orbital interactions [4].…”

Section: Introductionmentioning

confidence: 99%

SU(4) Heisenberg model on the honeycomb lattice with exchange-frustrated perturbations: Implications for twistronics and Mott insulators

et al. 2019

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The SU(4)-symmetric spin-orbital model on the honeycomb lattice was recently studied in connection to correlated insulators such as the eg Mott insulator Ba3CuSb2O9 and the insulating phase of magic-angle twisted bilayer graphene at quarter filling. Here we provide a unified discussion of these systems by investigating an extended model that includes the effects of Hund's coupling and anisotropic, orbital-dependent exchange interactions. Using a combination of mean-field theory, linear flavor-wave theory, and variational Monte Carlo, we show that this model harbors a quantum spin-orbital liquid over a wide parameter regime around the SU(4)-symmetric point. For large Hund's coupling, a ferromagnetic antiferro-orbital ordered state appears, while a valence-bond crystal combined with a vortex orbital state is stabilized by dominant orbital-dependent exchange interactions.

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“…It was recently proposed that a Coulomb-impurity lattice on a graphene substrate can realize an SU(4)-symmetric spin-orbital model with a tunable coupling constant. 58 We hope that our study stimulate a search for the spin nematic state in real materials including them.…”

Section: Discussionmentioning

confidence: 87%

Spin nematics in frustrated spin-dimer systems with bilayer structure

2019

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We study frustrated spin-1/2 dimer systems with bilayer structure in two dimensions, where spins are ferromagnetically coupled in dimers. Our model includes frustrated two-spin exchange interactions as well as four-spin interaction. We pay particular attention to the spin nematic phase, which does not exhibit any magnetic (spin-dipole) order but has a spin-quadrupolar long-range order. Employing a perturbation calculation, a mean-field approximation, and a numerical manyvariable variational Monte Carlo method, we determine ground-state phase diagrams on various two-dimensional lattices. It is found that the model exhibits the spin nematic phase with ferroquadrupolar order in a wide parameter region, in addition to conventional magnetically-ordered phases. In particular, it is shown that even when the four-spin interaction is absent, frustrated two-spin exchange interactions can realize the spin nematic phase as a result of strong interdimer correlations. It is also found that the phase transitions between the spin nematic phase and antiferromagnetic phases can be continuous. Furthermore, we present some exact arguments that various phases including the spin nematic phase and the vector chiral (p-type nematic) phase emerge from an SU(4) symmetric point in the model by the addition of appropriate perturbative interactions. The spin nematic phase generated from the SU(4) point is connected with the spin nematic phase numerically found in the system with only two-spin interactions.arXiv:1907.08373v1 [cond-mat.str-el] 19 Jul 2019 and nontrivial degeneracies in the mean-field solutions are discussed in Appendices A, B, and C, respectively. J 4 J d J J 2. Transition between spin-nematic and A-type AFM phases

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Designing Quantum Spin-Orbital Liquids in Artificial Mott Insulators

Cited by 7 publications

References 49 publications

Ferromagnetic Mott state in Twisted Graphene Bilayers at the Magic Angle

Ferromagnetic Mott state in Twisted Graphene Bilayers at the Magic Angle

SU(4) Heisenberg model on the honeycomb lattice with exchange-frustrated perturbations: Implications for twistronics and Mott insulators

Spin nematics in frustrated spin-dimer systems with bilayer structure

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