Journal
PHYSICAL REVIEW X
Volume 10, Issue 3, Pages -Publisher
AMER PHYSICAL SOC
DOI: 10.1103/PhysRevX.10.031034
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Funding
- Simons Collaboration on Ultra-Quantum Matter - Simons Foundation [651440]
- Simons Investigator grant
- German National Academy of Sciences Leopoldina through the LPDS Leopoldina Fellowship [2018-02]
- ERC Synergy Grant UQUAM
- DOE, Office of Basic Energy Sciences [DE-AC02-05-CH11231]
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In magic angle twisted bilayer graphene (TBG), electron-electron interactions play a central role, resulting in correlated insulating states at certain integer fillings. Identifying the nature of these insulators is a central question, and it is potentially linked to the relatively high-temperature superconductivity observed in the same devices. Here, we address this question using a combination of analytical strong-coupling arguments and a comprehensive Hartree-Fock numerical calculation, which includes the effect of remote bands. The ground state we obtain at charge neutrality is an unusual ordered state, which we call the Kramers intervalley-coherent (K-IVC) insulator. In its simplest form, the K-IVC order exhibits a pattern of alternating circulating currents that triples the graphene unit cell, leading to an orbital magnetization density wave. Although translation and time-reversal symmetry are broken, a combined Kramers time-reversal symmetry is preserved. Our analytic arguments are built on first identifying an approximate U(4) x U(4) symmetry, resulting from the remarkable properties of the TBG band structure, which helps select a low-energy manifold of states that are further split to favor the K-IVC state. This low-energy manifold is also found in the Hartree-Fock numerical calculation. We show that symmetry-lowering perturbations can stabilize other insulators and the semimetallic state, and we discuss the ground state at half-filling and give a comparison with experiments.
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