Wed, 05/08/2019 - 11:00am to 12:00pm
DeM 251
Material Science Seminar
Dr. Jian Kang (Florida State University/High Magnetic Field Laboratory)

Nonlocal interactions beyond Hubbard model in twisted bilayer graphene near magic angle

Abstract: Recent experiments on the twisted bilayer graphene have discovered the
correlated insulator phase and the neighboring superconductivity with
certain fillings of the charge carriers. To understand the correlation
effects, we build the localized Wannier states (WSs) for the four
narrow bands around the charge neutrality point and construct the
corresponding low energy tight binding model. Furthermore, we project
the gate-screened Coulomb interaction onto the constructed localized
WSs. Due to the nontrivial topological properties of the four narrow
bands, the projected interaction becomes nonlocal and contains new
terms beyond the cluster Hubbard model. These new terms lead to strong
correlations between different lattice sites even without the
hoppings. At the one-quarter filling, the largely degenerate ground
states are found to be SU(4) ferromagnetic in the strong coupling
limit. The kinetic terms select the ground state in which the two
valleys with opposite spins are equally mixed, with vanishing magnetic
moment per particle. Our results suggest the fundamental difference
between the twisted bilayer graphene and other unconventional
superconductors described by the Hubbard model.

1.    Jian Kang and Oskar Vafek, Phys. Rev. X, 8, 031088 (2018).
2.    Jian Kang and Oskar Vafek, arXiv:1810.08642. PRL accepted