Time: 14:00pm, Dec. 19 (Wed.), 2018,
Venue: Room S401, Kavli ITS, UCAS Teaching Building [View Map]
Speaker: Dr. Jian Kang
National High Magnetic Field Laboratory, Tallahassee, FL, USA
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.
About the Speaker
Jian Kang, a postdoc at National High Magnetic Field Laboratory, Florida. He obtained Ph.D. at Johns Hopkins University in 2013, under the advice of Prof. Zlatko Tesanovic. In 2013 – 2017, he was a postdoc in Prof. Rafael Fernandes's group at the University of Minnesota. During that period, he was focusing on the interplay between electronic nematicity and magnetism, superconductivity and disorder in iron-based superconductors. Currently, he is working on the unconventional superconducting and correlated insulating phases recently discovered in the twisted bilayer graphene. His main research field lies in theories of correlated electron systems and their collective behavior.