Time: 10:00am (UTC/GMT+8:00, Beijing/Shanghai), May 14 (Fri.), 2021

Venue: Rm. 830, M Building, Institute of Physics, CAS

Speaker: Jian Kang (Soochow Univ.)

Abstract:

The Bistritzer-MacDonald (BM) model predicted the existence of the narrow bands in the twisted bilayer graphene (TBG), and nowadays is a starting point for most theoretical works.  In this talk, this model with Coulomb interactions will be reviewed from a rather different perspective. We treat this model as an effective field theory defined at atomic energy scale and find that the theory flows toward a fixed point in which the bands are exactly flat [1]. Additionally, our theory explains the consistency of the magic angle between the theoretical value and the experimentally discovered one. With the Hamiltonian obtained, we further derived the exact excitation spectrum in the strong coupling limit at the even integer fillings [1]. Combined with the variational method, we studied the Landau fan, the electron mass measured by quantum oscillations, and the recently discovered cascade transitions near the integer fillings [2]. Our results not only qualitatively agree with the measurements, but also reveal the emergence of the Fermi liquid at the fractional fillings.

1.    Oskar Vafek and Jian Kang, Phys. Rev. Lett, 122, 257602 (2020).

2.    Jian Kang, B. Andrei Bernevig, and Oskar Vafek, arXiv:2104.01145.

About the Speaker:

Jian Kang is currently a professor at Soochow university since 2019. He obtained his Ph. D. at Johns Hopkins University in 2013, under the advice of Prof. Tesanovic. In 2013 – 2017, he was a postdoc in Prof. Fernandes's group at the University of Minnesota. In 2017-2019, he moved to the National high magnetic field laboratory in Florida, working with Prof. Vafek on the superconducting and correlated insulating phases discovered in twisted bilayer graphene. His main research field lies in theories of correlated electron systems and their collective behaviors.

Invited by Dr. Yi Zhang