Time: 09:30 am (UTC/GMT+08:00, Beijing/Shanghai), Jul. 2 (Thur.), 2020

Online Meeting Room (zoom.com):  Click here to join the meeting

Meeting ID: 980 2903 1727

Speaker: Prof. Kam Tuen Law (HKUST)

Abstract:

Recently, quantum anomalous Hall effect with spontaneous ferromagnetism was observed in twisted bilayer graphenes (TBG) near 3/4 filling by Goldhaber-Gordon’s [1] and Young’s [2] groups respectively. Importantly, it was observed that an extremely small current can switch the direction of the magnetization. This offers the prospect of realizing low energy dissipation magnetic memories.  However, the mechanism of the current-driven magnetization switching is poorly understood as the charge currents in graphene are generally believed to be non-magnetic. In this talk, we explain the observed current induced magnetization effect [3].

In particular, we demonstrate that in TBG, the twisting and substrate induced symmetry breaking allow an out of plane orbital magnetization to be generated by a charge current through the magneto-electric effect. Moreover, the large Berry curvatures of the flat bands give the Bloch electrons large orbital magnetic moments so that a small current can generate a large orbital magnetization. We further demonstrate how the charge current can switch the magnetization of the ferromagnetic TBG near 3/4 filling as observed in the experiments. We also predict that how similar current induced magnetization can appear in several related systems.

1. A. Sharpe etc.  Science 365, 605 (2019).
2. M. Serlin etc. Science 367, 895 (2020).
3. Wen-Yu He, David Goldhaber-Gordon, K. T. Law, Nature Communications 11, 1650 (2020).