Title: Non-adiabaticity meets non-Abelian: multiple-band effects in Hall conduction 

Time: 15:10-16:10, Monday, 2021/09/13

The link for the online seminar : 
Google Meet joining info
https://meet.google.com/htj-dwhd-zpz

Abstract:
The rich and diversified studies on the general phenomena of Hall conductions have largely focused on the linear and the first-few order response coefficients over the past few decades. These response coefficients are readout using infinitesimal external fields to probe the intrinsic properties, such as Berry curvature and its higher-order derivatives, of the materials in question. 

If one wishes to steer the transport dynamics via an external electric field for device applications, other than infinitesimal, a finite external field is to be applied. This will have interesting consequences for materials, in which there are more than one band near the fermi energy,  e.g. in bilayer graphene and twisted MoSe2 homobilayers, etc. On one hand, the non-perturbation nature of the external electric field can induce strong coherent coupling among the bands near the fermi energy, giving rise to non-adiabatic dynamics. On the other hand, non-Abelian gauge structure can also emerge as a multiple-band effect. The interplay of these two facets for the Hall conduction with multiple-band effects is an arena yet to be explored. 

In this talk, I will show our initial attempt to resolve the above open issue from the two interwoven faces of multiple band effects in generic quantum materials. An example taken from the well-known AB-stacked bilayer graphene unambiguously illustrates the unique role of finite-electric-field-induced non-adiabatic dynamics in revealing the non-abelian characters of Berry curvatures. The contributions from the diagonal and the off-diagonal elements of non-Abelian Berry curvature to the valley Hall currents are shown to be qualititively distinct. This provides us with a transport-based and non-geometric view toward the long-studied non-Abelian properties due to multiple-band effects.

References:
C. Li*, M. W.-Y. Tu* and W. Yao, 2D Mater. 8, 045012 (2021).
M. W.-Y. Tu,* C. Li, H. Yu and W. Yao, Phys. Rev. B 102, 045423 (2020).
M. W.-Y. Tu,* C. Li, H. Yu and W. Yao*, 2D Mater. 7, 045004 (2020).