Berry phase and pseudospin winding number in bilayer graphene

Article is made available in accordance with the publisher's policy and may be subject to US copyright law. Please refer to the publisher's site for terms of use. The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. Ever since the novel quantum Hall effect in bilayer graphene was discovered, and explained by a Berry phase of 2π [K. S. Novoselov et al., Nat. Phys. 2, 177 (2006)], it has been widely accepted that the low-energy electronic wave function in this system is described by a nontrivial Berry phase of 2π , different from the zero phase of a conventional two-dimensional electron gas. Here, we show that (i) the relevant Berry phase for bilayer graphene is not different from that for a conventional two-dimensional electron gas (as expected, given that Berry phase is only meaningful modulo 2π), and (ii) what is actually observed in the quantum Hall measurements is not the absolute value of the Berry phase but the pseudospin winding number.