Damien Leech: Valley-Polarised Tunnelling Currents in a Vertical Heterostructure of Monolayer and Bilayer Graphene

J. J. P. Thompson, D. J. Leech, M. Mucha-Kruczynski Department of Physics, University of Bath, Claverton Down, Bath BA2 7AY, United Kingdom Van der Waals heterostructures of two-dimensional atomic crystals opened up the possibility of exploring resonant quantum tunnelling in the limit of not only atomically thin barriers [1], but also with conservation of in-plane momentum [2]. In previous works, the source and drain were effectively monolayer thin so that all electrons tunnelled through an identical barrier. Here, using an example of a monolayer graphene/hexagonal boron nitride/bilayer graphene vertical heterostructure, we discuss theoretically effects arising in a device with a multilayer source electrode, so that electrons tunnel through a barrier with an effective width depending on their distance from the drain. In monolayer graphene, the wavefunction is a superposition of Bloch states on two sublattices lying on the same plane, in bilayer graphene electrons live on four sites distributed across two layers. In the presence of an external perpendicular magnetic field, electronic states in each of the two otherwise equivalent Brillouin zone corners occupy one of the layers preferentially. We show that such an arrangement results in valley-polarised tunnelling currents due to the difference in distances to arrive at the monolayer graphene electrode. We predict valley polarisations of the order of 80% in magnetic fields B ~1T and because of the chiral nature of the electronic states in graphene, this valley-polarisation can be tuned between the two valleys via the manipulation of applied voltages only. [1] L. Britnell et al., Nature Communications, 4, 1794 (2013). [2] A. Mishchenko et al., Nature Nanotechnology, 9, 808 (2014). Damien Leech is currently a PhD student in Physics at the University of Bath, where he also completed his undergraduate degree. He is primarily interested in the electronic structure of vertically-stacked heterostructures of 2D atomic crystals.