Valley Current Multiplexer in Bilayer Graphene: A Direct Measurement of Anomalous Velocity

Description

The issue of power (heat) dissipation is a major bottleneck in the design and improvement of modern digital electronics. Several new technological directions towards low dissipation and dissipation-less computation are being explored. One such direction is to move away from charge based computing (Electronics) to using other material degrees of freedom: most famously spin (Spintronics). However, recently with the investigation into graphene and graphene-like systems opened up the possibility of utilizing another degree of freedom: the valley degree of freedom (Valleytronics). Valley is a quantum number defined in an electronic system where the energy dispersion contains energetically degenerate but non-equivalent local minima due to a certain crystal structure. Here specifically, two-dimensional honeycomb lattice systems such as graphene. Breaking spatial inversion symmetry of these systems introduces a Berry curvature and enables electrical control of the valley degree of freedom. In a dual-gated bilayer graphene device inversion symmetry can be broken in a controllable manner by an application of a perpendicular electric field via the top and bottom gates. Berry curvature acts as an out of plane pseudo-magnetic field in momentum space and has opposite sign for the K and K’ valleys of graphene. Previously, in such a device, pure valley current has been demonstrated in the insulating regime. Valley Current is a dissipationless current. These results were an initial step in a new potential field known as Valleytronics. Here, we propose a valley current-based multiplexer as a follow-up device. Unlike the case of real magnetic field, where charged particles are expected to move in circular trajectories, the effect of Berry curvature is to add a transverse element to the particle velocity known as anomalous velocity. This specific effect is yet to be experimentally observed. Therefore in a ballistic device with a single injection terminal and several collection terminals located at different angles, changing the Berry curvature (and parallel Electric field) would enable one to select which output terminal will receive the flow of current, giving one access to anomalous velocity. Manipulation of the Valley degree of freedom is still in it’s infant stages such a device would be a first step towards a Valleytronics based framework, with the ultimate goal being a demonstration of a valley transistor.