Nonequilibrium valley polarization in graphene nanoconstrictions

We recently showed, using tight-binding calculations, that nonequilibrium valley polarization can be realized in graphene, when the current is injected through a valley filter: a ballistic point contact with zigzag edges. Here we demonstrate that the effect is surprisingly robust against changing the crystallographic orientation of the filter axis. Namely, the output current remains polarized unless a point contact has perfect armchair edges, at which two sublattices are equally represented. The polarization is inverted when the filter orientation crosses the armchair line and, subsequently, the dominating sublattice index of the terminal atoms changes. In a bent graphene strip, the valley-polarized current can be directed towards one edge providing a theoretical possibility to observe a zero-magnetic-field analogue of the well-known, Hall effect. For the valley valve, built of two valley filters in series and controlled elecrostatically by a gate voltage, the conductance-to-gate characteristic is inverted when a pi/3 vertex is placed between two filters. (C) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.