  Zigzag phosphorene nanoribbons are metallic owing to the edge states, whose
energies are inside the gap and far from the bulk bands. We show that --
through electrical manipulation of edge states -- electron propagation can be
restricted to one of the ribbon edges or, in case of bilayer phosphorene
nanoribbons, to one of the layers. This finding implies that edge and layer can
be regarded as tunable equivalents of the spin-one-half degree of freedom,
i.e., the pseudospin. In both layer- and edge-pseudospin schemes, we propose
and characterize a pseudospin field-effect transistor, which can generate
pseudospin-polarized current. Also, we propose edge- and layer-pseudospin
valves that operate analogously to conventional spin valves. The performance of
valves in each pseudospin scheme is benchmarked by the pseudomagnetoresistance
(PMR) ratio. The edge-pseudospin valve shows a nearly perfect PMR, with
remarkable robustness against device parameters and disorder. These results may
initiate new developments in pseudospin electronics.
