Switchable valley functionalities of an n   −   n ⁻   −   n junction in 2D crystals

Abstract

We show that an n − n − − n junction in 2D semiconductors can flexibly realize two basic valleytronic functions, i.e. valley filter and valley source, with gate controlled switchability between the two. Upon carrier flux passing through the junction, the valley filter and valley source functions are enabled respectively by intra-and inter-valley scatterings, and the two functions dominate respectively at small and large band-offset between the n and n − regions. It can be generally shown that, the valley filter effect has an angular dependent polarity and vanishes under angular integration, by the same constraint from time-reversal symmetry that leads to its absence in one-dimension. These findings are demonstrated for monolayer transition metal dichalcogenides and graphene using tight-binding calculations. We further show that junction along chiral directions can concentrate the valley pump in an angular interval largely separated from the bias direction, allowing efficient havest of valley polarization in a cross-bar device.

Valley filtering and valley-polarized collective modes in bulk graphene monolayers

“…The angle-averaged transmission can have a net valley polarization exceeding 75% for a 20period barrier. For an n-n − -n junction based on 2D graphene, valley filter and valley source functions were demonstrated theoretically [40] at small and large band-offset between the n and n − -regions, which are enabled respectively by intra-and inter-valley scatterings and depend on the junction orientation.…”

Valley-Selective Klein Tunneling through a Superlattice Barrier in Graphene

“…On the other hand, a counterintuitive role of intervalley scattering in pumping valley polarization has been revealed in recent theoretical studies. 29,30 Here we discover that, at a superlattice barrier in single-layer graphene, intervalley scattering can selectively block the Klein tunneling in a chosen valley while retaining the perfect transmission in the other. This valley selectivity is made possible by the staggered pseudospin gaps created by intervalley backscattering in the superlattice barrier.…”

“…A variety of schemes have been explored for producing valley current in graphene by introducing edges, 13 inversion symmetry breaking, [14][15][16][17][18] line defects and topological interfaces, [19][20][21][22][23][24] or exploiting strains 25,26 and trigonal warping. 27 In the context of Klein tunneling, possibilities to engineer valley polarized transmission at selected oblique angles are discovered, 9,28 but the angle-averaged transmission cannot carry valley polarization as a consequence of time-reversal symmetry, 29 limiting such schemes to ballistic devices and angle resolved operations. All the above mechanisms exploit intravalley process only, whereas intervalley scattering is generally considered as a deleterious cause of error for the valley functionalities.…”

Valley Polarization and Valleyresistance in a Monolayer Transition Metal Dichalcogenide Superlattice

“…In fact, the valley-polarized current can be pumped simply by the ubiquitous nonmagnetic disorders, not relying on any specific material property, because the intervalley backscattering with the distinct momentum transfers cause a net transfer of population from one valley to another [31]. Based on the intra-and inter-valley scatterings, the switchable valley filter and valley source functions may be realized from the lateral junctions in graphene and transition metal dichalcogenides [32]. However, the exploration towards practical valley polarization for potential valleytronic devices remains an ongoing challenge.…”