Phys. Rev. Appl. 4, 014002 (2015)

The electronic band structure of certain semiconductors can contain conically shaped valleys. When there are two non-equivalent valleys, such as in graphene, the system has an additional quantum number, known as the valley index. In much the same way that the electron spin is used in spintronic devices, valleytronic devices aim to make use of the valley index.

Many of the materials that offer this valley degree of freedom are single layers, but how do these valley electronic states move between layers? Ryosuke Akashi and colleagues have now shown that the stacking geometry can control how many dimensions the valley states travel in.

Using monolayers of a typical transition metal dichalcogenide, the authors showed that the motion of valley electronic states was confined to just one of the two-dimensional monolayers for a particular type of stacking, known as 3R stacking. For 2H stacking, on the other hand, the valley states could jump between different monolayers, and travel in three dimensions.