valleytronics

Valleytronics is a prospective field of electronics that seeks to use the valley degree of freedom of electrons as a carrier of information. In crystalline solids with multiple inequivalent energy extrema—valleys—in the electronic band structure, the valley index can serve as an additional binary degree of freedom, analogous to spin or charge.

Valleys occur at distinct points in the Brillouin zone, such as the K and K' points in many two-dimensional materials. In certain materials, notably monolayer transition metal dichalcogenides, inversion symmetry is broken and strong spin–orbit coupling links valley index to spin, giving rise to spin–valley locking. This enables valley-selective excitation and detection using light of definite circular polarization, as well as Berry-curvature–driven transport phenomena such as the valley Hall effect.

Common material platforms include 2D TMDCs like MoS2, WS2, WSe2 and related compounds, where optical selection rules allow valley polarization and PL carries valley information. Valleytronic devices conceptually include valley filters, valves, and valley-based transistors, as well as optoelectronic components that exploit selective emission or detection from a single valley. Electrical methods aim to generate and sense valley currents via nonlocal transport and Hall-type effects.

Major challenges include maintaining valley coherence at room temperature, suppressing intervalley scattering, achieving efficient and scalable generation and readout of valley polarization, and integrating valleytronic elements with conventional CMOS technology. Progress is incremental, with proof-of-principle demonstrations of valley-selective devices and hybrid spin–valley schemes.

Outlook remains focused on exploring valley phenomena and developing hybrid spin–valley devices, with potential applications in low-power information processing and novel optoelectronic components.