One of several low points in a material’s energy landscape where electrons settle, each belonging to a different direction of motion. In some electrons in different valleys can be told apart, so the valley could carry information, much as or charge does.
Going deeper
The corners of a honeycomb Brillouin zone fall into two inequivalent sets, K and K′. In a monolayer TMDC these valleys respond to opposite circular polarisations, so light can address one of them and the valley index becomes a usable binary label.
What a valley is
A valley is a local minimum of the or maximum of the valence band at a particular momentum. Silicon has six equivalent conduction-band valleys; what makes materials different is that theirs come in two sets, at the K and K′ corners of the Brillouin zone, that are related by time reversal but not by any lattice translation or rotation of the crystal.
Because of that, an electron carries a valley index alongside its charge and spin. In graphene the two valleys are degenerate and hard to address separately. In a , where is broken, they acquire opposite and opposite orbital magnetic moments, which makes them optically distinguishable.
Addressing a valley with light
Angular momentum conservation ties the of circularly polarised light to the valley: right-handed light drives transitions at K, left-handed light at K′. Shining one handedness on monolayer MoS2 therefore creates carriers mainly in one valley, and the emitted light keeps the same handedness – the observation, reported by two groups in 2012, that opened valleytronics.
The polarisation is not complete and falls with temperature, because scattering mixes the valleys. In tungsten-based monolayers, where the spin ordering of the valence band differs, valley polarisation survives better. Spin and valley are locked together by , so flipping a valley also requires flipping a spin, which is what makes these states comparatively long-lived.
What could be done with it
If the valley index can be written, moved and read, it could carry information like charge or spin. Writing is done optically or by injecting carriers from a magnetic layer; reading uses the valley Hall effect, where opposite Berry curvature pushes the two valleys to opposite edges, or the circular polarisation of emitted light. Magnetic fields and proximity to a magnet split the valleys in energy, giving a valley Zeeman effect that can bias one over the other.
The obstacles are lifetime and temperature. Valley polarisation of free decays within picoseconds at room temperature, though localised excitons and resident carriers live far longer. As with , the demonstrations so far are physics experiments rather than devices, and most work at low temperature.
For specialists
A local band extremum at a distinct momentum (K and K′ in graphene and TMDCs); the valley index can act as a binary degree of freedom.