Into the quantum valley

If information can be encoded in the wavelike motion of electrons in two-dimensional semiconductors then a new approach to quantum computing - so-called "valleytronics" - might be possible. Feng Wang and colleagues at Lawrence Berkeley National Laboratory are mapping out their route and have demonstrated that the optical Stark effect can be used to selectively control photoexcited electrons/hole pairs, excitons, in different energy valleys. Wang et al., (2015) Science, 346, 1205-1208; DOI: 10.1126/science.1258122]

In spintronics, information is encoded in quantum spin number, but in valleytronics the two energy valleys associated with electron waves - momentum and quantum valley number can be used to encode information.

"This is the first demonstration of the important role the optical Stark effect can play in valleytronics," Feng explains. The Stark effect is a split and shift of spectral lines due to an external electric field. In the optical equivalent, it describes the energy shift in a two-level system induced by a non-resonant laser field. "Our technique, which is based on the use of circularly polarized femtosecond light pulses to selectively control the valley degree of freedom, opens up the possibility of ultrafast manipulation of valley excitons for quantum information applications," Wang explains.

Wang's team has focused on 2D semiconductors with the MX2 format. A transition metal monolayer of, for instance, molybdenum (Mo) or tungsten (W), sandwiched between two layers of a chalcogen, such as sulfur (S) or selenium (Se). These atomically thin 2D materials have the same hexagonal honeycombed lattice as graphite, or indeed graphene. Unlike graphene, MX2 materials have a natural band gap that would make them useful in transistors and other electronic devices.

The team has previously reported experimental observations with photo-excited MX2 materials displaying an ultrafast charge transfer of less than 50 femtoseconds. This transfer rate is on a par with that observed in graphene. Wang explains their current research: "Using ultrafast pump-probe spectroscopy, we were able to observe a pure and valley-selective optical Stark effect in WSe2 monolayers from the non-resonant pump that resulted in an energy splitting of more than 10 milli-electron volts between the K and K' valley exciton transitions. As controlling valley excitons with a real magnetic field is difficult to achieve even with superconducting magnets, a light-induced pseudo-magnetic field is highly desirable."

Valleytronics could offer advantages in data processing speeds over conventional electronics. It also precludes the kind of interference from magnetic fields that could be a problem for spintronics. "Coherent manipulation of valley polarization should open up fascinating opportunities for valleytronics," Wang adds.

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the popular science book "Deceived Wisdom".