Into the valley

We are another step closer to "valleytronics", which could lead to faster and more efficient computers and data storage, according to Berkeley Lab and University of California Berkeley researchers who have found a new way to encode electrons in a two-dimensional semiconductor. [Y. Ye et al., Nature Nanotechnol. (2016) DOI :10.1038/nnano.2016.49]

Electrons are contained in one set of two distinct valleys, giving them a new "degree of freedom". That is an alternative way to harness the properties of the electron aside from charge in conventional electronics and quantum spin in spintronics. The landscapes of the electronic valleys are the energy peaks and troughs of the electronic bands. In a two-dimensional semiconductor such as a transition metal dichalcogenide (TMDC), there are two distinct valleys (K and K') of opposite spin and momentum. As ever, if there are two options, they can represent the 1s and 0s of binary code if they can be controlled in such a way as to represent the reading and writing of bits. Electrically navigating such valleys has been difficult so far.

Now, the US scientists have demonstrated experimentally how to electrically generate and control valley electrons in TMDCs, a particularly important step for valleytronics as TMDCs will mesh neatly into device fabrication technology in a way that other experimental materials and semiconductors might not. In their demonstration, the team coupled a host ferromagnetic semiconductor with a TMDC monolayer. Electrical spin injection from the ferromagnetic semiconductor localized the charge carriers to one momentum valley in the TMDC monolayer.

"This is the first demonstration of electrical excitation and control of valley electrons," says Xiang Zhang who worked with Yu Ye, Jun Xiao, Hailong Wang, Ziliang Ye, Hanyu Zhu, Mervin Zhao, Yuan Wang, Jianhua Zhao and Xiaobo Yin. It might now be possible to exploit all three degrees of freedom of the electron - charge, spin, and valley - in a single system, meaning that an electron could actually be used to encode three bits of information rather than one. Indeed, the proof of principle was in the fact that the team could electrically excite and confine the charge carriers in only one of two sets of valleys. This was achieved by manipulating the injected carrier's spin polarizations, in which the spin and valley are locked together in the TMDC monolayer. Reading the system involved the use of circularly polarized electroluminescence.

"Our research solved two main challenges in valleytronic devices. The first is electrically restricting electrons to one momentum valley. The second is detecting the resulting valley-polarized current by circular polarized electroluminescence," explains Ye. "Our direct electrical generation and control of valley charge carriers, in TMDC, opens up new dimensions in utilizing both the spin and valley degrees of freedom for next-generation electronics and computing."

"Next, we will study the valley scattering mechanisms involved in the processes and improve the electrically valley excitation efficiency," Ye told Materials Today.

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