Scientists push valleytronics one step closer to reality

Scientists push valleytronics 1 step closer to reality
This schematic shows a TMDC monolayer coupled with a host ferromagnetic semiconductor, which is an experimental approach developed by Berkeley Lab scientists that could lead to valleytronic devices. Valley polarization can be directly determined from the helicity of the emitted electroluminescence, shown by the orange arrow, as a result of electrically injected spin-polarized holes to the TMDC monolayer, shown by the blue arrow. The black arrow represents the direction of the applied magnetic field. Credit: Berkeley Lab

Scientists with the U.S. Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) have taken a big step toward the practical application of "valleytronics," which is a new type of electronics that could lead to faster and more efficient computer logic systems and data storage chips in next-generation devices.

As reported online April 4 in the journal Nature Nanotechnology, the scientists experimentally demonstrated, for the first time, the ability to electrically generate and control valley electrons in a two-dimensional semiconductor.

Valley electrons are so named because they carry a valley "degree of freedom." This is a new way to harness electrons for information processing that's in addition to utilizing an electron's other degrees of freedom, which are quantum spin in spintronic devices and charge in conventional electronics.

More specifically, electronic valleys refer to the energy peaks and valleys in electronic bands. A two-dimensional semiconductor called transition metal dichalcogenide (TMDC) has two distinguishable valleys of opposite spin and momentum. Because of this, the material is suitable for valleytronic devices, in which information processing and storage could be carried out by selectively populating one valley or another.

However, developing valleytronic devices requires the electrical control over the population of valley electrons, a step that has proven very challenging to achieve so far.

Now, Berkeley Lab scientists have experimentally demonstrated the ability to electrically generate and control valley electrons in TMDCs. This is an especially important advance because TMDCs are considered to be more "device ready" than other semiconductors that exhibit valleytronic properties.

"This is the first demonstration of electrical excitation and control of valley electrons, which will accelerate the next generation of electronics and information technology," says Xiang Zhang, who led this study and who is the director of Berkeley Lab's Materials Sciences Division.

Zhang also holds the Ernest S. Kuh Endowed Chair at the University of California (UC) Berkeley and is a member of the Kavli Energy NanoSciences Institute at Berkeley. Several other scientists contributed to this work, including Yu Ye, Jun Xiao, Hailong Wang, Ziliang Ye, Hanyu Zhu, Mervin Zhao, Yuan Wang, Jianhua Zhao and Xiaobo Yin.

Their research could lead to a new type of electronics that utilizes all three degrees of freedom—charge, spin, and valley, which together could encode an electron with eight bits of information instead of two in today's electronics. This means future computer chips could process more information with less power, enabling faster and more energy efficient computing technologies.

"Valleytronic devices have the potential to transform high-speed data communications and low-power devices," says Ye, a postdoctoral researcher in Zhang's group and the lead author of the paper.

The scientists demonstrated their approach by coupling a host ferromagnetic semiconductor with a monolayer of TMDC. Electrical spin injection from the ferromagnetic semiconductor localized the to one momentum valley in the TMDC monolayer.

Importantly, the scientists were able to 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.

The two sets of valleys emit different . The scientists observed this circularly polarized light, which confirmed they had successfully electrically induced and controlled valley electrons in TMDC.

"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," says 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."


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More information: Electrical generation and control of the valley carriers in a monolayer transition metal dichalcogenide, Nature Nanotechnology, DOI: 10.1038/nnano.2016.49
Journal information: Nature Nanotechnology

Citation: Scientists push valleytronics one step closer to reality (2016, April 4) retrieved 15 July 2019 from https://phys.org/news/2016-04-scientists-valleytronics-closer-reality.html
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Apr 04, 2016
Their research could lead to a new type of electronics that utilizes all three degrees of freedom—charge, spin, and valley, which together could encode an electron with eight bits of information instead of two in today's electronics.


Eight different values (3 bits) vs. two values (1 bit) today. But this misunderstands completely what is going on. The goal is to use valleytronics to store qubits in quantum computers. One qubit can be in an almost infinite set of states. You can only read one bit per qubit, but qubits can interact. A quantum computer is a device for interacting the qubit states so that the final state is probably the right answer. How does that help? The problems to be solved by quantum computers are mostly NP problems, where if you know the right answer it is easy to prove it. If you don't have the right answer, run the quantum computer again.


Apr 04, 2016
The next step is to be able to switch the electrons back and forth between valleys.

Apr 05, 2016
When we talk about electronic charge 1 bit then we talk about a group of electrons. (a lot)
Do we also talk about a group of electrons with all the same state for this 3 bits, or just one?

You would think it's probably easier to decrease the number of electrons needed to store 1 bit than to add more than 2 statuses to the electrons?

Apr 05, 2016
There's a context switch in the article from "some electrons" to "an electron" that's kind of misleading, I think. This technology is a long way from being able to do quantum computing.

@Kedas:
When we talk about electronic charge 1 bit then we talk about a group of electrons. (a lot)
Do we also talk about a group of electrons with all the same state for this 3 bits, or just one?
I think that the fact that the valleys are of opposite spin creates the linkage between valleytronics and spintronics, and I think that being able to selectively populate one valley or the other creates the linkage between all three. So I think we're talking about having all three bit states in a single population of electrons. But see above, I think it's a long way from there to quantum computing, although this technology does definitely allow 8 bits worth of data in one population of electrons, rather than three, so it's definitely a power gain,

[contd]

Apr 05, 2016
[contd]
and given that power dissipation (heat) is the biggest problem in current semiconductor design, this could let us shrink things by 3x (or put 3x more on a single chip). So it looks like a win if they can actually make it all work together.

You would think it's probably easier to decrease the number of electrons needed to store 1 bit than to add more than 2 statuses to the electrons?
Not if what I've said above is correct.

I'm going to have to pull up the paper (if I can get it) and have a look to see exactly what's involved here; not something I'm going to do right away, but I'll put it on my round-tuit list.

Good questions!

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