From UC Riverside: “New research finding gives valleytronics a boost”

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From UC Riverside

October 28, 2019
Iqbal Pittalwala

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A dark exciton (light blue) in monolayer WSe2 is found to decay into light (red) and atomic vibrations (blue) with opposite circular polarization. Credit: Erfu Liu, UC Riverside.

An international research team led by physicists at the University of California, Riverside, has revealed a new quantum process in valleytronics that can speed up the development of this fairly new technology.

Valleytronics, a portmanteau of “valley” and “electronics,” uses local energy minima — or valleys — in the electronic band structure of semiconductors. Current semiconductor technology uses electronic charge or spin to store and process information. In some semiconductors, however, valleys of the electrons are used to encode, process, and store information. Valleytronic systems have the potential to offer information processing schemes that are superior to charge- and spin-based semiconductor technologies.

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Joshua Lui (left), Erfu Liu (center), and Jeremiah van Baren. (UCR/Stan Lim)

The UC Riverside-led research team focused on monolayer tungsten diselenide (WSe2), a two-dimensional semiconductor with two distinct electronic valleys. Excited electrons tend to relax and accumulate in one of the valleys to acquire a valley index (K or K’). The valley indices can be used to represent 1 and 0 to encode information — just as electric charge is used in current technology.

Excitons and trions can also occupy the valleys in monolayer WSe2 and be used to transmit valley information. An exciton is a quantum bound state of an electron and an electron hole. A trion is a quantum bound state of three charged particles. Monolayer WSe2 hosts bright and dark excitons or trions with different spin configurations; bright decay rapidly into light, while dark decay slowly into light.

“Development of valleytronics requires stable valley states and easy identification of the valley indices,” said Chun Hung “Joshua” Lui, an assistant professor in the Department of Physics and Astronomy at UC Riverside, who led the research [Physical Review Research]. “Dark excitons and trions in monolayer WSe2 have much longer lifetime and better valley stability than the common bright excitons and trions. The dark excitons and trions, therefore, serve as excellent candidates for valleytronic applications.”

Lui explained that until now no method could read the valley indices of the dark excitons and trions because their light emission from either valley has exactly the same energy and polarization, making the two valleys indistinguishable from each other. Lui’s research team has now overcome this obstacle by identifying a measurable physical quantity that can distinguish the two valley indices of dark excitons and trions.

“We observed a new decay process of dark excitons and trions in monolayer WSe2, which allows us to identify their valley indices,” Lui said. “A dark exciton or trion can decay into a pair of photon and phonon with a distinctive valley signature.”

A photon is a quantum of an electromagnetic wave. It can have linear or chiral polarization when the electromagnetic field oscillates or rotates. The rotational direction of the electromagnetic field determines whether a chiral photon is right-handed or left-handed. Similarly, a phonon is a quantum of atomic vibration in the material. Atomic vibration usually involves linear oscillation of atoms. But in some special cases, the atoms can rotate to produce the so-called chiral phonons. The atomic rotation direction determines whether a chiral phonon is right-handed or left-handed.

“We found that the dark exciton in the K valley decays into a right-handed photon and a left-handed phonon, whereas the dark exciton in the opposite K’ valley decays into a left-handed photon and a right-handed phonon,” Lui said. “The handedness of the emitted photon is a clear signature of the valley indices of the dark excitons and trions.”

Lui added that the ability to read the dark-state valleys could facilitate the exploration of dark-state valley dynamics and applications in valleytronic technology.

Lui was joined in the study by Erfu Liu, a postdoctoral researcher in Lui’s lab and the first author of the research paper, and graduate student Jeremiah van Baren of UC Riverside; Takeshi Taniguchi and Kenji Watanabe of the National Institute for Materials Science, Japan; and Yia-Chung Chang of the Research Center for Applied Sciences, Academia Sinica, Taiwan.

See the full article here .

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The University of California, Riverside is one of 10 universities within the prestigious University of California system, and the only UC located in Inland Southern California.

Widely recognized as one of the most ethnically diverse research universities in the nation, UCR’s current enrollment is more than 21,000 students, with a goal of 25,000 students by 2020. The campus is in the midst of a tremendous growth spurt with new and remodeled facilities coming on-line on a regular basis.

We are located approximately 50 miles east of downtown Los Angeles. UCR is also within easy driving distance of dozens of major cultural and recreational sites, as well as desert, mountain and coastal destinations.

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From LBNL: “Valleytronics Discovery Could Extend Limits of Moore’s Law”

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Berkeley Lab

April 13, 2018
John German
jdgerman@lbl.gov
(510) 486-6601

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Valleytronics utilizes different local energy extrema (valleys) with selection rules to store 0s and 1s. In SnS, these extrema have different shapes and responses to different polarizations of light, allowing the 0s and 1s to be directly recognized. This schematic illustrates the variation of electron energy in different states, represented by curved surfaces in space. The two valleys of the curved surface are shown. No image credit.

Research appearing today in Nature Communications finds useful new information-handling potential in samples of tin(II) sulfide (SnS), a candidate “valleytronics” transistor material that might one day enable chipmakers to pack more computing power onto microchips.

The research was led by Jie Yao of the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) and Shuren Lin of UC Berkeley’s Department of Materials Science and Engineering and included scientists from Singapore and China. The research team used the unique capabilities of Berkeley Lab’s Molecular Foundry, a DOE Office of Science user facility.

For several decades, improvements in conventional transistor materials have been sufficient to sustain Moore’s Law – the historical pattern of microchip manufacturers packing more transistors (and thus more information storage and handling capacity) into a given volume of silicon. Today, however, chipmakers are concerned that they might soon reach the fundamental limits of conventional materials. If they can’t continue to pack more transistors into smaller spaces, they worry that Moore’s Law would break down, preventing future circuits from becoming smaller and more powerful than their predecessors.

That’s why researchers worldwide are on the hunt for new materials that can compute in smaller spaces, primarily by taking advantage of the additional degrees of freedom that the materials offer – in other words, using a material’s unique properties to perform more computations in the same space. Spintronics, for example, is a concept for transistors that harnesses the up and down spins of electrons in materials as the on/off transistor states.

Valleytronics, another emerging approach, utilizes the highly selective response of candidate crystalline materials under specific illumination conditions to denote their on/off states – that is, using the materials’ band structures so that the information of 0s and 1s is stored in separate energy valleys of electrons, which are dependent on the crystal structures of the materials.

In this new study, the research team has shown that tin(II) sulfide (SnS) is able to absorb different polarizations of light and then selectively reemit light of different colors at different polarizations. This is useful for concurrently accessing both the usual electronic and valleytronic degrees of freedom, which would substantially increase the computing power and data storage density of circuits made with the material.

“We show a new material with distinctive energy valleys that can be directly identified and separately controlled,” said Yao. “This is important because it provides us a platform to understand how valley signatures are carried by electrons and how information can be easily stored and processed between the valleys, which are of both scientific and engineering significance.”

Lin, the first author of the paper, said the material is different from previously investigated candidate valleytronics materials because it possesses such selectivity at room temperature without additional biases apart from the excitation light source, which alleviates the previously stringent requirements in controlling the valleys. Compared to its predecessor materials, SnS is also much easier to process.

With this finding, researchers will be able to develop operational valleytronic devices, which may one day be integrated into electronic circuits. The unique coupling between light and valleys in this new material may also pave the way toward future hybrid electronic/photonic chips.

Berkeley Lab’s “Beyond Moore’s Law” initiative leverages the basic science capabilities and unique user facilities of Berkeley Lab and UC Berkeley to evaluate promising candidates for next-generation electronics and computing technologies. Its objective is to build close partnerships with industry to accelerate the time it typically takes to move from the discovery of a technology to its scale-up and commercialization.

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From LBL: “Scientists Push Valleytronics One Step Closer to Reality”

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Berkeley Lab

April 4, 2016
Dan Krotz
510-486-4019
dakrotz@lbl.gov

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.

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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)

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.

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From left, Xiang Zhang, Yu Ye, Jun Xiao, and Yuan Wang are part of a team of scientists that made a big advance in valleytronics.

Their research could lead to a new type of electronics that utilize 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 charge carriers 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 circularly polarized light. 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.”

The research was supported by the Office of Naval Research Multidisciplinary University Research Initiative program, the National Science Foundation, China’s Ministry of Science and Technology, and the National Science Foundation of China.

See the full article here .

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