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Odds are good for unique 2-D compound

Engineers at Rice University and Texas A&M University have found a 2-D material that could make computers faster and more energy-efficient.

Their material is a derivative of perovskite—a crystal with a distinctive structure—that has the surprising ability to enable the valleytronics phenomenon touted as a possible platform for information processing and storage.

The lab of materials scientist Jun Lou of Rice's Brown School of Engineering synthesized a layered compound of cesium, bismuth and iodine that is adept at storing the valley states of electrons, but only in the structure's odd layers.

These bits can be set with polarized light, and the even layers appear to protect the odd ones from the kind of field interference that bedevils other perovskites, according to the researchers.

A Princeton-led study has revealed an emergent electronic behavior on the surface of bismuth crystals that could lead to insights on the growing area of technol...

These valleys are not literal pits in the crystal but rather are like pockets of low energy where electrons prefer to rest.

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Electrical Control of the “Valley”

Researchers show that they can electrically switch which energy valley electrons occupy in a solid, a key step toward enabling a “valleytronics” approach to information applications.

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The future of data encoding may lie with a new paradigm known as valleytronics. While traditional electronic devices manipulate the charge of electrons, those that use valleytronics employ the electrons’ orbital angular momentum. In a new study, Kin Fai Mak of Cornell University and colleagues report achieving such control via electrical inputs, an elusive goal deemed essential for high-speed, low-cost devices [1].

The “valley” in valleytronics refers to two dips that appear in plots of a solid’s energy bands at different electron momenta. By putting all the electrons into one valley or the other, those two dips could be used to represent the two values of bits (0 and 1) in information applications. While this “valley sorting” is unresponsive to electrical fields, it can be influenced by changes in ambient magnetism. Mak and collaborators found a way to control the valley populations in one material by electrically manipulating the magnetism of an adjacent material.

Oriented hexagonal boron nitride foster new type of information carrier

Valleytronics gives rise to valley current, a stable, dissipationless current which is driven by a pseudo-magnetic field, Berry curvature. This gives rise to valletronics based information processing and storage technology. A pre-requisite for the emergence of Berry curvature is either a broken inversion symmetry or a broken time-reversal symmetry. Thus two-dimensional materials such as transition metal dichalcogenides and gated bilayer graphene are widely studied for valleytronics as they exhibit broken inversion symmetry.

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For most of the studies related to graphene and other two-dimensional materials, these materials are encapsulated with hexagonal boron nitride (hBN), a wide band gap material which has comparable lattice parameter to that of graphene. Encapsulation with hBN layer protects the graphene and other two-dimensional materials from unwanted adsorption of stray molecules while keeping their properties intact. hBN also acts as a smooth twodimensional substrate unlike SiO2 which is highly non-uniform, increasing the mobility of carriers in graphene. However, most of the valleytronics studies on bilayer graphene with hBN encapsulation has not taken into account the effect of hBN layer in breaking the layer symmetry of bilayer graphene and inducing Berry curvature.

Silver sawtooth creates valley-coherent light for nanophotonics

Scientists at the University of Groningen used a silver sawtooth nanoslit array to produce valley-coherent photoluminescence in two-dimensional tungsten disulfide flakes at room temperature. Until now, this could only be achieved at very low temperatures. Coherent light can be used to store or transfer information in quantum electronics. This plasmon-exciton hybrid device is promising for use in integrated nanophotonics (light-based electronics). The results were published in Nature Communications on 5 February.

Tungsten disulfide has interesting electronic properties and is available as a 2-D material. "The electronic structure of monolayer tungsten disulfide shows two sets of lowest energy points or valleys," explains Associate Professor Justin Ye, head of the Device Physics of Complex Materials group at the University of Groningen. One possible application is in photonics, as it can emit light with valley-dependent circular polarization—a new degree of freedom to manipulate information. However, valleytronics requires coherent and polarized light. Unfortunately, previous work showed that photoluminescence polarization in tungsten disulfide is almost random at room temperature.

Excitons pave the way to higher-performance electronics

After developing a method to control exciton flows at room temperature, EPFL scientists have discovered new properties of these quasiparticles that can lead to more energy-efficient electronic devices.

They were the first to control exciton flows at room temperature. And now, the team of scientists from EPFL's Laboratory of Nanoscale Electronics and Structures (LANES) has taken their technology one step further. They have found a way to control some of the properties of excitons and change the polarization of the light they generate. This can lead to a new generation of electronic devices with transistors that undergo less energy loss and heat dissipation. The scientists' discovery forms part of a new field of research called valleytronics and has just been published in Nature Photonics.

Excitons are created when an electron absorbs light and moves into a higher energy level, or "energy band" as they are called in solid quantum physics. This excited electron leaves behind an "electron hole" in its previous energy band. And because the electron has a negative charge and the hole a positive charge, the two are bound together by an electrostatic force called a Coulomb force. It's this electron-electron hole pair that is referred to as an exciton.

New Research Finding Gives Valleytronics a Boost

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…

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On the cusp of valleytronics

Research into harnessing two-dimensional (2-D) materials for everyday devices has had some ups and downs. However, the emerging field of valleytronics is using energy troughs to offer renewed potential.

According to Johnson Goh, a senior scientist at A*STAR's Institute of Materials Research and Engineering, 2-D and other very thin materials could soon be using valleytronics to transmit information. Goh claims that a combination of increasingly affordable 2-D material production methods and the application of techniques such as valleytronics could quickly act together to shrink device sizes and power consumption needs.

The basic idea is to pass information through 2-D and other very thin conducting materials using the energy 'valleys' (or energy extrema) in their conduction and valence bands (the energy bands around which electrons orbit an atom's nucleus). Information, says Goh, can be transmitted by controlling an electron's association with a valley—a manipulation that can be achieved using electric fields, magnetic fields and circularly polarised light.

For example, in molybdenum disulfide, which is a 2-D material, the presence of two inequivalent valleys means information can be stored in a binary manner based on which valley an electron is residing in: one valley could represent a zero, while the other could represent a one. This information can then be used for computation or memory.

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