Valleytronics.com: the valleytronics experts

Scientists from several research institutes in Germany and the UK have demonstrated that few-cycle linearly polarized pulses can induce a high degree of valley polarization.

The mechanism to induce such polarization does not rely on the optical selection rules, and therefore can be in principle used in inversion symmetric materials, such as TMD bilayers or graphene. This could enable the design of ultrafast valleytronic devices.

Researchers from Sweden's Uppsala university and from UK's Element Six have demonstrated electrical valley control in 3D dual-gate diamond field-effect transistors.

The team has fabricated valley transistors on single-crystalline diamond plates with ultra-low nitrogen impurity concentrations. A dual-gate configuration was used to gain a high degree of freedom of the control of the valley currents. The control of location and time of the valley-polarized currents was achieved by adding multiple drain electrodes.

Researchers from ETH Zurich and Aalto University showed that sandwiching two slightly rotated layers of graphene between a ferromagnetic insulator provides a unique setting for new electronic states.

The combination of ferromagnets, graphene's twist engineering, and relativistic effects force the "valley" property to dominate the electrons behavior in the material. In particular, the researchers showed how these valley-only states can be tuned electrically, providing a materials platform in which valley-only states can be generated.

Researchers from Rice University and Texas A&M University have discovered that a 2D derivative of perovskite has a good potential for valleytronics applications.

The researchers synthesized a layered compound of cesium, bismuth and iodine that is able to store 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.

Researchers from Cornell University managed to control the valley (orbital angular momentum) of electrons in a material by using electrical inputs to manipulate the magnetism of an adjacent material.

The device is built from a 2D tungsten diselenide (WSe₂), a material whose energy landscape has valleys, atop a few atomic layers of chromium triiodide ( CrI₃), a material whose magnetism can be electrically altered. The researchers are now looking for an alternative electrically-controlled magnetism material that will behave in a similar way at room temperatures.

They then changed the voltage across the CrI3 layers and measured the population of the WSe2 valleys using a technique that monitored the spin direction of light that the WSe2 emitted when illuminated by a laser. They found that the direction changed when the voltage was applied, indicating a switch in the semiconductor’s filled valley. The CrI3 layer is magnetic only at around 60 K, so the team says that their next step is to find a material that would allow valley sorting at room temperature.

Researchers from Fusain University and the Chinese National University of Defense Technology demonstrated that 2D all-dielectric PhC slabs without in-plane inversion symmetry can be used to efficiently separate valley exciton emission of a 2D WS2 monolayer in the far field at room temperature.

Left: PhC slabs with C4 symmetry and without in-plane inversion symmetry. By breaking the in-plane inversion symmetry, the polarization states of PhC can cover entire Poincaré sphere's two poles. Right: Illustration of photoluminescence of WS2 monolayer on the PhC slab without in-plane inversion symmetry.

This is the first report of effective valley separation in TMDCs by using PhCs. This method could be extended to manipulate valley exciton emission of other TMDCs monolayers. The ability of this PhC slabs to transport valley information from the near field to the far field would help to develop photonic devices based on valleytronics.

Researchers from the University of California, Riverside, has observed light emission intervalley transmissions. The researchers say that this light emission can be used to read valley information from Valleytronics devices in the future.

The researhers observed the phenomenon in monolayer tungsten diselenide (WSe2) - a promising valleytronic material that possesses two valleys with opposite dynamic characteristics in the band structure, and can interact strongly with light.

Researchers from the University of Groningen managed to produce valley-coherent photoluminescence at room temperatures - by using a silver sawatooth nanoslit array in two-dimensional tungsten disulfide flakes.

So-called "coherent light" can be used to store or transfer information in quantum electronics, and the researchers say that their plasmon-exciton hybrid device model may be promising for future integrated nanophotonics applications.

Researchers from the University of California, Riverside,have developed a new method to read the valley indices of the dark excitons and trions. The researchers used monolayer (2D) tungsten diselenide (WSe2), a semiconductor with two distinct electronic valleys.The material hosts bright and dark excitons or trions with different spin configurations.

The researchers say that dark excitons and trions in monolayer WSe2 have much longer lifetime and better valley stability than the common bright excitons and trions - which makes them excellent candidates for valleytronic applications. But up until now there was no method to read the valley indices of the dark excitons and trions because their light emission from either valley has exactly the same energy and polarization. By identifying a measurable physical quantity that can distinguish the two valley indices of dark excitons and trions, the team was able to devise a method to read the valley indices.

The International Institute of Physics in Brazil recently uploaded this interesting talk by Stephan Roche from the ICREA and CINN who discusses valleytronics in 2D materials (such as graphene):

This talk was given as part of the IIP's 2D Materials: From Fundamentals to Spintronics workshop which took place in early October.