A global team of researchers have observed a new microscopic mechanism enabling precise control of the magneto-optical properties of excitons in alloys of 2D semiconductors, potentially enabling new valleytronics applications.

As part of this study, high-quality monolayers of mixed alloys MoxW1-xSe2 with precisely controlled chemical composition were investigated. The samples were synthesized in the Czech Republic and encapsulated between flakes of hexagonal boron nitride fabricated in Japan. For a series of samples with varying molybdenum and tungsten content, systematic photoluminescence measurements were carried out at a temperature of 10 kelvin and in strong magnetic fields reaching up to 30 tesla at the Laboratoire National des Champs Magnétiques Intenses in Grenoble. An analysis of the emitted light in circular polarizations enabled highly accurate determination of the neutral exciton g-factor.

Researchers at IIT Bombay developed a simple method that uses a single linearly polarized laser pulse with a slight skew to control and read quantum valley states in ultra-thin 2D semiconductors.

The researchers say that current techniques to control the quantum valley state required complex laser setups using circularly polarized light and multiple laser pulses. On top of that, the control was often incomplete or hard to measure. This means that reliable and reversible switching between the two valley states remained a major challenge. The team has now shown that a single linearly polarized laser pulse can both control and read the valley state of electrons.

Researchers at the Okinawa Institute of Science and Technology (OIST) have managed to directly observe dark excitons in TMD 2D materials for the first, which may be a first step towards the use of this phenomena in valleytronics devices.

The researchers believe that dark excitons have great potential as information carriers, because they are inherently less likely to interact with light, and hence less prone to degradation of their quantum properties. On the other hand, this this invisibility also makes them very challenging to study and manipulate. The researchers now believe they have opened a route to the creation, observation, and manipulation of dark excitons.

Researchers from Tohoku University, the National Institutes for Quantum Science and Technology (QST), and Cambridge University have managed to create a monolayer tin sulfide (SnS) material, which could be suitable for spin-valleytronics applications.

 The researchers explain that SnS is special because it can conduct electricity and respond to light in unique ways. But io far it has been challenging to selectively form SnS from base tin (Sn) and sulfur (S) - sometimes other materials are produced instead, such as SnS2 instead. Now the researchers developed an easier and safer process that can reliably produce entire sheets of SnS.

Researchers from Delft University of Technology demonstrated a device that generates electrical currents with a precisely controlled number of electrons at each valley. The researchers say that when electrons are squeezed through a narrow channel, they emerge as two jets which are valley polarized. By steering these jets with a magnetic field, the researchers can regulate which jet enters through a second opening, gaining control on how many electrons end up in a specific valley.

The researchers, led by Josep Ingla-Aynes, based their device on bi-layer graphene.

Researchers from the Max Born and Max Planck institutes have shown that the few cycle limit of circularly polarized light is imbued with an emergent vectorial character that allows direct coupling to the valley current. The underlying physical mechanism involves the emergence of a momentum space valley dipole, the orientation and magnitude of which allows complete control over the direction and magnitude of the valley current.

The researchers demonstrate this effect via minimal tight-binding models both for the visible spectrum gaps of the transition metal dichalcogenides as well as the infrared gaps of biased bilayer graphene.

Researchers from the US DOE Brookhaven National Laboratory, together with Northrop Grumman, have found a way to maintain valley polarization at room temperature using novel materials and techniques. 

The researchers used a chiral lead halide perovskite material (R/S-NEAPbI3). The researchers layered 500 nanometer thick flakes onto a monolayer molybdenum disulfide (MoS2), to create what is known as a heterostructure. Using a linearly polarized laser to excite the heterostructure the researchers fabricated and then measured the light that was emitted from the molybdenum disulfide TMD using a confocal microscope. They have discovered that the new material is promising for valletronics applications.

Researchers from CNRS in France demonstrated that Platinum diselenide (PtSe₂) is a promising 2D material for a terahertz (THz) range valleytronics device.

The researchers explain that PtSe₂ is promising as, unlike other transition metal dichalcogenides (TMDs), its bandgap can be uniquely tuned from a semiconductor in the near-infrared to a semimetal with the number of atomic layers. This gives the material unique THz photonic properties that can be layer-engineered. In this research, the main demonstration was that a controlled THz nonlinearity - tuned from monolayer to bulk - can be realized in wafer size polycrystalline through the generation of ultrafast photocurrents and the engineering of the bandstructure valleys. 

An international team of scientists led by a group at the University of St Andrews and the University of Manchester, report on a giant valley-selective Ising coupling in the surface layer of an intercalated transition metal dichalcogenide, V_1/3NbS₂.

Using angle-resolved photoemission spectroscopy measurements, the researchers proved the surface electronic structure of the semimetal. The researchers discovered an alternating pattern of enhancement and quenching of valley-spin polarization of the host NbS2 layers due to the intercalated Vanadium ions, equivalent to the application of a 250 T magnetic field. The researchers say that this is a major step forward in valleytronics, opening up new opportunities for the development of advanced electronic devices.