NRL researchers demonstrate how one can control the degree of valley polarization in monolayer TMDs

Researchers from the US NRL laboratory have experimentally shown why different TMDs feature different degree of valley polarization. Specifically the researchers uncovered the connection between photoluminescence (PL) intensity and the degree of valley polarization.

WS2 optical excitation and PL intensity map (NRL)

The researchers used monolayer TMDs (transition metal dichalcogenides), mainly WS2 and WSe2. Samples that exhibited low PL intensity exhibited a higher degree of valley polarization. This means that if one controls the defects and nonradiative recombination sites in a monolayer TMD than once could create a material with a high or low degree of valley polarization.

Researchers prove that it is possible to create spin-valley half-metal materials

Researchers from the RAS in Russia and RIKEN in Japan have proved the existence of a new class of materials, spin-valley half-metals. This discovery could lead to devices that enable both valleytronics and spintronics.

Spin-valley half-metal image (MIPT)

In "regular" half-metals, all the electrons that participate in electric currents have the same spin - and so the current is always spin-polarized. These materials have interesting applications for spintronics devices. In the new class of materials now proven theoretically to be possible, there are two valleys present - one providing electrons, one providing holes.

A new research center in Germany to study valleytronics using graphene and other 2D materials

RWTH Aachen University and AMO GmbH launched a new joint research center with a focus on the science and applications of graphene and related 2D materials. The new center will addressing the challenges of future technology including high-frequency electronics, flexible electronics, energy-efficient sensing, photonics as well as spintronics and valleytronics.

The new center has five founding principal investigators, all members of the $1 billion Graphene Flagship project.

Researchers manage to efficiently generate long-lived valley polarization in 2D heterostructures

Researchers from the US, Japan and Korea report an efficient generation of microsecond-long-lived valley polarization in WSe2/MoS2 heterostructures by exploiting the ultrafast charge transfer processes in the heterostructure that efficiently creates resident holes in the WSe2 layer.

Ultrafast charge transfer process in the WSe2/MoS2 heterostructure

The researchers say that the valley-polarized holes exhibit near-unity valley polarization and ultralong valley lifetime. The researchers observed a valley-polarized hole population lifetime of more than 1 μs and a valley depolarization lifetime of more than 40 μs at 10 K. The researchers say that near-perfect generation of valley-polarized holes combined with the ultralong valley lifetime may open up new opportunities for novel valleytronics and spintronics applications.

Researchers demonstrate how light can be used to manipulate electron valley

Researchers from the City College of New York have demonstrated how to manipulate the electron valley property in 2D semiconductors. This could be seen as a step towards the realization of valleytronics-based logic gates.

Valley control in 2D semiconductors (CCNY)

To control the valley, the researchers used a light trapping structure called a microcavity. The microcavity gives rise to half-light-half matter quasi-particles which have the fingerprint of the valley property. A laser can be used to optically control the quasi-particles in order to access the electrons occupying specific valley.

Researchers manage to split the energy levels between valleys in a 2D semiconductor

Researchers from the University at Buffalo discovered a new way to split the energy levels between the valleys in a two-dimensional semiconductor. The researchers used a ferromagnetic compound to pull the valleys apart and keep them at different energy levels - which enables an increase in the separation of the energy in the valley by a factor of 10 compared to an external magnetic field.

Valley energy seperation on WSe2 over EuS (U of Buffalo)

The whole device is a two-layered hterostructure made from a 10-nm thick film of europoium sulfide (EuS) which is magnetic and a single layer (<1nm) layer of tungsten diselenide (WSe2) on top. The magnetic field of the bottom layer forced the energy separation of the valleys in the WSe2.

New polaritons multi-valley model may help valleytronics research

Researchers from the Institute for Basic Science (IBS) in Korea published a new paper that models the behavior of polaritons in microcavities, nanostructures made of a semiconductor material sandwiched between special mirrors (Bragg mirrors).

Plot of polaritons' energy dispersion vs momentum in a semiconductor microcavit

The new research helps to understand polaritons, microcavities and have potential implications for future valleytronics devices - such taht use valleys of polaritons. The IBS researchers have generated a theoretical model for valley polarization.

Researchers demonstrate the nematic quantum hill effect on crystal bismuth

Researchers from Princeton University and the University of Texas-Austin have demonstrated that electrons on a crystal bismuth exhibit the Nematic Quantum Hall Liquid effect at very low temperatures.

Nematic quantum Hall liquid on the surface of bismuth

This is the first time that a quantum fluid of electrons is visualized. As valleytronics aims to exploit the energy level of electrons in relation to their momentum, this is an important step toward realizing this technology.

Researchers use bi-layer graphene to create a device that control electron flow based on the valley degree of freedom

Researchers from Penn State University demonstrated a new device, based on bi-layer graphene, that provides an experimental proof of the ability to control electron-flow by the valley degree of freedom. This is still an early-stage development, but could be seen as an important step towards valleytronics.

Bi-layer graphene based valleytronics experiment (Penn State)

The device is built from a bi-layer graphene, and gates above and below the graphene layer. Adding an electric field perpendicular to the plane opens a bandgap in the bi-layer graphene, and a physical gap (70 nanometer in height) is left, in which one-dimensional metallic states (wires) exists. These states act as valleytronics valves.

Researchers suggest a way to pump carriers valley-selectively

Researchers at the University of Geneva proposed a novel approach to optically ‘pump’ carriers valley-selectively using a high-purity bismuth crystal.

UNIGE bismuth optically pump carriers valley selectively

In bismuth there are three valleys and the electrons orbit in magnetic field that are strongly elongated along one direction due the interaction with the atomic lattice. Each valley has its own direction - which means that the direction can be used to discriminate between the valleys.