Electronics to Valleytronics

ValleytronicsThis 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. Researchers experimentally demonstrated, for the first time, the ability to electrically generate and control valley electrons in a two-dimensional semiconductor. A shorthand definition for valleytronics would be a movement away from exploiting the electrical charge of electrons as a means for storing information, and instead using the wave quantum number of an electron in a crystalline material to encode data. The aim of valleytronics is to find a faster way of encoding information than moving electrical charges around at high switching rates, which is the

basis of electronics. Whether valleytronics will eventually be a practical successor to electronics is a long way from being decided. It’s clear that valleytronics is going to depend heavily to 2-D materials and Valleytronics development could lead to new approaches for spintronics and quantum computing. Researchers have discovered that in the field of Valleytronics, valley polarization can make electron spin polarization in silicon transistors easier and this discovery could have an impact on the development of silicon-based “spintronic” devices and quantum computing.

Two-dimensional materials make “Valleytronics” possible. The latest cutting-edge electronics research increasingly depends on 2-D materials. Valleytronics is a portmanteau combining the terms valley and electronics. The term refers to the technology of control over the valley degree of freedom (a local maximum/minimum on the valence/conduction band) of certain semiconductors that present multiple valleys inside the first Brillouin zone—known as multivalley semiconductors. The term was coined in analogy to the blooming field of spintronics. While in spintronics the internal degree of freedom of spin is harnessed to store, manipulate and read out bits of information, the proposal for valleytronics is to perform similar tasks using the multiple extrema of the band structure, so that the information of 0s and 1s would be stored as different discrete values of the crystal momentum. The term is often used as an umbrella term to other forms of quantum manipulation of valleys in semiconductors, including quantum computation with valley-based qubits, valley blockade and other forms of quantum electronics.

In its simplest explanation, valleytronics represents an abandonment of exploiting the electrical charge of electrons as a means for storing information and instead uses the wave quantum number of an electron in a crystalline material to encode data. The “valley” in valleytronics is derived from the shape of the graph we get when plot the energy of electrons relative to their momentum. This creates a curve that features two valleys. Electrons move through the lattice of a 2-D semiconductor as a wave populating these two valleys, with each valley being characterized by a distinct momentum and quantum valley number. Manipulating these two valleys so that one is deeper than the other yields a way for the electrons to populate one of the two valleys. The positions into which electrons fall can be used to represent the zeroes and ones in digital computing.

Exploring valleytronics

Valleytronics is only a few years old, but until quite recently the mainstay material for achieving the effect has been diamonds. Last year, we saw the introduction of a new material calledrhenium disulfide that was in fact a 3-D material but behaved like a 2-D material. One of the applications cited for the material, besides photovoltaics, was valleytronics. Since then, the amount of research employing 2-D materials has been gradually increasing. First experimental evidence of valley blockade completes the set of Coulomb charge blockade and Pauli spin blockade and has been observed in a single atom doped silicon transistor. Several theoretical proposals and experiments were performed in a variety of systems, such as graphene, some Transition metal dichalcogenide monolayers, diamond, Bismuth, Silicon, Carbon nanotubes, Aluminium arsenide and silicene.   This year again researchers used the 2-D material known as tungsten diselenide in combination with a phenomenon known as the “optical Stark effect” to selectively control photo-excited electrons/hole pairs—excitons—in different energy valleys. This work is seen as a new pathway to achieving valleytronics. This is the first demonstration of the important role the optical Stark effect can play in valleytronics. The technique, which is based on the use of circularly polarized femtosecond light pulses to selectively control the valley degree of freedom, opens up the possibility of ultrafast manipulation of valley excitons for quantum information applications.

The coverage of the field has focused on efforts to achieve this effect with two-dimensional semiconductors such as graphene and tungsten diselenide.  However, some researchers have focused their research on valleytronics in silicon. The history of valleytronics in silicon is not one of achievement but more of an annoying curse. In silicon transistors, valleys cause electrons to lose speed. And in research for quantum-information-based devices, the valleys lead to decoherence, which can ruin the quantum state of so-called quantum computers. Therefore, researchers are looking at the behavior of electrons in the valleys of silicon-on-insulator quantum wells when exposed to a magnetic field. Conventional wisdom suggested that it would be more difficult to polarize the electrons after having polarized the valley, but the researchers discovered that the opposite was true.

Researchers were attempting to get the electrons to populate one valley more than the other to represent the ones and zeroes for digital logic. Electrons naturally want to settle into the lowest energy value and that could be in either of the two valleys. So, we need to find a way to generate a difference in the energies of the two electron valleys to get that digital logic.  The idea has been the need of a very powerful magnetic field to get even the most miniscule change.  The solution found that with metal dichalcogenides we could directly control the valley by using light. Being able to manipulate the valley degree of freedom in two-dimensional transition metal dichalcogenides would enable their application in the field of valleytronics.

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