Bilayer Graphene: A New Frontier in Valleytronics

Graphene is a material made up of a single layer of carbon atoms arranged in a two-dimensional honeycomb structure. When you stack two layers of graphene in a specific way, known as Bernal stacking, it forms Bilayer Graphene. This material has unique electrical properties that researchers are beginning to harness for advanced technologies, including Valleytronics.

Valleytronics is a new field of research focusing on the valleys in the electronic band structure of materials. In simple terms, these valleys are like local energy peaks where electrons can reside. Researchers want to use these valleys to store and process information, much like how we use charge or spin in traditional electronics. The challenge is that in many materials, including conventional semiconductors, it's hard to control these valleys.

With the discovery of graphene in 2004, scientists started looking at two-dimensional materials, which showed promise for valleytronics. Graphene has two valleys at the corners of its energy band structure, which are crucial for valleytronic applications. Early ideas focused on using graphene for electronic transport. However, atomic-scale defects in real devices mixed the valleys and made it hard to harness their potential.

One alternative approach is to use Light to excite Charge Carriers within the material. However, in pure graphene, the valleys have similar optical properties, making it difficult to measure and control valley polarization, which refers to the concentration of charge carriers in a particular valley. At higher light energies, a phenomenon called trigonal warping allows charge carriers from different valleys to spread out in space when light hits them. This effect could allow researchers to control the valleys, but at these higher energies, the valley states tend to mix due to scattering processes, reducing their usefulness for valleytronics.

To overcome these limitations, scientists can change the properties of graphene by placing it on certain substrates that break its inversion symmetry, such as boron nitride. This change opens a band gap in the material, giving rise to new optical properties. In gapped graphene, researchers can selectively excite electrons in different valleys using circularly polarized light, which is light that oscillates in a circular manner. This effect has been observed in other two-dimensional materials with a band gap, but in pure graphene, valley separation is more challenging.

The focus has shifted to bilayer graphene, specifically the gapless Bernal-stacked version. Recent studies have highlighted the unique behaviors of bilayer graphene, revealing phenomena such as many-body phase transitions and superconductivity. Researchers have found that charge carriers in bilayer graphene can be separated spatially when low-energy light is shone on them, thanks to the anisotropic arrangement of the valleys. This means that the charge carriers from different valleys move towards different sides of the light spot. This separation is further enhanced when using linearly polarized light, which can align the momentum of the charge carriers.

Even when a band gap is introduced to bilayer graphene, researchers noticed that the valley-dependent properties persist. In gapped bilayer graphene, the behavior of light can also distinguish between the valleys. This means that it’s possible to detect how many charge carriers belong to each valley based on the type of circular polarization they emit.

The researchers proposed using these features to create new kinds of devices called optovalleytronic devices. These devices would take advantage of the ability to separate charge carriers based on their valley index. Researchers have outlined two experimental setups to demonstrate this concept.

In the first setup, uniformly gapped bilayer graphene is exposed to linearly polarized light. This light excites charge carriers in the material. Because of the unique band structure of bilayer graphene, charge carriers from opposite valleys will spread away from the light spot, leading to a spatial separation. As the charge carriers reach the edges of the material, they can emit light of different circular polarizations based on which valley they came from. This means that by measuring the emitted light at the edges, scientists can gather information about the valley polarization.

In the second setup, a central region of gapless bilayer graphene is surrounded by regions with a band gap. The gapless area will allow charge carriers to move more freely, which means they can propagate into the gapped areas without significant mixing. Again, circularly polarized light emitted from the different gapped regions will have opposite handedness, enabling the detection of valley polarization.

Both setups focus on using the bulk properties of bilayer graphene and avoid the edge effects that often complicate valley manipulation in traditional devices. Researchers aim to work in the terahertz frequency range, which is important for future technologies.

Recent experiments have shown that valley states in bilayer graphene can exist for a long time, lasting much longer than traditional spin states in materials. This longevity makes bilayer graphene a strong candidate for potential applications in the emerging field of quantum valleytronics.

In conclusion, the unique properties of bilayer graphene allow for effective valley manipulation using light. Charge carriers from different valleys can be spatially separated, and their valley index can be preserved, providing a promising route for future optovalleytronic devices. As research continues, bilayer graphene may become a key player in the future of advanced electronic technologies, offering new ways to process and store information.