The way electrons flow in materials determine its electronic properties. For example, when a voltage is sustained across a conducting material, electrons start flowing, generating an electrical current. These electrons are often thought to flow in straight paths, moving along the electric field, much like a ball rolling down a hill. Yet these are not the only tra­jectories electrons can take: when a magnetic field is applied, the electrons no longer travel in straight paths along the electric field, but in fact, they bend. The bent electronic flows lead to transverse signals – the Hall responses. Now, an international team of researchers report that circular polarized light can induce bent electronic flows in bilayer graphene. The study has been carried out by a team including ICFO scientists Jianbo Yin, David Barcons, Iacopo Torre, led by Frank Koppens, in collaboration with Cheng Tan and James Hone from Columbia University, Kenji Watanabe and Takashi Taniguchi from NIMS Japan and Justin Song from Nanyang Techno­logical University (NTU) in Singapore.

In quantum materials, such as bilayer graphene, the wave pattern of electrons can exhibit a complex winding often referred to as quantum geometry. “Frank and I talked about the possibility of harnessing quantum geometry in bilayer graphene to bend the flow of electrons with light instead of using magnetic fields”, Justin Song says. With this in mind, Jianbo Yin decided to take on the challenge of experi­mentally realizing this unusual pheno­menon. “Our device was very complicated to build. It took building many devices and flying to Columbia University to work with Cheng Tan and James Hone to improve the device quality”.

In bilayer graphene, there are two pockets of electron valleys (K and K’): when a perpendicular electric field is applied, the quantum geo­metrical properties of electrons in these two valleys can cause them to bend in opposite directions. As a result, their Hall effects are cancelled out. The team of scientists found that by applying circular polarized infrared light onto the bilayer graphene device, they were able to selectively excite one specific valley population of electrons in the material, which generated a photo­voltage perpen­dicular to the usual electron flow. As Koppens highlights, ”we now engineered the device and setup in such a way that current only flows with light illu­mination. With this, we were able to avoid the background noise that hampers measurements and achieve a sensitivity in the detection several orders of magnitude better than any other 2D material.” This development is signi­ficant because conventional photo­detectors often require large voltage biases that can lead to dark currents that flow even when there is no light.

Yin remarks that “we can control the bending of the electrons with the out-of-plane electric field we apply. We can change the bending angle of these electrons, which can be quantified by the Hall conduc­tivity. By controlling the voltage knob, the Berry curvature [one characteristic of quantum geometry], can be tuned, which can lead to a giant Hall conduc­tivity.” The results open a new realm of many detection and imaging appli­cations, as Koppens finally concludes. “Such discovery could have major impli­cations in appli­cations for infrared and terahertz sensing since bilayer graphene can be transformed from semimetal to semiconductor with a very small bandgap, so it can detect photons of very small energies. It may be also useful, for example, for imaging in space, medical imaging, e.g. for tissue skin cancer, or even for security appli­cations such as the quality inspection of materials”.

The possi­bilities are manifold and the next steps of research focused on new 2D materials, such as the moiré material twisted bilayer graphene, may find new ways of controlling electron flows and uncon­ventional opto-electronic properties. (Source: ICFO)

Reference: J. Yin et al.: Tunable and giant valley selective Hall effect in gapped bilayer graphene, Science 375, 1398 (2022); DOI: 10.1126/science.abl4266

Link: Quantum Nano-Optoelectronics, ICFO Institut de Ciencies Fotoniques, Barcelona Institute of Science and Technology, Castelldefels, Spain