An ICFO team, in collaboration with international partners, publishes in Nature a new method that, for the first time, achieves valley polarization in centrosymmetric bulk materials universally, without reliance on specific materials. This “universal technique” could have significant implications for controlling and analyzing various properties of both 2D and 3D materials. Such advancements could drive progress in fields like information processing and quantum computing.
Understanding Band Structures: The Foundation of Materials Science
Electrons inside solid materials can only take certain values of energy. Physicists refer to the allowed energy ranges as “bands,” with the space between them termed as “band-gaps.” Together, they constitute the “band structure” of the material, a unique characteristic of each specific material.
When physicists plot the band structure, they observe that the resulting curves resemble mountains and valleys. These local energy maxima or minima in the bands are technically called “valleys,” and the field that studies how electrons in the material switch from one valley to another is known as “valleytronics.”
Valleytronics: Harnessing Electron Valleys for Next-Generation Electronics
In standard semiconductor electronics, the electric charge of the electrons is the primary property used to encode and manipulate information. However, these particles possess other properties that could also be leveraged, such as the valley they are in. Over the past decade, valleytronics has aimed to control valley population (or valley polarization) in materials. This achievement could lead to the creation of classical and quantum gates and bits, significantly advancing computing and quantum information processing.
Previous attempts had drawbacks. For instance, the light used to manipulate and change valley polarization had to be resonant, meaning the energy of its photons had to correspond exactly to the energy of the band-gap of that particular material. Any small deviation reduced the method’s efficiency, making it challenging to generalize across materials with different band-gaps. Additionally, this process had only been successful for monolayer structures (2D materials, just one-atom-thick), posing limitations in size, quality, and engineering complexity.
Researchers at ICFO. From left to right: Dr. Lenard Vamos, Julita Poborska, Igor Tyulnev, and Prof. ICREA Jens Biegert. ©ICFO
Now, ICFO researchers Igor Tyulnev, Julita Poborska, Dr. Lenard Vamos, led by Prof. ICREA Jens Biegert, in collaboration with researchers from the Max-Born-Institute, the Max-Planck Institute for the Science of Light, and Instituto de Ciencia de Materiales de Madrid (ICMM-CSIC) have found a new universal method to induce valley polarization in centrosymmetric bulk materials. The discovery, published in Nature, unlocks the possibility of controlling and manipulating valley population without being restricted by the specific chosen material. Simultaneously, the method enables obtaining a more detailed characterization of crystals and 2D materials.
Valley polarization in bulk materials is possible
The experimental journey began with the leadership of ICREA Prof. Jens Biegert at ICFO, who initially aimed to experimentally induce valley polarization in 2D materials, aligning with the theoretical findings of Álvaro Jiménez, Rui Silva, and Misha Ivanov. To initiate the experiment, they conducted initial measurements on bulk MoS2, a material composed of stacked monolayers. Surprisingly, they observed the signature of valley polarization. Julita Poborska elaborates, “When we started this project, our theory collaborators deemed valley polarization in bulk materials nearly impossible.”
The theoretical team initially tailored their model for single 2D layers. Prof. Misha Ivanov explains, “At first, it seemed that adding more layers would complicate the selection of specific valleys. However, after the initial experimental results, we adapted the simulation to bulk materials, and it validated our observations remarkably well. We didn’t need to make any adjustments; it just worked.” Ultimately, Poborska concludes, “It turns out that valley polarization in bulk materials is feasible due to symmetry conditions.”
First author Igor Tyulnev details, “Our experiment involved generating an intense light pulse with a polarization that matched the internal structure.” This resulted in the creation of a “trefoil field,” whose symmetry aligned with the triangular sub-lattices of hetero-atomic hexagonal materials.
This symmetry-matched strong field disrupts the material’s space and time symmetry, with the resulting configuration dependent on the trefoil field’s orientation relative to the material. Tyulnev emphasizes, “By simply rotating the incident light field, we could modulate valley polarization,” marking a significant milestone in the field and validating a novel universal technique for controlling electron valleys in bulk materials.
Schematic of the valley polarization effect, where the incident trefoil field (in purple) causes valley polarization in the bulk material MoS2.
The experimental process
The experiment comprises three main steps: First, synthesizing the trefoil field; then characterizing it; and finally, producing valley polarization.
The researchers stress the precision needed for characterization, as they coherently combine two optical fields to create the trefoil field. One field must circularly polarize in one direction, while the other is the second harmonic of the first, polarized with the opposite handedness. They overlay these fields to trace the desired trefoil shape.
Three years after the initial experimental attempts, Igor Tyulnev expresses his excitement about the recent Nature publication. Its appearance in such a prestigious journal acknowledges the new universal method, which, as he states, “can be used not only to control the properties of a wide variety of chemical species, but also to characterize crystals and 2D materials”.
Prof. ICREA at ICFO Jens Biegert remarks: “Our method may provide an important ingredient to engineer energy efficient materials for efficient information storage and fast switching. This addresses the pressing need for low-energy consumption devices and increased computational speed. I cannot promise that what we have provided is THE solution, but it is probably one solution to this big challenge”.
Original article
Tyulnev, I., Jiménez-Galán, Á., Poborska, J. et al. Valleytronics in bulk MoS2 with a topologic optical field. Nature 628, 746–751 (2024). https://doi.org/10.1038/s41586-024-07156-y
