A recent study, led by Klaas-Jan Tielrooij, who heads the Ultrafast Dynamics in Nanoscale Systems Group, has been published in the journal 'Science Advances'. This work provides new insights into electron-phonon interactions occurring within bilayer graphene. The majority of the research was conducted at ICN2 with worldwide collaboration.
Prior studies have revealed various insights in terms of electron-phonon interactions. Back in 2018, researchers at the Massachusetts Institute of Technology (MIT) had already discovered the potential to alter the electronic properties of two stacked graphene layers through a subtle rotation. They indicated that slightly twisting graphene layers at a specific ‘magic angle’ (referred to as MATBG henceforth) can transform the bilayer into either a flawless insulator or superconductor. In fact, despite this misalignment between the layers being extremely small (just a bit over 1 degree of rotation), the potential for achieving superconductivity or insulation is an astonishing outcome.
Nevertheless, a gap persisted in the understanding of the way these two components (electrons and phonons) exchange in a very specific scenario - within the MATBG. Tielrooij, leader of the Ultrafast Dynamics in Nanoscale Systems Group at ICN2 and Associate Professor in the Applied Physics Department of Eindhoven University of Technology (TU/e), has spearheaded an investigation with international collaborators that unveils a remarkable breakthrough. In their work, Tielrooij and the team aimed to delve more deeply into the connection between electrons and phonons in this MATBG. Comprehending this is crucial for the development of future optoelectronic devices. More specifically, they utilized measurements of voltage changes over time and frequency to directly examine and understand how the vibrations of phonons influence the cooling process and the loss of energy in hot electrons.
The results and future implications
This discovery was primarily conducted at ICN2. However, the worldwide collaboration played a crucial role in its completion. Tielrooij and the team obtained the magic-angle twisted samples from Dmitri Efetov’s group at Ludwig-Maximilians-Universität in Munich. This group was the pioneering group in Europe capable of producing such samples and conducting photomixing measurements. Theoretical contributions from MIT in the US and the Tokyo Institute of Technology in Japan were also pivotal to the research’s success.
Moreover, the work established the twist angle as an effective method for regulating energy relaxation and electronic heat flow. This explained the reasons and mechanisms behind the loss of energy by electrons. They noticed that the energy dissipates rapidly in the MATBG – occurring within the picosecond timeframe, equivalent to one millionth of one millionth of a second. This discovery highlights a significantly accelerated process compared to a single layer of graphene. The research leader notes, “At low temperatures, it’s very difficult for electrons to lose energy to phonons, yet it happens in the MATBG.”
The future implications of these findings, as highlighted by Tielrooij, extend to the realm of charge transport dynamics and hold promise for the development of advanced ultrafast optoelectronics devices. Particularly advantageous in low-temperature environments, bilayer graphene emerges as a fitting candidate for applications in space and quantum technologies. The identified interactions between electrons and phonons are anticipated to exert a significant influence on the electronic and optoelectronic properties of this material, foreshadowing its potential role in future device technologies.
Reference
Ultrafast Umklapp-assisted electron-phonon cooling in magic-angle twisted bilayer graphene
Jake Dudley Mehew, Rafael Luque Merino, Hiroaki Ishizuka, Alexander Block, Jaime Díez Mérida, Andrés Díez Carlón, Kenji Watanabe, Takashi Taniguchi, Leonid S. Levitov, Dmitri K. Efetov, Klaas-Jan Tielrooij
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