Researchers shed light on electron-phonon interactions using 'magic angle' graphene

Researchers from ICN2, ICFO, Tokyo Institute of Technology, TU Eindhoven, National Institute for Material Sciences, MIT and Ludwig Maximilian University have studied electron-phonon interactions and found a remarkable speedup in cooling of twisted bilayer graphene near the 'magic angle': The cooling time is a few picoseconds from room temperature down to 5 kelvin, whereas in pristine bilayer graphene, cooling to phonons becomes much slower for lower temperatures. 

“We sought to understand how electrons and phonons ‘talk’ to each other within two twisted graphene layers,” says Klaas-Jan Tielrooij, associate professor at the Department of Applied Physics and Science Education at TU/e and the research lead of the new work.


Their experimental and theoretical analysis indicates that this ultrafast cooling is a combined effect of superlattice formation with low-energy moiré phonons, spatially compressed electronic Wannier orbitals, and a reduced superlattice Brillouin zone. This enables efficient electron-phonon Umklapp scattering that overcomes electron-phonon momentum mismatch. These results establish twist angle as an effective way to control energy relaxation and electronic heat flow. 

Electrons are the well-known charge and energy carriers associated with electricity, while a phonon is linked to the emergence of vibrations between atoms in an atomic crystal. “Phonons aren’t particles like electrons though, they’re a quasiparticle. Yet, their interaction with electrons in certain materials and how they affect energy loss in electrons has been a mystery for some time,” notes Tielrooij. “These interactions can have a major effect on the electronic and optoelectronic properties of devices, made from materials like graphene, which we are going to see more of in the future.”

“Depending on how the layers of graphene are rotated and doped with electrons, contrasting outcomes are possible. For certain dopings, the layers act as an insulator, which prevents the movement of electrons. For other doping, the material behaves as a superconductor – a material with zero resistance that allows the dissipation-less movement of electrons,” says Tielrooij.

Better known as twisted bilayer graphene, these outcomes occur at the so-called magic angle of misalignment, which is just over one degree of rotation. “The misalignment between the layers is tiny, but the possibility for a superconductor or an insulator is an astounding result.”

For their study, the team wanted to learn more about how electrons lose energy in magic-angle twisted bilayer graphene(MATBG). To this end, they used a material consisting of two sheets of monolayer graphene (each 0.3 nanometers thick), placed on top of each other, and misaligned relative to each other by about one degree. Then, using two optoelectronic measurement techniques, the researchers were able to probe the electron-phonon interactions in detail, and they made some staggering discoveries.

“We observed that the energy vanishes very quickly in the MATBG – it occurs on the picosecond timescale, which is one-millionth of one-millionth of a second!” says Tielrooij.

This observation is much faster than for the case of a single layer of graphene, especially at ultracold temperatures (specifically below -73 degrees Celsius). “At these temperatures, it’s very difficult for electrons to lose energy to phonons, yet it happens in the MATBG.”

The reason that the electrons lose the energy so quickly through interaction with the phonon has been explored by the team, which uncovered a whole new physical process.

“The strong electron-phonon interaction is a completely new physical process and involves so-called electron-phonon Umklapp scattering,” adds Hiroaki Ishizuka from Tokyo Institute of Technology in Japan, who developed the theoretical understanding of this process together with Leonid Levitov from Massachusetts Institute of Technology in the US.

Umklapp scattering between phonons is a process that often affects heat transfer in materials, because it enables relatively large amounts of momentum to be transferred between phonons.

“We see the effects of phonon-phonon Umklapp scattering all the time as it affects the ability for (non-metallic) materials at room temperature to conduct heat. Just think of an insulating material on the handle of a pot for example,” says Ishizuka. “However, electron-phonon Umklapp scattering is rare. Here though we have observed for the first time how electrons and phonons interact via Umklapp scattering to dissipate electron energy.”

Looking at the future of these materials, Tielrooij said: “As the material is only being studied for a few years, we’re still some way from seeing magic-angle twisted bilayer graphene having an impact on society.”

But there is a great deal to be explored about energy loss in the material. “Future discoveries could have implications for charge transport dynamics, which could have implications for future ultrafast optoelectronics devices,” says Tielrooij. “In particular, they would be very useful at low temperatures, so that makes the material suitable for space and quantum applications.”

Posted: Feb 13,2024 by Roni Peleg