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What did the electron ‘say’ to the phonon in the graphene sandwich?

February 12, 2024

(Nanowerk News) Electrons carry electrical energy, while vibrational energy is carried by phonons. Understanding how they interact with each other in certain materials, such as in a two-layer graphene sandwich, will have implications for future optoelectronic devices.

Key takeaways

  • Twisted layers of graphene exhibit unique electrical properties.
  • Electron-phonon interactions are crucial for energy loss in graphene.
  • Discovery of a new physical process involving Umklapp scattering of electrons and phonons.
  • Potential implications for ultrafast optoelectronics and quantum applications.
Illustration showing the control of energy relaxation with rotation angle. (Image: Courtesy of the authors)

The investigation

Recent work has revealed that layers of graphene twisted together by a small “magic angle” can act as a perfect insulator or superconductor. But the physics of electron-phonon interactions is a mystery. As part of a worldwide international collaboration, TU/e ​​researcher Klaas-Jan Tielrooij has led a study on electron-phonon interactions in graphene layers. And they have made a surprising discovery.

What did the electron say to the phonon between two layers of graphene?

This might sound like the beginning of a physics meme with a hilarious punchline to follow. But according to Klaas-Jan Tielrooij this is not the case. He is an associate professor in the Department of Applied Physics and Science Education of TU/e ​​and research leader of the new work published in Scientific advances (“Umklapp-assisted ultrafast electron phonon cooling in magic angle twisted bilayer graphene”).

“We are trying to understand how electrons and phonons ‘talk’ to each other inside two twisted layers of graphene,” says Tielrooij.

Electrons are the well-known carriers of charge and energy associated with electricity, while a phonon is linked to the appearance of vibrations between atoms in an atomic crystal.

“However, phonons are not particles like electrons, they are a quasiparticle. However, their interaction with the electrons of certain materials and how they affect the energy loss of the electrons has been a mystery for some time,” says Tielrooij.

But why would it be interesting to learn more about electron-phonon interactions? “These interactions can have an important effect on the electronic and optoelectronic properties of devices, made from materials such as graphene, which we will see more of in the future.”

Twistronics: Preview of the year 2018

Tielrooij and his collaborators, who reside in Spain, Germany, Japan and the United States, decided to study electron-phonon interactions in a very particular case: within two layers of graphene, where the layers are slightly misaligned. .

Graphene is a two-dimensional layer of carbon atoms arranged in a honeycomb network that has several impressive properties, such as high electrical conductivity, high flexibility and high thermal conductivity, and is also almost transparent.

In 2018, the World Breakthrough of the Year in Physics award went to Pablo Jarillo-Herrero and his colleagues at MIT for their pioneering work in twistronics, where adjacent layers of graphene are twisted very slightly relative to each other to change the electronic properties of graphene. .

Spin and surprise!

“Depending on how the graphene layers are rotated and doped with electrons, contrasting results are possible. In certain dopings, the layers act as an insulator, preventing the movement of electrons. In the case of other doping, the material behaves like a superconductor, a material with zero resistance that allows the movement of electrons without dissipation,” says Tielrooij.

Better known as twisted bilayer graphene, these results occur at the so-called magic angle of misalignment, which is just over one degree of rotation. “The misalignment between the layers is small, but the possibility of creating a superconductor or an insulator is a surprising result.” Example of graphene layers twisted together. Image: Klaas-Jan Tielrooij

How electrons lose energy

For their study, Tielrooij and the team wanted to learn more about how electrons lose energy in magic angle twisted bilayer graphene, or MATBG for short.

To achieve this, they used a material composed of two sheets of monolayer graphene (each 0.3 nanometers thick), placed on top of each other and misaligned with each other by approximately one degree.

Then, using two optoelectronic measurement techniques, the researchers were able to probe electron-phonon interactions in detail and made some astonishing discoveries.

“We observed that energy decays very quickly in the MATBG: it occurs on the picosecond time scale, which is a millionth of a millionth of a second!” says Tielrooij.

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

Why do electrons lose energy?

So why do electrons lose energy so quickly when interacting with phonons? Well, it turns out that researchers have discovered a completely new physical process.

“The strong interaction between electrons and phonons is a completely new physical process and consists of the so-called Umklapp scattering between electrons and phonons,” adds Hiroaki Ishizuka of the Tokyo Institute of Technology (Japan), who developed the theoretical understanding of this process together with Leonid Levitov from Massachusetts. United States Institute of Technology.

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

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

Challenges solved together

Tielrooij and his collaborators may have completed most of the work while he was in Spain at the Catalan Institute of Nanoscience and Nanotechnology (ICN2), but as Tielrooij points out. “International collaboration was essential to make this discovery.”

So how did all the collaborators contribute to the research? Tielrooij: “First, we needed advanced manufacturing techniques to manufacture the MATBG samples. But we also needed a deep theoretical understanding of what happens in the samples. On top of that, ultrafast optoelectronic measurement setups were also required to measure what is happening in the samples.”

Tielrooij and the team received the magic angle twisted samples from Dmitri Efetov’s group at the Ludwig-Maximilians-University of Munich, which was the first group in Europe capable of fabricating such samples and which also performed photomixing measurements, while working theoretically at MIT in the United States and the Tokyo Institute of Technology in Japan proved crucial to the success of the research.

At ICN2, Tielrooij and his team members Jake Mehew and Alexander Block used state-of-the-art equipment, particularly time-resolved photovoltage microscopy, to make their measurements of electron phonon dynamics in the samples.

The future

So what will the future of these materials look like? According to Tielrooij, don’t expect anything too soon.

“Given that the material has only been studied for a few years, we are still far from seeing magic angle twisted bilayer graphene having an impact on society.”

But much remains 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 optoelectronic devices,” says Tielrooij. “In particular, they would be very useful at low temperatures, making the material suitable for space and quantum applications.”

The research by Tielrooij and the international team represents a real breakthrough when it comes to how electrons and phonons interact with each other.

But we’ll have to wait a little longer to fully understand the consequences of what the electron said to the phonon in the graphene sandwich.

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