March 1, 2024

What did the electron “say” to the phonon in the graphene sandwich?

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Illustration showing control of energetic relaxation with torsion angle. Credit: Science Advances (2024). DOI: 10.1126/sciadv.adj1361

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Illustration showing control of energetic relaxation with torsion angle. Credit: Science Advances (2024). DOI: 10.1126/sciadv.adj1361

A collaboration led by TU/e ​​and the Catalan Institute of Nanoscience and Nanotechnology involving researchers from around the world has the answer and why, and the results have been published in the journal Science Advances.

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.

Recent work has revealed that layers of graphene twisted relative to each other 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 led a study on electron-phonon interactions in graphene layers. And they made a surprising discovery.

What did the electron say to the phonon between two layers of graphene? This might sound like the start of a physics meme with a hilarious joke to follow. But that is not the case, according to Klaas-Jan Tielrooij. He is an associate professor in the Department of Applied Physics and Science Education at TU/e ​​and research leader of the new work published in Science Advances.

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

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

“However, phonons are not particles like electrons, they are a quasi-particle. However, their interaction with electrons in certain materials and how they affect the energy loss in electrons has been a mystery for some time,” notes 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 like graphene, which we will look at more of in the future.”

Tielrooij and his collaborators, based around the world in Spain, Germany, Japan and the USA, decided to study electron-phonon interactions in a very particular case – inside two layers of graphene, where the layers are slightly misaligned. .

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

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

“Depending on how the graphene layers are rotated and doped with electrons, contrasting results are possible. For certain dopings, the layers act as an insulator, which prevents electron movement. For other dopings, 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 tiny, but the possibility of a superconductor or insulator is a surprising result.”

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 consisting of two sheets of monolayer graphene (each 0.3 nanometers thick), placed one on top of the other and offset relative to each other by about one degree.

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

“We observed that energy disappears very quickly in MATBG – it occurs on the picosecond timescale, 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°C). “At these temperatures, it is very difficult for electrons to lose energy to phonons, but this happens in MATBG. We observed that energy disappears very quickly in MATBG – it occurs on the picosecond time scale, which is a millionth of a millionth of a second.”

Why electrons lose energy

So why are electrons losing energy so quickly through interaction with phonons? Well, it turns out that researchers have discovered an entirely new physical process.

“The strong electron-phonon interaction is a completely new physical process and involves so-called Umklapp electron-phonon scattering,” adds Hiroaki Ishizuka from the Tokyo Institute of Technology in Japan, who developed the theoretical understanding of this process together with Leonid Levitov, of Massachusetts. US 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 dispersion all the time, as it affects the ability of (non-metallic) materials at room temperature to conduct heat. Just think of an insulating material in the handle of a pan, for example,” says Ishizuka. “However, electron-phonon Umklapp scattering is rare. Here, however, we observe for the first time how electrons and phonons interact through Umklapp scattering to dissipate electron energy. Strong electron-phonon interaction is a completely new physical process and involves so-called Umklapp electron-phonon scattering.”

Challenges solved together

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

So how did all the collaborators contribute to the research? Tielrooij says: “First, we needed advanced manufacturing techniques to make the MATBG samples. But we also needed a deep theoretical understanding of what’s happening in the samples. Additionally, ultrafast optoelectronic measurement setups were needed to measure what’s happening in the samples as well. International collaboration proved essential in making this discovery.”

Tielrooij and team received the magic-angle-twisted samples from Dmitri Efetov’s group at the Ludwig-Maximilians-Universität in Munich, which was the first group in Europe capable of making such samples and which also performed photomixing measurements while working theoretically at MIT. in the US and at 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 cutting-edge equipment, particularly time-resolved photovoltage microscopy, to perform their measurements of electron-phonon dynamics in the samples.

The future

So what does the future look like for these materials? According to Tielrooij, don’t expect anything anytime soon.

“As the material has only been studied for a few years, we are still a long way from seeing magical angle-twisted bilayer graphene having an impact on society.”

But there is a lot to be explored about energy loss in the material.

“Future discoveries could have implications for the dynamics of charge transport, which could have implications for future ultrafast optoelectronic devices,” says Tielrooij. “In particular, they would be very useful at low temperatures, which makes the material suitable for space and quantum applications.”

Tielrooij and the international team’s research is a true breakthrough in terms of how electrons and phonons interact with each other.

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

More information:
Jake Dudley Mehew et al, Umklapp-assisted ultrafast electron-phonon cooling in magic-angle twisted bilayer graphene, Science Advances (2024). DOI: 10.1126/sciadv.adj1361

Diary information:
Science Advances

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