Jun 11 2020

What Is Twistronics

It’s hard to keep up with all the latest science and technology, even for an enthusiast. Entire new fields are emerging, and it can be challenging for the non-expert to wrap their head around all the new concepts. Here is my attempt to quickly tackle a relatively new idea in physics – twistronics.

The term refers to tuning the properties of 2-dimensional materials by stacking them and rotating the layers with respect to each other. This is a lot harder than it sounds – graphene (2d carbon in a hexagonal configuration, like chicken wire) for example likes to align itself and will resist such twisting. Further, it is difficult to make pure 2d sheets without errors or contamination. But some theoretical physicists were predicting that interesting things might happen at certain “magic” angles of rotation, such a 1.1 degrees. It was then left to experimental physicists to make it happen.

This has all been happening very quickly, over the last few years. It was in 2018, in fact, that physicist Pablo Jarillo-Herrero published the first study showing the properties of graphene “devices” with a twist angle of 1.1 degrees. What he found at this magic angle was something, he now reports, that he dared not predict or even hope for, one of the holy grails of material science – superconductivity.

A superconducting material is essentially one that allows for the flow of electrons without any resistance, and therefore no loss of energy as heat. Superconductivity would transform our electrified world and allow for the creation of much more powerful and efficient electronics. Physicists have created a number of superconducting materials, but at low temperature. The trick is to make so-called high-temperature superconducting material. This first became a popular sensation in the 1980s, with the discovery of superconducting ceramics. At the time, if you believed they hype, it seemed like a matter of just years before we would have room-temperature superconductors, with devices sitting on every desktop. Reality proved much more difficult.

The current record for the highest temperature superconductor was made in 2019, at -23 degrees C. This is still amazing, with many potential applications. But it is still a long way from room temperature, or basically the temperature at which most devices would be operating, without elaborate and expensive cooling systems. We still may get there, but it is taking longer than everyone thought (which is usually the case).

So has Jarillo-Herrero finally achieved this goal? No, but he did create a proof of concept. He has demonstrated that the magic angle of twistronics is a thing. The primary benefit of this is to allow for the exploration of new physics. How, if, and when this will all translate into gadgets remains to be seen. But this only gets us up to 2018. After the 2018 study many labs around the world took up twistronics, which Jarillo-Herrero describes as an “explosion” of research. I am writing about this today because of a new study, applying the effect to photons:

“While photons — the quanta of light — have very different physical properties than electrons, we have been intrigued by the emerging discovery of twistronics, and have been wondering if twisted two-dimensional materials may also provide unusual transport properties for light, to benefit photon-based technologies,” said Andrea Alù, founding director of the CUNY ASRC’s Photonics Initiative and Einstein Professor of Physics at The Graduate Center. “To unveil this phenomenon, we used thin layers of molybdenum trioxide. By stacking two of such layers on top of each other and controlling their relative rotation, we have observed dramatic control of the light guiding properties. At the photonic magic angle, light does not diffract, and it propagates very confined along straight lines. This is an ideal feature for nanoscience and photonic technologies.”

The material at the magic angle allows for light to be conducted along a straight line at specific frequencies and without resistance, rather than just propagating in all directions.  It seems obvious that this technology, if perfected and commercialized (always a big if) can be used in photonics – devices that use photons of light instead of electrons as their primary information carrying medium. The goal of photonics is to produce faster, smaller, and cooler computing devices.

Here is a more technical description of what all this means:

“Following our previous discovery published in Nature in 2018, we found that biaxial van der Waals semiconductors like α-MoO3 and V2O5 represent an emerging family of material supporting exotic polaritonic behaviors,” said A/Prof Qiaoliang Bao, “These natural-born hyperbolic materials offer an unprecedented platform for controlling the flow of energy at the nanoscale.”

I don’t pretend to understand this, but I can unpack some of the concepts. Van der Waals forces are a class of forces that are very short range and very weak acting between close non-charged atoms, molecules or surfaces. They can create transient weak attraction, for example, at certain distances. The now classic example of this is the gecko. They can walk on ceilings because their feet contain many tiny projections (setae) that collectively produce enough van der Waals attraction to support their weight.

A “polariton” is a quasiparticle, meaning it is not really a particle but rather it is a phenomenon that acts like a particle. When you excite a particle, like an electron, the excitation itself can propagate and have some particle-like properties, so it is referred to as a quasi-particle. A polariton is simply the quasiparticle that emerges from exciting a photon.

A hyperbolic material is one that is anisotropic, meaning it has different properties in different configurations, and has unique physical properties at the nanoscale. That is, I think, the big picture here. This is one manifestation of the fact that physicists can now design and create materials that interact with subtle forces at the nanoscale, causing unique properties. This is all a relatively new and exciting area of physics. Again, if nothing else, this is creating a technique for studying new physics itself. But the hope is this new physics can be exploited to create new classes of technology.

We may be in the same place as when the first researchers were working out the physics of electricity. No one at the time imagined it would transform our world as it has. Twistronics, metamaterials, and 2d materials may have as profound an effect on our technology, but it’s hard to tell from this early stage. Still, the advances are quite exciting (pun intended).

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