Researchers examine twisted bilayer graphene's intriguing interactions with light

In 'magic angle' graphene, especially near the angle of 1 degree, the electrons slow down dramatically, favoring interactions between the electrons. Such interactions give rise to a new type of superconductivity and insulating phases in twisted bilayer graphene. Along with many other fascinating properties discovered in the past three years, this material has proven to display extremely rich physical phenomena, but most importantly, it has shown to be an easily controllable quantum material. Up until now, the interaction between twisted bilayer graphene and light was shown to have fascinating outcomes on a theoretical level, but no experiment has so far been able to clearly show how this interaction works.

In a recent work, ICFO researchers Niels Hesp, Iacopo Torre, David Barcons-Ruiz and Hanan Herzig Sheinfux, led by ICREA Prof. at ICFO Frank Koppens, in collaboration with the research groups of Prof. Pablo Jarillo-Herrero (MIT), Prof. Marco Polini (University of Pisa), Prof. Efthimios Kaxiras (Harvard), Prof. Dmitri Efetov (ICFO) and NIMS (Japan), have found that twisted bilayer graphene can be used to guide and control light at the nanometer scale. This is possible thanks to the interaction between light and the collective movement of the electrons in the material.

Moiré graphene may assist in harnessing Bloch oscillations

For many years, scientists have been trying to harness Bloch oscillations, an exotic kind of behavior by electrons that could introduce a new field of physics and important new technologies. Now, MIT physicists report on a new approach to achieving Bloch oscillations in recently introduced graphene superlattices. Graphene's electronic properties undergo an interesting transformation in the presence of an “electric mesh” (a periodic potential), resulting in new types of electron behavior not seen in pristine materials. In their recent work, the scientists show why graphene superlattices may be game changers in the pursuit of Bloch oscillations.

Normally, electrons exposed to a constant electric field accelerate in a straight line. However, Quantum Mechanics predicts that electrons in a crystal, or material composed of atoms arranged in an orderly fashion, can behave differently. Upon exposure to an electric field, they can oscillate in tiny waves—Bloch oscillations. “This surprising behavior is an iconic example of coherent dynamics in quantum many-body systems,” says Leonid Levitov, an MIT professor of physics and leader of the current work. Levitov is also affiliated with MIT’s Materials Research Laboratory.

Graphene oxide foam helps filter toxins from drinking water

MIT-led research team uses graphene oxide foam in a device that can extract uranium and other heavy metals from tap water.

Using graphene foam to filter toxins from drinking water image

Some kinds of water pollution, such as algal blooms and plastics that foul various bodies of water, are found in plain sight. However, other contaminants are not quite as visible, which potentially makes them more dangerous. Among these invisible substances is uranium. Leaching into water resources from mining operations, nuclear waste sites, or from natural subterranean deposits, the element can reach taps worldwide.

'Magic angle' trilayer graphene found to act as rare "spin-triplet" superconductor

Researchers at MIT and Harvard University have previously found that graphene can have exotic properties when situated at a 'magic angle'. Now, a new study by some of the members of the same team shows that this material could also be a "spin-triplet" superconductor – one that isn't affected by high magnetic fields – which potentially makes it even more useful.

"The value of this experiment is what it teaches us about fundamental superconductivity, about how materials can behave, so that with those lessons learned, we can try to design principles for other materials which would be easier to manufacture, that could perhaps give you better superconductivity," says physicist Pablo Jarillo-Herrero, from the Massachusetts Institute of Technology (MIT).

Researchers create tunable superconductivity in magic-angle twisted trilayer graphene

When two sheets of graphene are stacked atop each other at just the right angle, the layered structure morphs into an unconventional superconductor, allowing electric currents to pass through without resistance or wasted energy. This “magic-angle” transformation in bilayer graphene was observed for the first time in 2018 in the group of Pablo Jarillo-Herrero at MIT. Since then, scientists have searched for other materials that can be similarly twisted into superconductivity, but for the most part, no other twisted material has exhibited superconductivity other than the original twisted bilayer graphene.

Stacking order imageIllustrations of A-tw-A stacking (a) and A-tw-B stacking (b). Image from Nature

In a recent paper, Jarillo-Herrero and his group reported observing superconductivity in a sandwich of three graphene sheets, the middle layer of which is twisted at a new angle with respect to the outer layers. This new trilayer configuration reportedly exhibits superconductivity that is more robust than its bilayer counterpart.