The study reveals a Moiré nematic phase in the twisted double-double graph

The study reveals a Moiré nematic phase in the twisted double-double graph

Scanning tunneling microscopy images of the controllable nematic phase of electrons in the Moiré graph. To the left is the normal phase where the electrons are not arranged. To the right is the nematic phase, where clear streaks are observed. Credit: Rubio-Verdú et al.

Moiré superlattices are systems in which two sheets of a material are placed on top of each other with a small angular displacement, resulting in a characteristic pattern known as the Moiré pattern. In recent years, many physicists and materials scientists have studied the properties of these materials, as they may exhibit superconductivity and other interesting phases or characteristics.

Researchers at Columbia University and other institutes around the world have recently revealed the presence of a Moiré nematic phase in twisted double-layer graphene, a graphene-based Moiré superlattice. This finding, presented in a paper published in Natural physics, could pave the way for a better understanding of these highly studied material structures.

“My group and several others have been interested in the properties of twisted van der Waals materials such as graphene after it was discovered that these materials may host interesting quantum phases as superconductivity in 2018,” Abhay N. Pasupathy, one of The researchers who conducted the study told Phys.org. “The simple question we wanted to answer is what other kinds of interesting quantum phases could these materials exhibit? So in a way, ours was an open search without a preconceived goal.”

A nematic phase, like the one Pasupathy and his colleagues observed in the twisted bilayer graph, is a condition first seen by the spontaneous arrangement of molecules in parallel patterns, where they all face in the same direction. This phase is often observed in liquid crystals, but it can also be found in other materials.

“Imagine you bought a lot of baguettes from the store and you don’t organize them well, just throw them in a giant bag,” Pasupathy explained. “If you look into the bag, each baguette will point in a random direction. If you now start shaking the bag and try to make it more compact, you will find that many of the baguettes will line up with each other and will all point. in the same direction. That direction could have been anything, but in the end they choose a direction that they all have to line up in. “

The example from Pasupathy summarizes what happens in a nematic phase, a condition in which individual particles in a material spontaneously line up in a particular direction and produce a directional order. Nematous phases in liquid crystals have been utilized to create many modern technologies, such as LCD screens. In the case of the material observed by scientists at Columbia, the constituent particles are electrons.

“The electrons are not pointed like a baguette, but instead one can think of how electrons move in a solid,” Pasupathy said. “If electrons choose to move more along one direction than another, this would be the corresponding nematic phase.”

To perform their experiments, Pasupathy and his colleagues used an accurate imaging technique called scanning tunneling microscopy. This technique allows scientists to carefully study individual atoms inside a material and the motion of electrons contained in them.

“Using this method, we were able to ‘see’ that the electrons were in a nematic phase,” Pasupathy explained. “We discovered that when the conditions were just right, the electrons in the material spontaneously entered the nematic phase, and that they could be tuned in and out of this phase at will by applying voltages. Right now, the implication of this work is to show how electrons in these materials can interact with each other in unusual ways, giving rise to unexpected properties. “

The results collected by this team of researchers show that by applying specific stresses to the twisted bilayer graph, one can cause it to spontaneously enter a nematic phase. In addition, they suggest in their paper that this phase is not related to the specific structure of the graphene lattice, but instead is a phenomenon that can spontaneously appear in all Moiré lattices.

Currently, many physicists and materials scientists are discussing and investigating the possible causes of superconductivity and other interesting phases observed in van der Waals materials and Moiré superlattices. One possibility is that the repulsion between electrons in these materials plays a crucial role in the emergence of these phases.

Although the results of this recent study do not offer conclusive evidence that this is the case, they do show that interactions between electrons can actually affect the properties of van der Waals materials. In the future, this may inspire similar studies examining the nematic phase, which researchers are observing more in depth, as well as the role electrons may play in inducing it.

“We now plan to explore other materials in the same category as the twisted double layer graph,” Pasupathy added. “At this point, research is still fundamental science, where we are trying to discover new properties of these materials.”


Nematicity is a new piece in the double two-layer graph phase diagram puzzle


More information:
Carmen Rubio-Verdú et al, Moiré nematic phase in twisted double-double graph, Natural physics (2021). DOI: 10.1038 / s41567-021-01438-2

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Citation: Study reveals a Moiré nematic phase in twisted double two-layer graph (2022, 31 January) retrieved 31 January 2022 from https://phys.org/news/2022-01-unveils-moir-nematic-phase-bilayer.html

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