New Structure Of Graphene

Graphene is a two-dimensional carbon based material, consisting of an atomic thick carbon layer, which can be produced by stripping from the same graphite found in the pencil lead. The ultrathin material consists entirely of carbon atoms arranged in simple hexagonal patterns. Since the first separation of graphene in 2004, scientists have found that graphene exhibits many extraordinary properties in its monolayer form.

In 2018, scientists at the Massachusetts Institute of Technology (MIT) found that if two graphene layers were stacked at a very specific "magical" angle, the twisted double-layer structure could show strong superconductivity. In this widely sought for material state, the current can flow with zero energy loss. Recently, the same group of researchers found a similar superconducting state in twisted three-layer graphene-a structure made up of three graphene layers stacked at a precise, new "magic angle.".

Now, the team reports that four and five layers of graphene can be twisted and stacked with new "magic angles" to generate strong superconductivity at low temperatures. This latest discovery, published in the journal Nature Materials on July 7, 2022, establishes various twisted and stacked configurations of graphene as the first known "family" of multilayer magic angle superconductors. The team also identified similarities and differences between members of the graphene family.

This new discovery can be used as a blueprint for designing practical room temperature superconductors. If the properties among family members can be replicated in other natural conductive materials, they can be used, for example, to transmit electricity without dissipation losses, or to build friction free maglev trains.

"The magic horned graphene system is now a legitimate 'family' that goes beyond several systems," said Jeong min (Jane) park, the lead author and a graduate student in the Department of physics at MIT. "Having this family is particularly interesting because it provides a way to design robust superconductors."

Park's MIT co authors include Cao Yuan, Xia Liqiao, sun Shuwen and Pablo Jarillo Herrero, physics professors of Cecil and Ida green, and Kenji Watanabe and Takashi Taniguchi of the National Institute of Materials Science in Tsukuba, Japan.

Jarillo Herrero's team was the first to discover magic horned graphene in the form of a double-layer structure of two graphene sheets, one on top of the other, slightly offset at a precise angle of 1.1 degrees. This twisted configuration, known as a molar superlattice, transforms the material into a strong and durable superconductor at ultra-low temperatures.

The researchers also found that the material exhibits an electronic structure called a "flat band" in which electrons in the material have the same energy regardless of their momentum. In this flat band state, and at very low temperatures, the normally fanatical collective of electrons slows down enough to pair in so-called Cooper pairs - the fundamental component of superconductivity that can flow through the material without resistance.

Although researchers have observed that twisted bilayer graphene exhibits both superconductivity and flat band structure, it is not clear whether the former comes from the latter.

"There is no evidence that flat band structures lead to superconductivity. Since then, other groups have produced other twisted structures from other materials, which have some flat banded structures, but they don't really have strong superconductivity. So we wonder. Can we make another flat tape superconductor?" Park said

The results from the University of graphene will also show that the calculation of 1.6 degrees of twisted graphene from the University's point of view will also show that they have a three degree superconductivity problem. They continue to show that if stacked and twisted in the right way, there should be no limit to the number of graphene layers that exhibit superconductivity in their predicted angle. Finally, they demonstrated that they could mathematically associate each multilayer structure with a common flat band structure - a powerful proof that flat bands can lead to strong superconductivity.

"They found that there might be layers of this entire graphene structure, to an infinite layer, that might correspond to a similar mathematical expression of the flat band structure," Park said.

Not long after this work, Jarillo Herrero's team found that superconductivity and flat ribbons did appear in the twisted three-layer graphene sheet, stacked like a cheese sandwich, with the middle cheese layer moving 1.6 degrees relative to the sandwiched outer layer. But there are subtle differences between the three-layer structure and its double-layer counterpart.

"This leads us to ask, where are the two structures appropriate in terms of the entire material category, and are they from the same family?" Park said.

An unconventional family

In the current study, the team hopes to increase the number of graphene layers. They made two new structures, four and five graphene layers. Each structure is stacked alternately, similar to a twisted three-layer graphene shift cheese sandwich.

The team placed these structures in refrigerators below 1 kelvin (about - 273 degrees Celsius), passed current through each structure, and measured the output under various conditions, similar to testing its two-layer and three-layer systems.

Overall, they found that four and five layers of twisted graphene also exhibit strong superconductivity and a flat band structure. These structures also have other similarities with their three-layer counterparts, such as their reactions to magnetic fields of different intensities, angles and directions.

These experiments show that the twisted graphene structure can be considered as a new family or a common superconducting material. Experiments have also shown that there may be a "black sheep" in the family. The original twisted double-layer structure, although sharing key attributes, also shows subtle differences with its "brothers and sisters". For example, the team's previous experiments have shown that the superconductivity of the structure is broken at lower magnetic fields and more inhomogeneous with the rotation of the magnetic field compared with the multilayer structure.

The team simulated each type of structure to find an explanation for the differences between family members. They concluded that the fact that the superconductivity of twisted bilayer graphene disappears under certain magnetic field conditions is only because all the physical layers exist in the structure in the form of "non mirror image". In other words, there are no two layers in the structure that are mirror opposite to each other, while the multilayer structure of graphene shows some mirror symmetry. The discovery of strong electron driven mechanisms in the whole graphene family indicates that superconductivity is driven by the same mechanism.

"It's quite important," Park points out. "If you don't know this, one might think that bilayer graphene is more traditional than multilayered structure. But we suggest that the whole family may be unconventional, robust superconductors."