Graphene continues to amaze the scientists with its extensive properties
The “magic angle” between graphene layers exhibited strange insulation & superconducting properties
Ever since the discovery of the “wonder material” in 2004, scientists have been intrigued by the range of properties exhibited by Graphene — a single layer of carbon atoms linked in a hexagonal honeycomb pattern. The one that stands out among the rest is the strength of the material despite having a simple two-dimensional structure. Although not a metal, it conducts electricity at ultrahigh speeds, better than most metals.
These are not the only properties though — back in 2018, researchers at MIT discovered that when graphene layers are stacked on top of each other, their electrical properties change. They figured that when two sheets of graphene were stacked together at a “magic angle” of 1.1 degrees, it could act as an insulator or a superconductor at the same time. This monumental discovery helped launch a new field known as “twistronics” — the study of electronic behavior in twisted graphene and other materials.
Last year, Stanford researchers added to the long line of breakthroughs for graphene, showing that graphene arranged in a specific way can generate a magnetic field. This was the practical proof of magnetism which was only theorized previously. The magnetism achieved was different from Ferromagnetism, which is the most common type found in materials. Known as Orbital Ferromagnetism, it came about as a result of the electrons’ orbital motions lining up.
“These two studies are aiming to better understand the puzzling physical behavior of magic-angle twistronics devices. Once understood, physicists believe these devices could help design and engineer a new generation of high-temperature superconductors, topological devices for quantum information processing, and low-energy technologies.” ~ Yuan Cao, Researcher
Building on the study from two years ago, MIT scientists are now reporting the latest advancements in graphene twistronics. The same team that discovered the paradoxical properties of Graphene in 2018, has now conducted two new studies to examine the twisted bilayer graphene structures (two layers at a 0-degree angle from each-other), even more closely.
The team approached researchers at the Weizmann Institute for Science, who had developed a precise atomic-level scanning technique they call “scanning nano-SQUID,” where SQUID stands for Superconducting Quantum Interference Device. Employing this technique, they tested the effects of different angles between graphene sheets, measuring angle differences down to 0.002 degrees.
In the first study, they found that the insulating and superconductive properties were more pronounced when the angle between the graphene sheets stayed closer to 1.1 degrees. They found that the structure exhibited exotic properties were more pronounced in narrow angels than in structures with a wider range of twist angles.
The second study aimed at analyzing the same properties in a four-layer twisted graphene structure as compared to two layers in the first study. When they twisted four sheets of graphene to the magic angle, it became an insulator just like with two sheets, albeit with a difference. This time, they could fine-tune its insulating ability by utilizing certain electric and magnetic fields — something that was not possible with its two-layered predecessor.
The team believes it is still very early stages of research in the field of twisted graphene system and the scientific community is still in awe of this new discovery. However, it is showing some great promise for the production of exotic materials of the future.