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John O | March 2018

Rotated at a perfect angle graphene can be an insulator or superconductor

By Josh Perry, Editor


Researchers from the Massachusetts Institute of Technology (MIT) and Harvard University, both in Cambridge, Mass., have discovered that graphene can be tuned as both an electrical insulator and as a superconductor.


Physicists at MIT and Harvard University have found that graphene, a lacy, honeycomb-like sheet of carbon atoms, can behave at two electrical extremes. (MIT)


According to a report from MIT, the researchers created superconductivity by forming a super-lattice of two graphene sheets stacked together and rotated at a “magic angle” of 1.1 degrees.


“As a result, the overlaying, hexagonal honeycomb pattern is offset slightly, creating a precise moiré configuration that is predicted to induce strange, ‘strongly correlated interactions’ between the electrons in the graphene sheets,” the report explained. “In any other stacked configuration, graphene prefers to remain distinct, interacting very little, electronically or otherwise, with its neighboring layers.”


At the magic angle, the graphene sheets acted as Mott insulators, with no conductive behavior, but when voltage was applied to the sheets the small amount of electrons added to the super-lattice “broke out of the initial insulating state and flowed without resistance, as if through a superconductor.”


To create the graphene super-lattice, researchers exfoliated a flake of graphene from graphite and picked up half of the flake with a glass slide coated with adhesive polymer and boron nitride. The glass was rotated slightly to pick up the other half to create an offset pattern.


“The team repeated this experiment, creating several ‘devices,’ or graphene superlattices, with various angles of rotation, between 0 and 3 degrees,” the report continued. “They attached electrodes to each device and measured an electrical current passing through, then plotted the device’s resistance, given the amount of the original current that passed through.”


At 1.1 degrees, the super-lattice displayed the properties of a Mott insulator, which is an interesting class of materials that have the energy band structure of electrical conductors but behave as insulators. Previous research has demonstrated that Mott insulators can be doped into superconductors at high temperatures, so the MIT team attempted to do the same to the super-lattice by adding voltage.


“Individual electrons bound together with other electrons in graphene, allowing them to flow where before they could not,” the article said. “Throughout, the researchers continued to measure the electrical resistance of the material, and found that when they added a certain, small amount of electrons, the electrical current flowed without dissipating energy — just like a superconductor.”


The research was recently published in Nature. The abstract stated:


“van der Waals heterostructures are an emergent class of metamaterials that consist of vertically stacked two-dimensional building blocks, which provide us with a vast tool set to engineer their properties on top of the already rich tunability of two-dimensional materials.


“One of the knobs, the twist angle between different layers, has a crucial role in the ultimate electronic properties of a van der Waals heterostructure and does not have a direct analogue in other systems such as MBE-grown semiconductor heterostructures. For small twist angles, the moiré pattern that is produced by the lattice misorientation creates a long-range modulation.


“So far, the study of the effect of twist angles in van der Waals heterostructures has been mostly concentrated in graphene/hexagonal boron nitride twisted structures, which exhibit relatively weak interlayer interaction owing to the presence of a large bandgap in hexagonal boron nitride. 


“Here we show experimentally that when two graphene sheets are twisted by an angle close to the theoretically predicted ‘magic angle’ the resulting band structure near charge neutrality becomes flat owing to the strong interlayer coupling.


“These flat bands exhibit insulating phases at half-filling, which are not expected in a non-interacting picture. We show that the half-filling states are consistent with a Mott-like insulator state that can arise from electrons localized in the moiré superlattice. These unique properties of magic-angle twisted bilayer graphene may open a new playground for exotic many-body quantum phases in a two-dimensional platform without magnetic field.


“The easy accessibility of the flat bands, the electrical tunability, and the bandwidth tunability though twist angle may pave the way towards more exotic correlated systems, such as unconventional superconductors or quantum spin liquids.”

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