Researchers Find Highly Correlated Electron Properties In Stacks Of Graphene

A team of researchers led by the University of Washington has shown that carefully constructed stacks of graphene can exhibit highly correlated electron properties. Evidently, this type of collective behaviour likely relates to the emergence of exotic magnetic states.

Highly correlated electron properties??
Correlated electron properties mean that all the electrons start doing the same thing or displaying the same properties at the same time.
That means you need not describe what an individual electron is doing but have to describe what all electrons are doing at once.
In simpler terms, it’s like having a room full of people wherein a change in any one person’s behaviour will cause everyone else to change accordingly.

A paper outlining the research appeared in Nature Physics journal on September 14^th

“We’ve created an experimental setup that allows us to manipulate electrons in the graphene layers in a number of exciting new ways,” said co-senior author Matthew Yankowitz, a University of Washington assistant professor of physics and of materials science and engineering.

The research team led by Yankowitz worked with bilayer graphene that is two sheets of graphene naturally layered together.

A lot of researchers in the past have stacked 2D materials and have discovered that, if we stack and rotate 2D materials in a particular configuration and expose them to extremely low temperatures, these stacked layers can exhibit exotic and unexpected properties.

Thus, Tthe team stacked one bilayer of graphene on top of another for a total of four graphene layers and twisted them so that the layout of carbon atoms between the two bilayers were slightly out of alignment.

With specific configurations of the electric field and charge distribution across the stacked bilayers, electrons display highly correlated behaviours.

The team found that at temperatures near absolute zero, they could “tune” the system into a type of correlated insulating state where it would not conduct any electrical charge. Near these insulating states, the team found pockets of highly conducting states with features resembling superconductivity.

As mentioned a lot of research teams are working on these things and some teams have recently reported these superconducting states. But the origins of these features remained a mystery.

Now, the current team’s work has found evidence for a possible explanation. They found that these states appeared to be driven by a quantum mechanical property of electrons called “spin“.

In regions near the correlated insulating states, they found evidence that all the electron spins spontaneously align. This may indicate a form of ferromagnetism is emerging and not superconductivity. But additional experiments would need to verify this.

Journal Reference:
Minhao He, Yuhao Li, Jiaqi Cai, Yang Liu, K. Watanabe, T. Taniguchi, Xiaodong Xu, Matthew Yankowitz. Symmetry breaking in twisted double bilayer graphene. Nature Physics, 2020; DOI: 10.1038/s41567-020-1030-6

Press Release: University of Washington