2019 BSJ Blog Spring 2019

Superconductivity at “Magic Angles”

by Meera Aravinth

Graphite, otherwise known as pencil lead, is an everyday material that most people never think about twice. For physicists, however, this mundane item has been a gateway to an interesting exploration of  2D materials and their properties. Graphene is a single layer of carbon atoms and can be formed by using Scotch tape to lift single layers from pencil lead. A layer of graphene is only one atom thick, and has a crystal lattice extending in the other two dimensions (imagine a sheet of paper), allowing it to be approximated as a 2D material.

A team of scientists from MIT, Harvard, and the National Institute for Material Science investigated a “2D superlattice” made of graphene. The material, called Magic Angle Twisted Bilayer Graphene (MA-TBG), is formed by stacking two layers of graphene on top of each other. The scientists found that when these layers are twisted out of alignment, various magnetic phenomena such as superconductivity occur.

Superconductivity is a state where a material’s electrical resistance goes to zero and it expels all magnetic flux fields. In this state, a material becomes a perfect conductor, where electrical currents can move through the material unimpeded. This phenomenon so far has only been observed at very low temperatures, as the superconducting state only emerges after the material passes through a specific critical temperature. Superconductors have many interesting properties and potential technological applications that make them of special interest. Many studies such as this one can help physicists better understand superconductivity, as well as continuing an exploration into the physics of 2D materials.

Of particular interest in the MIT study was the observation that the Bilayer Graphene entered a superconducting state when twisted to a 1.1 degree angle. MA-TBG entered a superconducting state at a temperature of  approximately 1.7K, and various “magic angles” were observed where the bilayer graphene displayed interesting magnetic properties.

This material has many interesting applications for the future. The dependence on very specific angles for the emergence of magnetic phenomena makes MA-TBG a  “highly tunable” platform for studying electron physics and superconductivity. Overall, MA-TBG has opened doors in the investigation of unconventional superconductivity in materials and has revealed many questions about how these magnetic phenomena emerge in nature.

 

References:

Yuan Cao, et al. Magic-angle graphene superlattices: a new platform for unconventional

superconductivity.  Nature 556, p. 43-50 (2018).