Two sheets of transparent plastic with similar line patterns printed on them will display surprisingly beautiful new patternes if you rotate one layer a bit on top of the other - a moiré-pattern occurs. The same happens in the nano-world.

All over the World materials researchers are at present focusing intensely on a specific two-dimensional material consisting of two one-atom thick layers of carbon atoms placed in close contact on top of each other, and with a definite angle of rotation between them. This material; graphene shows such a wealth of promising properties that the discovery resulted in a Nobel Prize in 2010. In the paper in question such a rotated double layer of graphene is under study. A special property of the double layer is that you can control the angle of rotation, and this induces a singularity in the electronic quantum states of the material. A singularity is a feature in the material where normal properties are radically changed. Normally a single layer of graphene behaves like a metal, but the occurrence of this specific singularity makes it possible to induce superconductivity in the two rotated layers, causing an electric current to traverse without energy loss.

In the paper in Advanced Materials PhD students Alfred Jones and Paulina Majchrzak from Søren Ulstrup's research group at IFA share first authorship. They have measured directly the electronic singularity in such a rotated double layer of graphene, and for the first time shown that it is possible to move the energy of the singularity, while using the material in a real device architecture, but at nano-size.

For the experiment the researchers took the two layers of graphene and slapped them onto each other on a semiconductive base. Using nanolithography these raw materials could then be connected electrically to a microchip, making them behave like a functional quantum component. Measurements of the singularity were made by inserting this micro chip in an ultra-high vacuum chamber at the synchrotron radiation source Diamond Light Source in England. Here it is possible to focus the x-ray radiation from the synchrotron to a beam of nano-dimensions and use it to directly measure images of the quantum states in the material.

By tuning the voltage drop over the microchip during the measurements the researchers were able to shift the singularity across a surprisingly wide range of energies in the device.

With this result it is now possible to predict what angles of rotation and voltages will be practically useful if you wish to build quantum components consisting of rotated layers of graphene in order to induce superconductivity in electric mini-circuits,, which will provide essential feedback to design future electronic components.

The final version of the paper is available via this link.