Two graphene double layers can be twisted in relation to each other (red/blue), and insulating states consisting electron-hole pairs (-/+) can form. Credit: Peter Rickhaus / ETH Zurich
From physics class, Ohm's law was well-known. The law states that current flow through conductors is determined by their resistance and voltage. The negatively charged carriers, which are electrons within the material, move in an unorganized fashion and almost independently from each other. It is more fascinating for physicists to see how the charge carriers interact with each other strongly enough that this simple picture does not hold true.
This is what happened in "Twisted Bilayer Graphene", which was discovered a few year ago. This material is composed of two layers of graphene with a single carbon atom each. The electrons can interact strongly with each other if two layers are slightly bent with respect to one another. The material can become superconducting, and thus conduct current without losses.
Klaus Ensslin, Thomas Ihn from the Laboratory for Solid State Physics at ETH Zurich and colleagues at the University of Texas at Austin (U.S.) have now discovered a new state in twisted double graphene layers. The material is no longer capable of conducting electric current because it has negatively charged electrons and positively-charged holes.
Twisted graphene layers
Peter Rickhaus, a postdoctoral researcher and the lead author of the recently published study in Science, says that "In conventional experiments in which graphene layers have been twisted by approximately one degree with regard to each other," the mobility of electrons is affected by quantum mechanical tunneling among the layers. "In our experiment, however, we twist the graphene layers by more than 2 degrees in relation to each other so that electrons cannot tunnel between them.
To measure electric current, twisted graphene is sandwiched between two dimensional insulators (left). The large voltage applied to the gate electrodes creates an electron-hole state (right). Credit: Peter Rickhaus / ETH Zurich
Coupling increases resistance
This is why electrons can be made in one layer of double-layered layers by applying an electric field. In the other, holes can be created. Both electrons as well as holes can conduct electricity. The graphene double layers would be expected to combine to make a better conductor with a lower resistance.
Folkert de Vries is a postdoctoral researcher in Ensslin’s team. He explains that in certain situations, however, the opposite can occur. "If you adjust the electric field so as to have the same amount of electrons and holes within the double layers, then the resistance suddenly rises sharply." Ensslin and his colleagues struggled for several weeks to understand this unexpected result. However, Allan H. MacDonald, an Austin theory colleague, finally gave them a clue: MacDonald had discovered a new type of density wave.
When electrons in a material conduct electric current collectively and spatially, so-called charge density wave are formed. These waves are also known as "charge density waves". The ETH researchers found that electrons and holes now form a collective density wave when they pair up by electrostatic attraction. The density wave now includes electrically neutral electron-hole pairs so that the double layers can no longer conduct electricity together.
New correlated state
Ensslin says, "That's an entirely new correlated state electrons and holes that has no overall charge." This neutral state is capable of transmitting information and conducting heat. It is also unique in that it can be controlled completely by the twisting angle or the applied voltage. Similar states have been observed with other materials where electron-hole pairs (also called excitons), are created by excitation using laser beam light. The electrons and holes in the ETH experiment are in their lowest energy state. This means they can live without spontaneous decay.
Possible application in quantum technologies
Enslin, who is an expert in investigating the electronic properties small-quantum systems, is already thinking about practical applications for the new correlation state. This will however require some preparatory work. You could capture the electron-hole pairs in a (Fabry Prot) resonator. This is difficult because neutral particles can't be controlled directly, such as with electric fields. On the other hand, the fact that the state of neutrality is electrically neutral could prove to be a benefit: It could be used to make quantum memories less sensitive to electric field noise.
Learn more about how ions get their electrons back
Peter Rickhaus and colleagues, Correlated electron hole state in twisted dual-bilayer graphene Science (2021). Information from Science Peter Rickhaus and colleagues, Correlated electron hole state in twisted dual-bilayer graphene. (2021). DOI: 10.1126/science.abc3534
