Physicists Have Created A Stable Version Of Superconducting Moire Graphene
American and Japanese physicists have found that the newly discovered superconducting properties of two superimposed pieces of graphene can be stabilized by connecting their sheets with another flat material – tungsten diselenide. Thanks to this, scientists will be able to learn the mechanisms of superconductivity in graphene, the researchers write in the scientific journal Nature.
"These experiments call into question the fact that some unusual properties of "moire" graphene indicate the existence of a form of superconductivity that is still unknown to us. On the contrary, now these properties can be easily explained using the classical theory of superconductivity or special manifestations of ferromagnetism, which are associated with the quantum Hall effect," one of the authors of the work, Professor Ronny Thomale of the University of Würzburg (Germany), commented on the discovery.
Graphene is a single layer of carbon atoms that are connected by a structure of chemical bonds similar to a honeycomb. For obtaining and studying the first samples of graphene, Russian natives Konstantin Novoselov and Andrey Geim received the Nobel prize in physics in 2010.
Two years ago, physicists at the Massachusetts Institute of Technology (MIT) accidentally turned graphene into an exotic "insulator-superconductor" by gluing two pieces of this material at a certain angle and obtaining a kind of moire pattern. With this position of the graphene sheets, the carbon atoms begin to strongly influence how the electrons move within the entire structure.
Due to this, due to the rotation of one of the graphene sheets at a certain angle, the current carriers begin to move without the loss of energy, like pairs of electrons in superconductors. If there are small deviations from this angle, the interaction of electrons creates an insurmountable barrier for other particles. The substances in which this happens are called "Mott insulators" by physicists.
Unstable graphene "sandwich"
This feature makes such materials unstable, which prevents scientists from studying them. The fact is that any small touches and deformations destroy superconductivity or radically change the properties of such a "layer" of graphene.
Physicists from the United States and Japan, led by Assistant Professor at the California Institute of Technology in Pasadena (USA) Stevan Nadj-Perde (Stevan Nadj-Perde), solved this problem by adding an additional layer to such a "sandwich" of carbon nanomaterial.
In the past, physicists have tried to stabilize moire graphene by attaching it to a gold or boron nitride substrate. This technique really worked, but all the superconducting properties of the material disappeared.
Nagy-Perde and his colleagues found that such problems can be avoided by inserting a layer of tungsten diselenide, another flat material, between boron nitride and moire graphene. In this case, as their observations showed, the opposite happens – the" sandwich "of graphene almost completely loses its insulating properties, and its superconducting properties continue to exist even with relatively large deviations from the "magic" angle.
As scientists suggest, this is since tungsten diselenide is a strong dielectric and the lattice of its atoms does not match the structure of graphene in size or structure. However, for boron nitride, arranged almost the same way as graphene, this is not typical, which unpredictably affects the distribution of electrons inside the graphene sheets.
"The creation of this material was a huge step forward in the study of superconductivity. In the past, attempts to measure the properties of different samples of moire graphene led to very different results, and scientists could not reproduce the results of experiments. Now we are close to the beginning of a new era in physics, related to the discovery of materials whose electronic and quantum properties can be flexibly controlled," Thomale concluded.