A pair of researchers, one with Peking University, the other with Princeton University, has found that the parameters of twisted-graphene’s excitation spectra correspond directly to attributes of the heavy fermion model. In their paper published in the journal Physical Review Letters, Zhi-Da Song and B. Andrei Bernevig describe building a model to show aspects of the Bistritzer-MacDonald model and then used it to demonstrate characteristics of twisted bilayer graphene. Aline Ramires with the Paul Scherrer Institute has published a News & Views piece in the journal Nature outlining the work by Bernevig and Song.
Graphene is a flat, 2D sheet of carbon and a subject of considerable research. One research effort four years ago involved placing one sheet of graphene on top of another and then twisting the top sheet. After much trial and error, those researchers found that twisting the top sheet a certain amount (1.05 degrees) led to the creation of a superconductor. That led them to refer to the twisted amount as a “magic angle.”
Since that time, other researchers have been studying the attributes of twisted bilayer graphene aligned at its magic angle. In this new effort, the researchers studied its excitation spectra and found it corresponded to the parameters of the fermion model.
Prior work has shown that twisted bilayer graphene at just the right orientation takes on some unique properties—one set of electrons, for example, moves around, which accounts for its conductivity. But another set of electrons remain fixed. The two contradictory characteristics of the material allow scientists to push a sample between an insulator and a superconductor.
To better understand why this happens, Song and Bernevig created a model of the system and then used it to carry out exact calculations describing the behavior of the material. They found that they were able to describe the structure of twisted bilayer graphene as it compared to heavy fermion materials. More work showed that the parameters of the material corresponded directly to the parameters of the heavy fermion model. Heavy fermion materials are those that are found at the bottom of the periodic table. More information: Zhi-Da Song et al, Magic-Angle Twisted Bilayer Graphene as a Topological Heavy Fermion Problem, Physical Review Letters (2022). DOI: 10.1103/PhysRevLett.129.047601
Aline Ramires, Twisted-graphene model draws inspiration from heavy elements, Nature (2022). DOI: 10.1038/d41586-022-02108-w Journal information: Physical Review Letters , Nature