Twisted bilayer graphene is a special type of stack of two single-atom-thick sheets of carbon atoms. In this stack, the atoms’ honeycomb lattices are rotated so they do not overlap. Scientists are very interested in this material because it can be a superconductor when the layers are rotated at a “magic” angle close to 1°. Superconducting materials can conduct electricity with no loss of energy as heat. Before now, scientists did not fully understand the underlying mechanism that results in this superconducting state. Researchers used a scanning tunneling microscope to “see” the electron interactions and pairings at the heart of this material’s novel properties. The scientists also modeled these electron–electron interactions.
Finding a superconducting material that works close to room temperature could revolutionize how we transmit electricity and produce electronic devices. “Unconventional” superconductors can do this at higher temperatures than other materials. Although scientists discovered these materials in 1986, they still don’t fully know how they work. This work shows how two layers of graphene placed on top of each other at a “magic” angle have properties similar to high-temperature superconductors. This material may offer new insights into how certain unconventional superconductors work.
The most common approach to investigate properties of magic-angle twisted bilayer graphene is to study how well the electricity conducts through this material. However, such an approach is incapable of providing information about the microscopic properties of the electrons, how they behave at different energies, and the spatial pattern of their wavefunctions (a mathematical expression that fully describes the quantum mechanical nature of the electron). Scientists fabricated devices made from this magic angle bilayer material. They then used a scanning tunneling microscope to study with high-resolution the electron’s wavefunctions and their energy levels. As they adjusted the density of electrons in the system by using an electrostatic gate in their samples, they were able to probe the interaction between electrons in a detailed manner. Their measurements show that at the electron density where superconductivity emerges in this system, the electrons’ interactions are so strong that their individual energy levels are very broad. This spectral broadening cannot be captured by the standard non-interacting model of electrons commonly used to understand simple metals and semiconductors. However, calculations of small clusters of highly interacting electrons, used to understand highly correlated materials such as the cuprate high-critical-temperature superconductors, produce similar findings to those seen in the experiments. The spectroscopic properties of the electron energy levels in twisted bilayer graphene show it is a strongly interacting system and a model material to understand how superconductivity emerges in the presence of such strong interactions. Solving the mystery of how superconductivity arises would help to develop materials for efficient power transmission and ushering in new technologies.
The research was primarily supported by the Department of Energy, the Office of Science, Basic Energy Sciences and Moore Foundation. Other support for the experimental work was provided by the National Science Foundation, ExxonMobil, the Princeton Catalysis Initiative, and Alexander von Humboldt Foundation. Support was provided by Japan through MEXT, A3 Foresight, and CREST, JST. Theoretical work was supported by the Princeton Center for Theoretical Science, DOE (Office of Science, Basic Energy Sciences), the Packard Foundation, and the National Science Foundation.
Publication: Y. Xie, B. Lian, B. Jäck, X. Liu, C.-L. Chiu, K. Watanabe, T. Taniguchi, B. A. Bernevig and A. Yazdani, “Spectroscopic signatures of many-body correlations in magic-angle twisted bilayer graphene,” Nature 572, 101 (2019). [DOI: 10.1038/s41586-019-1422-x]
Experiments explore the mysteries of ‘magic’ angle superconductors, Princeton University news press release