Using the Advanced Light Source (ALS) to study twisted bilayer graphene (TBG) systems, researchers found intriguing spectroscopic features in a superconducting “magic-angle” TBG—features that are absent in non-superconducting TBG.
The results provide crucial information on superconductivity in magic-angle TBG for next-gen electronics and advanced energy technologies.
Searching for the science behind the magic
Two-dimensional materials like graphene give scientists great flexibility in engineering electronic properties because they can be stacked like sheets of paper. Besides choosing what materials to stack and in what order, researchers can manipulate the electrical and optical properties of these stacks by controlling the twist angle between layers. Because of this versatility, two-dimensional materials provide an ideal platform for investigating the complex interplay between phenomena such as band topology, strong electron correlation, magnetism, and superconductivity, all of which are relevant to next-gen electronics and advanced energy technologies.
So-called “magic-angle” twisted bilayer graphene (MATBG) attracts broad research interest primarily because of its surprising and unusual superconducting properties, which resemble those of high-temperature superconductors. The superconductivity in MATBG devices is thought to arise from flat bands that form in a material’s electronic band structure when two layers of graphene are twisted at a “magic” angle of about 1.08 degrees. Flat bands indicate a high density of states, which significantly enhances electron interactions and the resulting potential for exotic phenomena. Despite intensive experimental efforts, the origin of MATBG superconductivity remains elusive.
Advanced micro-ARPES at the ALS
Angle-resolved photoemission spectroscopy (ARPES) is a powerful tool for probing the fine electronic structure of materials and has played a pivotal role in unraveling the mechanisms of high-temperature superconductivity and discovering novel topological quantum materials. However, due to the microscale dimensions of MATBG devices, traditional ARPES techniques, typically limited to a spatial resolution of hundreds of microns, cannot be directly applied. Persistent efforts have led to the development of advanced micro- and nano-ARPES techniques, extending ARPES research to sub-micrometer quantum materials and devices.
Here, researchers systematically characterized the electronic structure of several twisted bilayer graphene (TBG) devices using micro-ARPES at ALS Beamline 7.0.2 (MAESTRO), where a world-leading nano-ARPES system has also been developed. The high-quality moiré superlattice and superconductivity of these devices were characterized by a group at Princeton University.
Read more on ALS website
Image: Artistic depiction of a twisted bilayer graphene system. A very slight twist in the alignment of the two graphene layers creates a moiré pattern—a periodic modulation of the electronic environment that can give rise to exotic behaviors such as superconductivity. Advanced nanoscale probes provide important clues as to the connections between electronic structure and material properties.