Researchers from Columbia University, Harvard University, Japan's National Institute for Materials Science and Austria's University of Innsbruck have studied the structural and electronic properties of twisted trilayer graphene using low-temperature scanning tunneling microscopy at twist angles for which superconductivity has been observed.
The discovery of superconductivity in two layers of graphene arranges in the "magic angle" of 1.1 degrees has become quite famous. With just two atom-thin sheets of carbon, researchers discovered a simple device to study the resistance-free flow of electricity, among other phenomena related to the movement of electrons through a material. Adding a third layer of graphene improves the odds of finding superconductivity, but the reason was unclear. Now, the researchers of the new study reveal new details about the physical structure of trilayer graphene that help explain why three layers are better than two for studying superconductivity.
Using a microscope capable of imaging down to the level of individual atoms, the team saw that groups of atoms in some areas were scrunching up into what Simon Turkel, a Ph.D. student in the Pasupathy Lab, dubbed "twistons." These twistons appeared in an orderly fashion, allowing the device as a whole to better maintain the magic angles necessary for superconductivity to occur.
It's an encouraging result according to Joshua Swann, a Ph.D. student at Columbia who built the device for the study. "I've made 20 or 30 bilayer graphene devices and seen maybe two or three that superconducted," he said. "With three layers, you can explore properties that are hard to study in bilayer systems."
Those properties overlap with a class of complex materials called the cuprates, which superconduct at a relatively high temperature of -220 °F. A better understanding of the origins of superconductivity could help researchers develop wires that won't lose energy as they conduct electricity or devices that won't need to be kept at costly-to-maintain low temperatures.
In the future, researchers hope to link what they see in their scans with measurements of quantum phenom in trilayer devices. "If we can control these twistons, which all depend on the angle mismatch between the top and bottom layers of the device, we can do systematic studies of their effects on superconductivity," said Turkel. "It's an exciting open question."