Researchers detect 'twistons' that assist the magic angles necessary for superconductivity in trilayer graphene

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.

Researchers deepen understanding of unconventional superconductivity in trilayer graphene

Researchers from Science and Technology (IST) Austria, in collaboration with scientists from the Weizmann Institute of Science in Israel, have developed a theoretical framework of unconventional superconductivity, which addresses the questions raised by earlier work that detected unique superconductivity in 'magic angle' trilayer graphene.

Superconductivity relies on the pairing of free electrons in the material despite their repulsion arising from their equal negative charges. This pairing happens between electrons of opposite spin through vibrations of the crystal lattice. Spin is a quantum property of particles comparable, but not identical to rotation. The mentioned kind of pairing is the case at least in conventional superconductors. "Applied to trilayer graphene," co-lead-author from IST, Areg Ghazaryan, points out, "we identified two puzzles that seem difficult to reconcile with conventional superconductivity."

Researchers develop ultra-efficient 'clean' technique to control the properties of graphene

Researchers from Columbia University and collaborators from Korea's Sungkyunkwan University and Japan's National Institute for Materials Science have reported that graphene can be efficiently doped using a monolayer of tungsten oxyselenide (TOS) that is created by oxidizing a monolayer of tungsten diselenide.

The new results relied on a cleaner technique to manipulate the flow of electricity, giving graphene greater conductivity than metals such as copper and gold, and raising its potential for use in telecommunications systems and quantum computers.

Princeton team gains better understanding of superconductivity in 'magic angle' graphene

Princeton researchers have dissipated some of the mystery around 'magic angle' graphene's superconductivity by showing an uncanny resemblance between it and the superconductivity of high temperature superconductors. Magic graphene may hold the key to unlocking new mechanisms of superconductivity, including high temperature superconductivity.

Ali Yazdani, Professor of Physics and Director of the Center for Complex Materials at Princeton University, led the research. He and his team have studied many different types of superconductors over the years and have recently turned their attention to magic bilayer graphene. “Some have argued that magic bilayer graphene is actually an ordinary superconductor disguised in an extraordinary material,” said Yazdani, “but when we examined it microscopically it has many of the characteristics of high temperature cuprate superconductors. It is a déjà vu moment.”

Researchers discover a correlated electron-hole state in double-bilayer graphene

A team of researchers, led by Klaus Ensslin and Thomas Ihn at the Laboratory for Solid State Physics at ETH Zurich, together with colleagues at the University of Texas in Austin (USA), has observed a novel state in twisted bi-layer graphene. In that state, negatively charged electrons and positively charged (so-called) holes, which are missing electrons in the material, are correlated so strongly with each other that the material no longer conducts electric current.

An insulator made of two conductors imageImage by Peter Rickhaus / ETH Zurich (taken from Nanowerk)

“In conventional experiments, in which graphene layers are twisted by about one degree with respect to each other, the mobility of the electrons is influenced by quantum mechanical tunneling between the layers”, explains Peter Rickhaus, a post-doc and lead author of the study. “In our new experiment, by contrast, we twist two double layers of graphene by more than two degrees relative to each other, so that electrons can essentially no longer tunnel between the double layers.”