Manchester-led research offers advance in superconductors with a ‘twist’

An international research team led by The University of Manchester has revealed a nanomaterial that mirrors the “magic angle” effect originally found in a complex man-made structure known as twisted bilayer graphene – a key area of study in physics in recent years.

The new research shows that the special topology of rhombohedral graphite effectively provides an inbuilt “twist” and therefore offers an alternative medium to study potentially game-changing effects like superconductivity. “It is an interesting alternative to highly popular studies of magic-angle graphene” said graphene pioneer Professor Sir Andre Geim, a co-author of the study.

The team, led by Artem Mishchenko, Professor of Condensed Matter Physics at The University of Manchester published its findings today in the journal Nature.

“Rhombohedral graphite can help to better understand materials in which strong electronic correlations are important - such as heavy-fermion compounds and high-temperature superconductors”, said Professor Mishchenko. 

A previous step-forward in two-dimensional materials research was the curious behaviour that stacking one sheet of graphene atop one another and twisting it to a ‘magic angle’ changed the bilayer’s properties, turning it into a superconductor.

Professor Mishchenko and his colleagues have now observed the emergence of strong electron-electron interactions in a weakly stable rhombohedral form of graphite - the form in which graphene layers stack slightly differently compared to stable hexagonal form.

Interactions in twisted bilayer graphene are exceptionally sensitive to the twist angle. Tiny deviations of about 0.1 degree from the exact magic angle strongly supress interactions. It is extremely difficult to make devices with the required accuracy and, especially, find sufficiently uniform ones to study the exciting physics involved. The newly published findings on rhombohedral graphite has now opened an alternative route to accurately making superconductor devices.

Graphite, a carbon material made up of stacked graphene layers, has two stable forms: hexagonal and rhombohedral. The former is more stable, and has thus been extensively studied, while the latter is less so.

To better understand the new result, it is important to note that the graphene layers are stacked in different ways in these two forms of graphite. Hexagonal graphite (the form of carbon found in pencil lead) is composed of graphene layers orderly stacked on top of each other. The metastable rhombohedral form has a slightly different stacking order, and this slight difference leads to a drastic change in its electronic spectrum.