Graphene reveals surprising types of superconducting states

Graphene is considered one of the simplest yet most fascinating materials in physics: it consists of a single layer of carbon atoms. When several such layers are stacked in a specific natural arrangement, the resulting material can have extraordinary electronic properties.

An international research team involving the University of Basel has now observed several forms of superconductivity in a specific form of graphene known as rhombohedral graphene. In a superconducting state, electric current flows through a material without resistance. Normally, this state occurs only at very low temperatures, and magnetic fields can quickly destroy it.

More stable in magnetic fields

This was precisely the surprising finding in the experiments: The newly observed superconducting states persisted even in comparatively strong magnetic fields. In certain cases, the superconductivity became even more stable when exposed to a magnetic field.

This contradicts the behavior of classical superconductors, in which magnetic fields break up the electron pairs responsible for the resistance-free flow of current.

The samples studied consisted of four or five atomically thin layers of graphene. These layers are not artificially twisted, as in other graphene experiments, but follow a natural stacking arrangement in a high-purity crystal. The researchers were able to tune the electronic properties in a targeted way by controlling the number of electrons and the electric field in the material by applying electrical voltages.

Challenging measurement conditions

The group led by Prof. Dr. Dominik Zumbühl at the Department of Physics at the University of Basel played a key role in this work. The Basel team helped make it possible to carry out the measurements under particularly challenging conditions: at extremely low temperatures, with very weak electrical signals, and under controlled magnetic fields in different directions. Such measurements are crucial for making these delicate superconducting states visible in the first place and for distinguishing them from one another.

The study thus demonstrates that even a seemingly simple material like carbon can give rise to an entire family of unusual superconducting states. The discovery provides a new model system for better understanding unconventional superconductivity. Researchers are interested in such materials, among other reasons, because in the long term they could open up new avenues for lossless electricity transmission, novel electronic components, or topological quantum states.

This text is based on an MIT press release.