To conduct or to insulate?

A new study has discovered mysterious behaviour of a material that acts like an insulator in certain measurementsbut simultaneously like a conductor in others. In an insulator, electrons are largely stuck in one place, while in a conductor, the electrons flow freely. The results, published in the latest issue of Science, challenge current understanding of how materials behave.

Conductors, such as metals, conduct electricity, while insulators, such as rubber or glass, prevent or block the flow of electricity. By tracing the path that electrons follow as they move through a material, researchers led by the University of Cambridge found that it is possible for a single material to display dual metal-insulator properties at once—although at the very lowest temperatures, it completely disobeys the rules that govern conventional metals.

While it is not known exactly what causes this mysterious behaviour, one possibility is the existence of a potential third phase that is neither insulator nor conductor. The duelling metal-insulator properties were observed throughout the interior of the material, called samarium hexaboride (SmB6).

There are other recently-discovered materials which behave both as conductor and as insulator, but they are structured like a sandwich, so the surface behaves differently from the bulk. But the new study found that in SmB6, the bulk itself can be both conductor and insulator simultaneously.

“The discovery of dual metal-insulator behaviour in a single material has the potential to overturn decades of conventional wisdom regarding the fundamental dichotomy between metals and insulators,” said Suchitra Sebastian of the University’s Cavendish Laboratory, who led the research.

In order to learn more about SmB6 and various other materials, Suchitra Sebastian and her colleagues traced the path that the electrons take as they move through the material: the geometrical surface traced by the orbits of the electrons leads to a construction which is known as a Fermi surface. In order to find the Fermi surface, the researchers used a technique based on measurements of quantum oscillations, which measure various properties of a material in the presence of a high magnetic field to get an accurate “fingerprint” of the material.

SmB6 belongs to the class of materials called Kondo insulators, which are close to the border between insulating and conducting behaviour. Kondo insulators are part of a larger group of materials called heavy fermion materials, in which complex physics arises from an interplay of two types of electrons: highly localised ‘f’ electrons, and ‘d’ electrons, which have larger orbits. In the case of SmB6, correlations between these two types of electrons result in insulating behaviour.

Kondo insulators are part of a larger group of materials called heavy fermion materials. In the case of SMB6, correlations between these two types of electrons result in insulating behaviour. “It’s a dichotomy,” said Suchitra Sebastian. “The high electrical resistance of SmB6 reveals its insulating behaviour, but the Fermi surface we observed was that of a good metal.”

Compiled by R. Ramachandran