University of Cambridge researchers have discovered that a material already known for its peculiar electrical properties appears to behave as both a conductor and an insulator at the same time. This find could represent the discovery of an entirely new class of materials, challenging our current understanding of how metals behave.

Traditionally, insulators, conductors and semiconductors can be told apart by the size of their so-called band gap, which measures the amount of energy the material's electrons need before they can start moving freely through the solid and conduct electricity. Conductors have a band gap close to 0 eV (electronvolts), semiconductors range approximately between 1 and 9 eV, and anything above is usually considered an insulator.

More recently, so-called topological insulators have been discovered that can act as conductors and insulators at the same time, depending on the location of the electrons within the compound. More specifically, the interior or bulk of the material acts as an insulator, but its surface is conductive.

What the research team led by Prof. Suchitra Sebastian found, however, is something much more peculiar and explanation-defying. It appears that in the material samarium hexaboride (SmB6), the bulk itself can be both a conductor and an insulator at the same time.

Samarium hexaboride is a Kondo insulator, that is to say that it has a narrow band gap (of about 10 meV) and therefore is a good conductor at room temperature. However, at low temperatures of below 50 K, some complex and peculiar interactions between its electrons lead it to behave as an insulator. Although peculiar themselves, these materials, which straddle the line between insulators and conductors, are well understood by material scientists.

The puzzling thing about samarium hexaboride is that it adds even more strangeness to the mix. Measuring the electrical resistance of the compound indicates that the material behaves as an insulator; however, further analysis of the material's Fermi surface (an abstract boundary used to reliably predict the properties of materials) contradicts this, indicating that the material actually behaves as a good metal. And yet, at temperatures approaching absolute zero, the quantum oscillations of the material keep growing more and more in size as the temperature is lowered, a behavior that is in contrast with both the Fermi analysis and the rules that govern conventional metals.

Scientists don't yet know what may be causing this unusual behavior, and have suggested that this could be the first in a new class of materials which is neither a conductor or an insulator (or a semiconductor, for that matter). One hypothesis is that, because the material is right at the edge between conductor and insulator, it could be simply fluctuating back and forth between these two radically opposite behaviors.

"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," says Sebastian.

The results were published in the latest edition of the journal Science.