A newly identified material behaves as both a conductor and an insulator at the same time, a discovery that challenges our current understanding of how materials behave and suggests the existence of a new type of insulating state.
Writing in the July 2 edition of the journal Science, Dr. Suchitra Sebastian from the Cavendish Laboratory at the University of Cambridge and her colleagues report that the discovery of dual metal-insulator behavior in a single material appears to contradict a long-established dichotomy between metals and insulators.
The material, samarium hexaboride (SmB6), behaves like an insulator in some measurements, but simultaneously acts like a conductor in others, the researchers explained in a statement. In insulators, electrons tend to be held in one place, while in conductors, they flow freely.
Insulators block the flow of electricity, while conductors permit the transfer of charge across an object’s surface. In their new study, however, Dr. Sebastian’s team report that this new material, samarium hexaboride, appears to display both properties at the same time, though they note that at the lowest temperatures, it disobeys the rules governing conventional metals.
While other recently-discovered materials also behave both like a conductor and an insulator, the difference is that those substances have surfaces which behave differently from their cores, while in the case of samarium hexaboride, the interior of the material itself can be both an insulator and a conductor simultaneously, the authors noted.
Behavior could be example of a brand new quantum phase
The Cambridge-led team currently does not know just what’s causing this mysterious behavior, but one possibility is that there could be a third phase in which a material is neither a conductor nor an insulator. The discovery challenges the scientific community’s overall understanding of the distinction between metals and insulators.
To learn more about SmB6, the researchers followed the path taken by electrons as they moved throughout the material. In order to find the geometrical surface traced by the electrons’ orbits, a construction known as a Fermi surface, they used a technique centered around measurements of quantum oscillations in order to measure various characteristics of a material when a magnetic field is present. This allowed them to get an accurate “fingerprint” of the material.
In order for these quantum oscillations to be observed, the materials must be as close to pure as possible to limit the defects that electrons could collide with. For that reason, several of the tests involving SmB6 were conducted at the National High Magnetic Field Laboratory in Tallahassee, Florida, and the researchers found that the interplay of two different types of electrons (localized ‘f’ electrons and larger-orbit ‘d’ electrons) resulted in insulating behavior.
Thus, while the electrical properties of SmB6 demonstrated insulating behavior, Dr. Sebastian said that the Fermi surface her team observed “was that of a good metal.” Furthermore, when it approached temperatures of nearly 0 degrees Kelvin (-273 Celsius), it behaved in a way that was not characteristic of conventional metals, as the amplitude of its quantum oscillations continued to grow dramatically as the temperature lowered instead of leveling off.
The study authors came up with several possible reasons for this unusual behavior. It could be a new phase that is neither conductor nor insulator, they explained. Alternatively, it could be a back-and-forth fluctuation between the two states, or because the material has a small gap between its insulating and conducting behaviors, its electrons could be able to jump that gap.
“The crossover region between two different phases – magnetic and non-magnetic, for example – is where the really interesting physics happens,” Dr. Sebastian said. “Because this material is close to the crossover region between insulator and conductor, we found it displays some really strange properties – we’re exploring the possibility that it’s a new quantum phase.”
Update: Dr. Sebastian discusses her research with redOrbit
Following the initial publication of this article, redOrbit had the opportunity to discuss the research via email with Dr. Sebastian, who explained that, “in our current understanding, metals and insulators are dichotomous. They are by very definition the opposite of each other.”
“Electrons travel large distances in metals, conducting electrical current,” she added. “Electrons are not mobile in insulators, posing a large resistance to the flow of electrical current… So to find a material which simultaneously behaves like a metal in some properties, and like an insulator in some other properties, appears to overturn our current understanding of insulators.”
What are the potential benefits of having a material that can behave like both a conductor and an insulator? “It is too early to tell,” she said to redOrbit, “but there are potential benefits in terms of future electronic applications based on quantum materials” (such as SmB6).