In a world-first, researchers appear to have discovered a material that may act as both a conductor and an insulator simultaneously, turning the laws of physics on its head. Scientists at the University of Cambridge published the mysterious behaviour in the academic journal Science, and it still has physicists scratching their heads. The phenomenon appears to be due to new quantum effects.
Electrons in insulators are typically held in one place, resisting the flow of electricity. Electrons in conductors however flow much more freely over long distances. Yet, this discovery shows a material with electrons that may behave in both ways.
The peculiar material, samarium hexaboride (SmB6), is a well-known insulator. However, its ability to behave as both an insulator and a conductor might open up an entirely new realm of metals, challenging the current understanding of the physics of metals. The insulating material surprised scientists when a small sample was placed on a cantilever in a magnetic field and they observed activity on the screen that appeared to show electrons travelling long distances – a characteristic of conducting metals.
Samarium hexaboride is what is known as a Kondo insulator – it has a narrow band gap of around 10 meV, which makes it an effective conductor at room temperature. In other words, the electrons in samarium hexaboride are able to move freely through the solid at room temperature and therefore conduct electricity. However, at temperatures below -223 degrees Celsius, some unusual physics between its electrons make it act as an insulator.
A measurement of the electrical resistance of SmB6 shows it behaves as an insulator, however analysis of the Fermi surface (an abstract boundary used in physics to predict a material’s properties) shows the material actually acts as a decent electrical conductor. At temperatures approaching absolute zero (-273 degrees Celsius), the material’s quantum oscillations grow larger and larger as the temperature decreases, which seemingly is a huge change to the laws of conventional metals.
Quantum oscillations are an innate property of metals and may be mapped out to construct the Fermi surface, which depicts the geometry of the orbits of electrons in a material. It is a technique used to visualise the movement of electrons and better understand a conducting metal’s properties.
Scientists are currently trying to figure out what may be causing this behaviour and suggest it may be the first in a new category of materials which lie somewhere between insulators and conductors (and semiconductors). Some believe samarium hexaboride might be fluctuating between the two behaviours, by sitting on the border between the two classes.
“The discovery of dual metal-insulator behaviour in a single material has the potential to [change] decades of conventional wisdom regarding the fundamental dichotomy between metals and insulators,” explains Dr. Suchitra Sebastian, a Royal Society University Research Fellow in Physics at the Cavendish Laboratory in Cambridge. It appears that electrons in certain insulators may somehow act as if they are in a metal. To this effect, the unique behaviour of the material may be explained as a new quantum state that fluctuates between metal and insulator.
Sebastian also speculates that they might have discovered a new quantum phase of matter. In quantum physics, trillions of electrons in a material may act collectively to display significantly different properties from what they do alone. Samarium hexaboride might represent an “emergent” quantum phase of matter.
A number of creative theoretical explanations are surfacing to potentially explain the outcome. The team plans to conduct a number of experiments using high-quality crystals to further investigate the new physics underlying the discovery.
What experiments may be performed to explore this potentially new development in physics?