Superconductivity occurs when electrons in a metal pair up and move through the material without resistance. But there may be more to the story than we thought, as scientists in Germany have now discovered that electrons can also group together into families of four, creating a new state of matter and, potentially, a new type of superconductivity.
Conductivity is a measure of how easily electrons (and therefore electricity) can move through a material. But even in materials that make good conductors, like gold, electrons will still encounter some resistance. Superconductors, however, remove all such barriers and provide zero resistance at ultracold temperatures.
The reason electrons can move through superconductors so easily is because they pair up through a quantum effect known as Cooper pairing. In doing so, they raise the minimum amount of energy it takes to interfere with the electrons – and if the material is cold enough, its atoms won’t have enough thermal energy to disturb these Cooper pairs, allowing the electrons to flow freely with no loss of energy.
But in the new study, researchers from the Universities of Dresden and Würzburg in Germany made a fascinating discovery. In one particular type of superconductor, they found that Cooper pairs were themselves pairing up, forming families of four electrons.
"When we discovered that suddenly four electrons instead of two were forming a bond, we first believed it was a measurement error,” says Henning Klauss, lead researcher on the study. “But the more methods we used to confirm the result, the clearer it became that this had to be a new phenomenon: all data are consistent with the same result. Now we know that the four particle electron family in certain metals creates a completely new state of matter when cooled to ultra-low temperatures.”
The discovery was made in a superconducting metal containing barium, potassium, iron and arsenic, from a class known as iron pnictides. Such a phenomenon had been theoretically predicted about a decade ago, but no experimental evidence had arisen until now. The researchers then spent two years confirming the find, using seven different methods.
The team says that these electron families could open a new branch of superconductivity and a range of advanced devices that tap into it. But before then, there’s still plenty of work to be done to unravel how it works, which materials it occurs in, and how it can be invoked.
"One can assume that our results will lead to a whole new line of research, looking for other metals with four connected electrons, for example, or exploring how materials need to be changed to create an electron family," says Klauss. "In purely theoretical terms, a whole new type of superconductivity would also be possible with our electron family. The only thing that is certain is that iron pnictides are well suited for technologies such as quantum sensors due to their new aggregate state.”
The research was published in the journal Nature Physics.
Source: Technical University of Dresden