Jahn–Teller metals offer new hope for high-temperature superconductivity.
Levitating superconductor. Image: Wiki
It's only slightly less exciting than it sounds: a new state of matter. The discovery, which comes courtesy of an international team led by Kosmas Prassides of Tokohu University in Japan, offers a novel material with an unusual combination of properties—insulator, superconductor, metal, magnet. Of particular interest is the hint of high-temperature superconductivity, something of a materials science holy grail and a persistent physics mystery.
So, there are lots of different states of matter. We all know solids, liquids, gases, and, probably, plasmas, but beyond these there's an entire catalog of matter alternatives: Bose–Einstein condensate, degenerate matter, supersolids/superfluids, quark-gluon plasma, etc. The difference is that all those alternatives are lab-created and don't have much place out in the real world of nature. The Prassides group's new material is one of those states, a crystalline arrangement of carbon-60 molecules, better known as buckyballs, doped with rubidium atoms, which are used here to control and maintain distances between the buckyballs, tuning the material's properties/phases.
It's in this tuning that we find the new, previously unknown state or states of matter, which are known as a "Jahn–Teller metals" after the Jahn-Teller effect, which relates structural deformations among molecules found within a material to its electrical properties. Put simply, by applying or removing pressure, it's possible to boost the conductivity of what may have been an insulator at lower pressures. High pressure: conductivity.
This is what the rubidium atoms do: apply pressure. Usually when we think about adding pressure, we think in terms of squeezing something, forcing its molecules closer together by brute force. But it's possible to do the same thing chemically, tweaking the distances between molecules by adding or subtracting some sort of barrier between them—sneaking in some extra atoms, perhaps.
What happens in a Jahn–Teller metal is that as pressure is applied, and as what was previously an insulator—thanks to the electrically-distorting Jahn-Teller effect—becomes a metal, the effect persists for a while. The molecules hang on to their old shapes. So, there is an overlap of sorts, where the material still looks an awful lot like an insulator, but the electrons also manage to hop around as freely as if the material were a conductor.
"The surprising thing about this metal–insulator transition is that it involves an intermediate state never seen before," Hamish Johnston writes in Physics World. "The researchers have dubbed this a 'Jahn–Teller metal' because when the material is studied using infrared spectroscopy, the fulleride molecules clearly show rugby-ball distortions, which were only known to occur in insulators. However, nuclear magnetic resonance measurements clearly show that electrons are able to "hop" from one molecule to the next--which is the signature of a conducting metal."
This is all pretty important because this transition from insulator to metal is also a transition from insulator to potential superconductor. The resulting metal just needs low enough temperatures and all of a sudden its electrons start pairing up and skipping around, with the result being a sudden drop to exactly zero electrical resistance(!). This is obviously a very desirable property.
Its this pairing up of electrons, which are together known as Cooper pairs, that's crucial for superconductivity Basically, as the temperature of a given material drops, suddenly the once-negligible attractive force between electrons becomes very significant. Electrons that would formerly repel each other are attracted. These pairs may then "condense" into a single unified ground energy state, or state of lowest possible energy. In this state, electrons are no longer allowed to scatter or really do anything on their own. The result is superconductivity.
What's weird about Jehn-Teller metals is that so far we really have no idea what causes the electrons within them to pair up. In a conventional superconductor, they do it because they're swapping phonons, which are excitations ("quasiparticles") found within the molecular lattice of some material, and the effect is attraction. Again, this takes extremely cold temperatures.
If Jehn-Teller metals involve some other electron pairing mechanism, that might mean the possibility of superconductivity occurring at not-so-cold temperatures. Researchers just have to figure out what that other mechanism is: "The relationship between the parent insulator, the normal metallic state above [superconducting temperatures], and the superconducting pairing mechanism is a key question in understanding all unconventional superconductors," Prassides and co write in Science Advances.
So, it may be possible to "synthetically determine" some optimal molecular substructure for high- or higher-temperature superconductivity. As Prassides and co. conclude, "because synthetic chemistry allows the creation of new electronic structures distinct from those in the atoms and ions that dominate most known superconductors, this is strong motivation to search for new molecular superconducting materials."