Scientists have long theorised that there are other types of superconductor out there waiting to be discovered, and it turns out they were right: new research has identified a g-wave superconductor for the first time, a major development in this area of physics.
Superconductors are materials that offer no electrical resistance, so electricity can pass through them with close to 100 percent efficiency.
That sounds great when you think about the potential of super-efficient power grids that don't lose energy to heat. But there's a catch. Materials that are able to act in this way usually need to be cooled to ultra-low temperatures before the actual superconductivity starts happening.
Up until now though, almost all superconductors have been confirmed as 'spin-singlet', made up of Cooper pairs of electrons that combine a spin-up electron with a spin-down electron, removing the material's electrical resistance along the way.
There are currently two types of superconductivity that fit this description, s-wave and d-wave.
Put simply, the electrons in the s-wave Cooper pairs point directly at each other, cancelling out each other's angular momentum – which measures rotational energy and movement.
A different alignment in d-waves creates a positive angular momentum along one axis and negative along a second axis – giving it two units of angular momentum.
The newly discovered g-wave superconductor has a totally separate type of angular momentum than either s-wave or d-wave, and it was discovered through a super-detailed resonant ultrasound spectroscopy analysis of the metal strontium ruthenate.
"This experiment really shows the possibility of this new type of superconductor that we had never thought about before," says physicist Brad Ramshaw, from Cornell University. "It really opens up the space of possibilities for what a superconductor can be and how it can manifest itself."
"If we're ever going to get a handle on controlling superconductors and using them in technology with the kind of fine-tuned control we have with semiconductors, we really want to know how they work and what varieties and flavours they come in."
The team was actually looking for another type of superconductor that only exists as a hypothesis for now: the p-wave superconductor. Scientists think this could be a 'spin-triplet' where paired electrons have the same spin direction, creating anangular momentum of 1 – somewhere between s-wave and the more exotic d-wave.
Instead of finding p-wave superconductivity, they found a different kind of angular momentum altogether.
The spectroscopy scans looked at the symmetry of a strontium ruthenate crystal, building a new and completely custom setup to get the material cooled down to the necessary temperatures.
The elastic constants of the material – the speed of sound passing through it, essentially – revealed that strontium ruthenate is a two-component superconductor, capable of complex electron binding that needs a direction as well as a number to express it. That meant the material couldn't be classed as an s-wave, d-wave, or p-wave superconductor. It was something else.
"Resonant ultrasound really lets you go in and even if you can't identify all the microscopic details, you can make broad statements about which ones are ruled out," says Ramshaw.
"So then the only things that the experiments are consistent with are these very, very weird things that nobody has ever seen before. One of which is g-wave, which means angular momentum 4. No one has ever even thought that there would be a g-wave superconductor."
The discovery is another step forward in our understanding of superconductors. If we can scale up the technology and get it working at warmer temperatures, the potential benefits are huge: circuit boards and power grids that don't lose any electricity to heat as energy is transferred.
In terms of this team of researchers, in the future they'll be looking at other materials that might be capable of the elusive p-wave superconductivity. They're also going to be analysing strontium ruthenate further – it's a fascinating metal for all kinds of reasons.
"This material is extremely well studied in a lot of different contexts, not just for its superconductivity," says Ramshaw. "We understand what kind of metal it is, why it's a metal, how it behaves when you change temperature, how it behaves when you change the magnetic field."
"So you should be able to construct a theory of why it becomes a superconductor better here than just about anywhere else."
The research has been published in Nature Physics.