Channelling electricity with virtually no resistance under ambient conditions has been a focus for physicists for the better part of half a century.
Tantalizing opportunities may be found in copper-oxide ceramics known as cuprates, but achieving them requires first understanding how the superconducting phenomenon emerges at relatively warm temperatures.
Now, a team of researchers led by Canada's National Institute of Scientific Research (INRS) has uncovered evidence of a hidden rivalry between two quantum states that could help explain how these remarkable materials work their magic.
The discovery suggests a mysterious phenomenon known as a pseudogap may not facilitate superconductivity as some theories propose. Instead, the two states appear to be competing for control of the material's electrons.
It's a finding that could bring us closer to understanding one of the biggest mysteries in modern physics, and perhaps one day help engineers design superconductors that don't require elaborate cooling systems.
Superconductivity is to electrical power as empty roads and green lights are to a morning commute. As electrons push through the traffic of jiggling electromagnetic fields in a copper wire, they spend a portion of their energy in the form of heat.
Electrons in a superconductor glide through without resistance, retaining far more of their energy. The trick involves persuading electrons to abandon their individuality and form quantum partnerships known as Cooper pairs. Working together in a blurred state, these electron twins don’t suffer the impediments of their isolated siblings.
The advantages are huge. Think super-powerful magnets in medical imaging and particle physics experiments; faster circuits that won’t overheat; potential for nuclear fusion plasma confinement.
Unfortunately, getting electrons to cooperate in this fashion requires specific materials operating under some pretty chilly conditions. For conventional superconductors, temperatures typically need to plunge close to absolute zero.
Pumping insanely cold liquids around superconductors is a solution for technologies such as magnetic resonance imaging, but if we want all of the benefits of zero-resistance currents, we need to find something that can handle the heat.
In 1986, physicists Johannes Georg Bednorz and Karl Alexander Müller discovered a family of copper-oxide materials that could achieve superconductivity at decidedly less-freezing temperatures, demonstrating that the concept was at least feasible and could provide some clues on achieving higher temperature superconductivity.
Four decades later, physicists still aren’t entirely sure what makes their electrons behave so differently.
One clue may be found in a strange quantum state called the pseudogap.
"The pseudogap is one of the greatest mysteries in condensed-matter physics," says INRS physicist and senior author Fabio Boschini.
"Understanding how it interacts with superconductivity is essential if we want to explain why these materials can carry electricity without energy loss at unusually high temperatures."
Conductivity is determined largely by the effort it takes to move an electron through a system, a feat itself determined by the energy required for an electron to occupy a spot in a material. Given the right conditions, specific energy levels may be forbidden, creating a “gap” that electrons can’t move through.
A pseudogap isn’t quite so strict, leaving some wiggle room for electrons to squeeze through. Precisely what this means for superconductivity has been a matter of debate. Some physicists suspect the pseudogap provides space for electrons to enter superconducting partnerships. Others argue it's a rival arrangement that interferes with the process.
To find out which explanation fits, Boschini and his colleagues used a laser to temporarily disrupt superconductivity in a copper-oxide crystal while the material remained relatively cold. Using a form of spectroscopy, the team then tracked changes in the electrons' energies over trillionths of a second.
What they discovered was a quantum tug-of-war. When laser pulses disrupted superconductivity, the spectrum of energy levels developed an imbalance that looked just like a pseudogap. As superconductivity returned, the asymmetry disappeared.
Intriguingly, the pseudogap-like signature emerged even when the electrons had cooled below the temperature at which the material normally becomes superconducting. This suggests the signature wasn't a consequence of heating. Rather, the momentary absence of superconductivity seemed to allow a pseudogap to manifest.
The researchers also found evidence that features of both states could briefly coexist, suggesting that their relationship is more complicated than a simple winner-takes-all competition.
So, next step: build a room-temperature superconductivity machine... right?
Sadly, the findings don't settle the question of what causes the pseudogap or explain how cuprates achieve their unusually high superconducting temperatures.
Other recent studies show just how challenging this process could truly be. By resolving how the buzz of electrons in a cuprate lattice contributes to superconductivity, an international team of researchers led by physicists from Radboud University in the Netherlands and the University of Bristol in the UK has demonstrated just how much of the picture we’re still missing.
At low enough temperatures, Cooper pairs appear as a consequence of ripples passing through the lattice. Get the timing right, and these waves can bring electrons closer than their usual electrostatic repulsion would permit, allowing them to marry.
Something similar must also occur amid the virtual tempest of higher temperatures. The most likely candidates probably involve the electrons themselves, which swing back and forth under the influence of their charge and spin.
By etching electrical contacts onto a film of lanthanum cerium copper oxide layered to allow for a smooth temperature gradient, the researchers could perform a series of experiments that revealed superconductivity at varying values of temperature and magnetic field strength while electrons were being added.
This provided sufficient evidence that interactions between incoming electrons and the fluctuating spin of electrons in the material were, in fact, critical in creating the required waves at higher temperatures.
Next stop, superconductivityville… right?
There’s more to the story, however. Gaps where electrons can sit, called holes, can also induce superconductivity at not-so-cold temperatures, just in a slightly different way.
“Although electron- and hole-doped cuprates are made from the same cloth, they are stitched together in different patterns,” says co-lead author Nigel Hussey, a physicist at the University of Bristol.
Since the holes are where the real high-temperature superconductivity game is at, it’s important to understand how it works. Which future experiments will hopefully deliver.
Superconductivity at practical temperatures is clearly a complicated business. Yet it’s also clear that advances are being made. If we ever hope to build electrical systems that waste next to no energy without needing a freezer the size of a small building, filling in every gap – pseudo or electron – is important.
Both studies were published in Nature Physics.
Sources: EurekAlert and The Simons Foundation