Researchers close gaps in high-temperature superconductors

Channeling electricity with virtually no resistance under ambient conditions has been the focus of physicists for the better part of half a century.

Tantalizing opportunities can be found in copper oxide ceramics known as cuprates, but achieving them requires first understanding how the superconducting phenomenon arises at relatively warm temperatures.

Now, a team of researchers led by Canada’s National Institute for Scientific Research (INRS) has discovered evidence of a hidden rivalry between two quantum states that could help explain how these remarkable materials work their magic.

The discovery suggests that a mysterious phenomenon known as a pseudogap may not facilitate superconductivity as some theories propose. Instead, the two states appear to compete for control of the material’s electrons.

It’s a finding that could bring us closer to understanding one of the biggest mysteries of modern physics and may one day help engineers design superconductors that don’t require elaborate cooling systems.

Superconductivity is to electrical energy what empty roads and green lights are to a morning commute. As electrons make their way through the traffic of oscillating electromagnetic fields in a copper wire, they expend some of their energy as heat.

Electrons in a superconductor slide without resistance, retaining much more energy. The trick is to persuade the electrons to abandon their individuality and form quantum associations known as Cooper pairs. Working together in a blurred state, these electronic twins do not suffer from the impediments of their isolated siblings.

The advantages are enormous. Think super powerful magnets in particle physics and medical imaging experiments; faster circuits that won’t overheat; potential for confinement of nuclear fusion plasma.

Unfortunately, getting electrons to cooperate in this way requires specific materials that operate in fairly cold conditions. For conventional superconductors, temperatures typically need to fall close to absolute zero.

Pumping incredibly cold liquids around superconductors is one solution for technologies like MRI, but if we want all the benefits of zero-resistance currents, we need to find something that can withstand 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 subzero temperatures, demonstrating that the concept was at least feasible and could provide some clues to achieving superconductivity at higher temperatures.

Four decades later, physicists still aren’t entirely sure what makes their electrons behave so differently.

A clue can be found in a strange quantum state called a pseudogap.

“The pseudogap is one of the biggest mysteries of condensed matter physics,” says INRS physicist and lead author Fabio Boschini.

“Understanding how it interacts with superconductivity is essential if we want to explain why these materials can transport electricity without energy loss at unusually high temperatures.”

Conductivity is largely determined by the effort it takes to move an electron through a system, a feat determined by the energy required for an electron to occupy a place in a material. Given the right conditions, specific energy levels can be prohibited, creating a “gap” through which electrons cannot pass.

A pseudospace is not as strict and leaves some wiggle room for electrons to pass through. Precisely what this means for superconductivity has been a topic of debate. Some physicists suspect that pseudospace provides space for electrons to enter superconducting associations. Others argue that it is a rival agreement that interferes with the process.

To figure 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 tracked changes in electron energies over a trillionth of a second.

What they discovered was a quantum tug of war. When the laser pulses altered the superconductivity, the spectrum of energy levels developed an imbalance that resembled a pseudogap. When superconductivity returned, the asymmetry disappeared.

Interestingly, the pseudospace-like signature emerged even when the electrons had cooled below the temperature at which the material normally becomes superconducting. This suggests that the signature was not a consequence of warming. Rather, the momentary absence of superconductivity appeared to allow a pseudogap to manifest.

The researchers also found evidence that features of both states could briefly coexist, suggesting their relationship is more complicated than a simple winner-take-all competition.

So, next step: build a room-temperature superconductivity machine… right?

Unfortunately, the findings do not resolve the question of what causes the pseudogap or explain how cuprates reach their unusually high superconducting temperatures.

Other recent studies show how challenging this process could really be. By working out how the buzzing 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 United Kingdom has shown how much of the picture we are still missing.

At sufficiently low temperatures, Cooper pairs appear as a consequence of waves passing through the network. If the timing is right, these waves can bring electrons closer together than their usual electrostatic repulsion would allow, allowing them to marry.

Something similar must also happen in the midst of the virtual storm of higher temperatures. The most likely candidates probably involve the electrons themselves, which oscillate back and forth under the influence of their charge and spin.

By etching electrical contacts into a layered lanthanum-cerium-copper oxide film to allow for a smooth temperature gradient, the researchers were able to perform a series of experiments that revealed superconductivity at different values ​​of temperature and magnetic field strength while adding electrons.

This provided sufficient evidence that the interactions between the incoming electrons and the fluctuating spin of the electrons in the material were, in fact, critical for creating the waves required at higher temperatures.

Next stop, Superconductivityville… right?

However, there is more to the story. The gaps where electrons can sit, called holes, can also induce superconductivity at not-so-cold temperatures, just in a slightly different way.

“Although the electron- and hole-doped cuprates are made from the same fabric, they are stitched in different patterns,” says co-senior author Nigel Hussey, a physicist at the University of Bristol.

Since holes are the real game of high-temperature superconductivity, it’s important to understand how it works. What future experiments are expected to work.

Superconductivity at practical temperatures is clearly a complicated matter. However, it is also clear that progress is being made. If we ever hope to build electrical systems that waste almost no energy without needing a freezer the size of a small building, it is important to fill all the holes (pseudo or electrons).

Both studies were published in Nature physics.

Sources: EurekAlert and The Simons Foundation