Superconductivity is the ability of certain materials to channel electricity without the energy losses that heat ordinary wires. Electrons pair up into quantum partnerships known as Cooper pairs and glide through the material unimpeded. The payoff would be enormous: powerful magnets for medical imaging and particle physics, circuits that do not overheat, and potentially the confinement of plasma in nuclear fusion reactors. The catch is temperature. Most superconductors only work close to absolute zero, requiring elaborate and expensive cooling systems.
That changed in 1986, when physicists Johannes Georg Bednorz and Karl Alexander Müller discovered that a family of copper-oxide materials, known as cuprates, could superconduct at far less frigid temperatures. Four decades later, physicists still do not fully agree on why. One clue centres on a strange quantum state called the pseudogap, an incomplete version of the energy gap that normally blocks electrons from moving. Some theories suggest the pseudogap helps prepare electrons for superconducting partnerships; others argue it is a rival arrangement that interferes.
To test the two ideas, INRS physicist Fabio Boschini and his colleagues used a laser to momentarily disrupt superconductivity in a cold copper-oxide crystal, then tracked the electrons' energies over trillionths of a second with spectroscopy. When superconductivity was disrupted, an imbalance resembling a pseudogap appeared. As superconductivity returned, the asymmetry disappeared. The signature even emerged below the temperature at which the material normally becomes superconducting, suggesting it was not a side effect of heating. The two states briefly coexisted, complicating a simple winner-takes-all picture.
"The pseudogap is one of the greatest mysteries in condensed-matter physics," said Boschini, the study's senior author. "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."
A second study, led by physicists from Radboud University in the Netherlands and the University of Bristol in the UK, tackled a related question: what creates the waves that bind electrons into Cooper pairs at higher temperatures. By etching electrical contacts onto a film of lanthanum cerium copper oxide with a smooth temperature gradient, the team found evidence that fluctuations in the spin of electrons in the material are critical. They also noted that holes, gaps where electrons can sit, can induce superconductivity at not-so-cold temperatures in a slightly different way.
"Although electron- and hole-doped cuprates are made from the same cloth, they are stitched together in different patterns," said Nigel Hussey, a physicist at the University of Bristol and co-lead author. Since hole-doped cuprates are where the highest superconducting temperatures are found, understanding them is the next step.
The findings do not resolve what causes the pseudogap or fully explain cuprates, and the researchers acknowledge how much of the picture is still missing. But each experiment narrows the search for materials that could one day carry power with almost no waste, without the need for a freezer the size of a small building.