Enormous effort has been invested in making high-temperature superconductivity live up to its name. For some time, researchers in the field have suspected they are being blocked by a mysterious phenomenon known as the pseudogap. Now their suspicion has been confirmed for the first time, offering hope of finding ways to circumvent the obstacle.
Superconductivity occurs when electrical resistance drops to zero. For a long time after its 1911 discovery it was thought to be restricted to metals at temperatures close to absolute zero. However, in 1986 superconductivity was observed in ceramics. Further work led to temperatures warm enough to be achieved with liquid nitrogen (-196°C) and edging ever closer to room temperature.
While still very cold by everyday standards, cooling with liquid nitrogen is relatively affordable, and has made a variety of applications practical. The field is so exciting it drew two Nobel Prizes and took just 15 years to produce more than 100,000 peer reviewed papers.
Unfortunately however, superconducting materials lose their capacity either when it warms or if too much current is put through them, and even the highest critical current densities are still frustratingly low. For some time physicists have suspected that the problem is what is called the pseudogap.
Superconductivity requires paired charge carriers, usually electrons, and these must be “phase coherent,” with aligned quantum waves.
Superconductors have an energy gap, the energy required of photons to break a carrier pair apart. When electrons are sparce however, particularly in copper oxide superconductors, a state can exist where the energy gap varies depending on the direction from which they photons are coming. This is known as a pseudogap, a term which has been extended to materials in this state. In a pseudogap electron pairs still form, but they lack the coherence required for superconductivity.
Many superconductivity researchers have speculated that the pseudogap is an obstacle to achieving their goal of perfect energy transmission at ever higher temperatures, but this has not been clear, with some arguing the pseudogap might be friend, not foe.
In Nature Materials Stanford University scientists have published what lead author Dr Makoto Hashimoto calls “clear, smoking-gun evidence that the pseudogap phase competes with and suppresses superconductivity,”
Hashimoto used photons to knock electrons out of a copper oxide superconductor. He found that at the critical temperature where superconductivity vanishes the pseudogap makes use of electrons that at slightly lower temperatures would be pairing up to form superconductive carriers.
Credit: SLAC National Accelerator Laboratory. The pseudogap (PG) competes for electrons with superconductivity, making it hard to raise the temperatures at which superconductivity occurs.
"It's a complex, intimate relationship. These two phenomena likely share the same roots but are ultimately antagonistic,” says co-author Stanford's Professor Zhi-Xun Shen. “When the pseudogap is winning superconductivity is losing ground.”
The cause of the pseudogap remains unknown, but it is now clear that understanding how to remove or work around it is the key to superconductivity gains. If these can be achieved we may soon see the rise of levitating trains, more powerful computers, cheaper, more reliable electricity grids, and more widely available MRIs.