Superconductor research indicates new phase of matter

Posted by Emma Woollacott

Scientists believe they've found an answer to a puzzling gap in the electronic structures of some high-temperature superconductors - and that the answer could be a previously-undiscovered phase of matter.

This 'pseudogap' has been a stumbling block for researchers who are trying to find superconductors that operate at room temperature.

"A clear answer as to whether such a gap is just an extension of superconductivity or a harbinger of another phase is a critical step in developing better superconductors," says Zhi-Xun Shen of the Stanford Institute for Materials and Energy Science (SIMES).

"Our findings point to management and control of this other phase as the correct path toward optimizing these novel superconductors for energy applications, as well as searching for new superconductors."

Although researchers have developed 'high-temperature superconductors', even the warmest of these — the cuprates — must be chilled halfway to absolute zero before they will superconduct. Room-temperature versions are something of a Holy Grail.

One hallmark of a superconductor is the so-called 'energy gap' that appears when the material transitions into its superconducting phase.

The gap in electron energies arises when electrons pair off at a lower energy to do the actual job of superconducting electric current. When most of these materials warm to the point that they can no longer superconduct, the electron pairs split up, the electrons start to regain their previous energies, and the gap closes. But in the cuprates, the gap persists even above superconducting temperatures.

This 'pseudogap' doesn't fully disappear until a second critical temperature called T* is reached - and this can be 100 degrees higher than the temperature at which superconductivity begins.

Shen's team looked at a sample of a cuprate superconductor, examining electronic behavior at the sample's surface, thermodynamic behavior in the sample's interior, and changes to the sample's dynamic properties over time.

They found that electrons in the pseudogap phase are not pairing up, but reorganizing into a distinct order of their own. In fact, the new order is also present when the material is superconducting; it had been overlooked before, masked by the behavior of superconducting electron pairs.

Simply knowing the pseudogap indicates a new phase of matter provides a clear signpost for follow-up research, according to Ruihua He, a post-doctoral researcher at the Advanced Light Source.

"First to-do: uncover the nature of the pseudogap order," he says. "Second to-do: determine whether the pseudogap order is friend or foe to superconductivity. Third to-do: find a way to promote the pseudogap order if it's a friend and suppress it if it's a foe."