Superconducting ‘pseudogap’ is a new phase of matter

A team of researchers has discovered the strongest evidence yet that a puzzling gap in the electronic structures of some high-temperature superconductors could indicate a new phase of matter.

Understanding this ‘pseudogap’ has been a 20-year quest 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,” said 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,” he added.

Superconductors are materials that conduct electricity with 100 percent efficiency, losing nothing to resistance. However, they work only at extremely low temperatures.

Although researchers have developed ‘high-temperature superconductors’, even the warmest of them— the cuprates — must be chilled halfway to absolute zero before they will superconduct.

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-star’ is reached – and this can be 100 degrees higher than the temperature at which superconductivity begins.

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Shen and his team looked at a sample of a cuprate superconductor and examined 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 first author Ruihua He.

According to co-author Makoto Hashimoto, their work ‘makes the high-temperature superconductor roadmap much clearer than before, and a good roadmap is important for any big science project’.

The finding appears in the March 25 issue of Science.

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