Scientists are working hard to find ways to make superconductors operate at much warmer temperatures, but certain unanswered questions have been standing in their way for decades. One of the most pressing unanswered questions is called the "pseudogap." The quest to understand the pseudogap and determine if it helps or harms superconductivity is a key area of research for scientists all around the world.

A group of scientists has found some of the most compelling evidence yet that suggests the pseudogap might be an indicator of a new phase of matter.

"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," said Zhi-Xun Shen of the Stanford Institute for Materials and Energy Science (SIMES), a joint institute of the Department of Energy’s SLAC National Accelerator Laboratory and Stanford University.

The ultimate goal for superconductors, which are able to conduct energy with 100% efficiency, is to allow them to operate at room temperature. Currently the superconductors that are able to operate at the warmest temperatures are cuprates, and even they need to be cooled to half of absolute zero to become superconductive.

The team is studying a phenomenon that occurs when the superconductor warms to the temperature where it is no longer able to superconduct. The energy gap that appears as the superconductor first transitions into the superconducting phase in most materials ends when they warm; the electron pairs at this point split up and start to regain their previous energies.

In the cuprates, however, the gap continues above the superconducting temperatures and doesn’t fully disappear until a temperature that scientists call T-star is reached. The T-star temperature can be as much as 100 degrees warmer than the superconducting temperature of the material. The puzzle is that the electrons in this gap phase aren’t superconducting and scientist don’t know what the electrons are doing.

"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," Shen said. 

Shen and his team are working with the cuprates, looking at samples from the inside out, and using three measurement techniques that have never been used before on the material.

"There is much to be said about using the same material and three different techniques to tackle the problem," commented condensed matter physicist Sudip Chakravarty of the University of California Los Angeles, who was not involved in the research. "Even after decades of research this is a key unanswered question." 

Using these techniques the team has found that the electrons are not pairing up during the pseudogap. They are reorganizing into an order of their own and the team says that order is present when the material is superconducting and was overlooked before. This shows that the pseudogap indicates a new phase of matter and the goal now is to learn more about the phase and see if its helping or hurting the superconducting.

Ruihua He, a post-doctoral researcher at the Advanced Light Source and first author of the paper, outlined the next steps: "First to-do: uncover the nature of the pseudogap order. 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."