High temperature superconductor reveals new phase of matter

Understanding this 'pseudogap' has been a 20 year quest for researchers who are trying to control and improve the materials, with the ultimate goal of finding superconductors that operate at room temperature. "Our findings point to management and control of this other phase as the correct path toward optimising 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. Shen led the team of researchers that made the discovery; their findings appear in the March 25 issue of Science. Superconductors are materials that conduct electricity with 100% efficiency, losing nothing to resistance. Currently used in medical imaging, highly efficient electrical generators and maglev trains, they have the potential to become a truly transformative technology; energy applications would be just one beneficiary. This promise is hampered by one thing, though: they work only at extremely low temperatures. Although research over the past 25 years has developed 'high temperature superconductors' that work at warmer temperatures, even the warmest of them - the cuprates - must be chilled half way to absolute zero before they will superconduct. The prospect of being able to dramatically increase that working temperature, thus making superconductors easier and cheaper to use, has kept interest in the cuprates at the boiling point. But to change something there is a need to understand it, and the pseudogap puzzle has stood in the way. One hallmark of a superconductor is a 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 is the pseudogap, and it doesn't fully disappear until a second critical temperature called T* is reached. T* can be 100 degrees higher than the temperature at which superconductivity begins. As the electrons in the pseudogap state aren't superconducting Shen researched to find out what they were 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. In work done at SLAC's Stanford Synchrotron Radiation Lightsource, Lawrence Berkeley National Laboratory's Advanced Light Source and Stanford University, Shen's team looked at a sample of a cuprate superconductor from the inside out. They examined electronic behaviour at the sample's surface, thermodynamic behavior in the sample's interior, and changes to the sample's dynamic properties over time using a trifecta of measurement techniques never before employed together. The team found that electrons in the pseudogap phase were not pairing up. They reorganised into a distinct yet elusive order of their own. The new order was also present when the material was superconducting; it had been overlooked before, masked by the behaviour of superconducting electron pairs. Knowing the pseudogap indicates a new phase of matter opens the door to follow up research, such as uncovering the nature of the pseudogap order, determining whether the psuedogap is 'friend or foe' to superconductivity and finding a way to promote the pseudogap order if it's a 'friend and suppress it if it's a 'foe'. This advance was made possible by a strong collaboration between Shen's team and teams of researchers from SIMES (led by Aharon Kapitulnik), LBNL (led by Joseph Orenstein) and the ALS (led by Zahid Hussain), the sample growers from the National Institute of Advanced Industrial Science and Technology (led by Hiroshi Eisaki), as well as SIMES theorists Steve Kivelson and Thomas Devereaux. An unprecedented three-pronged study has found that one type of high-temperature superconductor may exhibit a new phase of matter. As in all superconductors, electrons pair off (bottom) to conduct electricity with no resistance when the material is cooled below a certain temperature. But in this particular copper-based superconductor, many of the electrons in the material don't pair off; instead they form a distinct, elusive order (orange plumes) that had not been seen before. (Illustration by Greg Stewart, SLAC.) Superconductors conduct electricity with 100 percent efficiency, losing none of it to resistance. The few high-temperature superconducting wires on the right conduct as much current as all the copper cables on the left. (Photo courtesy of American Superconductor.)