Slow atomic movements shed new light on unconventional superconductivity

While atoms are known to wiggle very fast, dopants added to a cuprate superconductor can also cause atoms to meander very slowly. An SLAC study shows this process, called atomic relaxation, offers a new way to explore quantum states in these puzzling materials.

Materials known as unconventional superconductors can conduct electricity with no loss at higher temperatures than regular superconductors. But after 40 years of research, those temperatures are still quite cold—about 140°C below the freezing point of water. Engineering them to operate in much warmer conditions—a development that could spur revolutions in energy, microelectronics and other fields—requires a much better understanding of how these complex materials work.

Almost all the research so far has focused on very fast processes that may contribute to superconductivity—for instance, natural, high-frequency vibrations known as phonons that rattle a material’s atomic latticework trillions of times per second.

Now researchers at the Department of Energy’s SLAC National Accelerator Laboratory have taken a new look from the opposite direction: They observed how an exceedingly slow process known as atomic relaxation changes in the presence of two of the quantum states that intertwine in cuprate superconductors.

The results suggest that the relaxation process is a promising tool for exploring and understanding those two states—charge density waves (CDWs), which are stripes of higher and lower electron density in the material, and the superconducting state itself, which switches on when the material chills below its transition temperature.

The research team described the results in the Proceedings of the National Academy of Sciences.

Atomic relaxation can be triggered by many things, including doping—the introduction of a new element into a material’s atomic lattice to change its electronic properties. Doping makes the semiconductors in modern electronics possible, and it’s commonly used in research on high-performance materials such as superconductors. In cuprates, it’s used to generate superconductivity.

Atoms of the dopant replace some of the atoms in the material. But since they’re a different size than the atoms all around them, the dopant atom can’t find a comfortable spot to settle. So it slowly meanders between its neighbors, nudging the neighboring atoms to do the same, and keeps on doing that until interrupted.

“This observation of slow atomic motion is a new way to look at things,” said SLAC lead scientist Joshua Turner, a principal investigator with the Stanford Institute for Materials Science and Engineering (SIMES) at SLAC. “It can tell us all sorts of interesting things about what the electrons are doing in systems and materials that many people have been studying for a long time.”

Using the Coherent Hard X-ray Scattering beamline (CHX) at the National Synchrotron Light Source II at DOE’s Brookhaven National Laboratory, the SLAC team found that atomic relaxation took about 1,000 seconds in the cuprate they studied.

But to their surprise, atoms meandered farther away from their average positions and relaxation slowed down in the presence of CDWs. Even more surprising, this remarkable effect reversed, and relaxation accelerated, as the cuprate approached its superconducting state.

“This insight gives scientists a whole new way to explore how these quantum states intertwine on slow time scales and to understand the fundamental forces that drive unconventional superconductivity,” said Lingjia Shen, an associate staff scientist with the Linac Coherent Light Source (LCLS) X-ray laser at SLAC who played a leading role in the research.