Scientists reveal that superconductivity and charge density waves are intrinsically related at the nanoscopic level, a new knowledge that could pave the way for the next generation of electronics and computers. Room-temperature superconductors have the potential to alter everything from electrical grids to particle accelerators to computers, but before they can be realized, researchers need to better understand how existing high-temperature superconductors work.
Researchers from the Department of Energy’s SLAC National Accelerator Laboratory, the University of British Columbia, Yale University, and others have taken a step in that direction by examining the fast dynamics of a material known as yttrium barium copper oxide, or YBCO.
The researchers write in Science that YBCO’s superconductivity is inextricably linked to another phenomena known as charge density waves (CDWs), or ripples in the density of electrons in the material. When the researchers turned off YBCO’s superconductivity, CDWs became stronger, as expected. They were shocked, however, to discover that the CDWs suddenly became more spatially structured, implying that superconductivity fundamentally changes the form of the CDWs at the nanoscale.
“A big part of what we don’t know is the relationship between charge density waves and superconductivity,” said Giacomo Coslovich, a staff scientist at the Department of Energy’s SLAC National Accelerator Laboratory, who led the study. “As one of the cleanest high-temperature superconductors that can be grown, YBCO offers us the opportunity to understand this physics in a very direct way, minimizing the effects of disorder.”
He added, “If we can better understand these materials, we can make new superconductors that work at higher temperatures, enabling many more applications and potentially addressing a lot of societal challenges — from climate change to energy efficiency to availability of fresh water.”
We conducted these tests at the LCLS because we need ultrashort X-ray pulses, which can only be produced in a few sites on the planet. We also needed soft X-rays, which have longer wavelengths than standard X-rays, to detect the CDWs directly.
Joshua Turner
Observing fast dynamics
The researchers investigated the kinetics of YBCO using SLAC’s Linac Coherent Light Source (LCLS) X-ray laser. They used infrared laser pulses to turn off superconductivity in the YBCO samples, and then bounced X-ray photons off those samples. The scientists pieced together a kind of picture of the CDWs’ electron ripples for each X-ray shot. They replicated the CDW’s rapid evolution by pasting those together.
“We conducted these tests at the LCLS because we need ultrashort X-ray pulses, which can only be produced in a few sites on the planet. We also needed soft X-rays, which have longer wavelengths than standard X-rays, to detect the CDWs directly” said Joshua Turner, a staff scientist and study co-author at the Stanford Institute for Materials and Energy Sciences. “In addition, the folks at LCLS are fantastic to work with.”
These LCLS experiments generated terabytes of data, which was difficult to handle. “LCLS beamline experts binned our massive volumes of data into a more manageable form so our algorithms could extract the feature properties,” said MengXing (Ketty) Na, a University of British Columbia graduate student and research co-author.
After lasers turned off the superconductivity, the scientists discovered that charge density waves within the YBCO samples became more correlated – that is, more electron ripples were periodic or spatially synchronized.
“Doubling the number of waves connected with a single flash of light is particularly astounding, because light normally has the opposite effect. If we push too hard, we can entirely disrupt the charge density waves with light “Coslovich explained.
The researchers then analyzed how regions of CDWs and superconductivity should interact given a number of fundamental assumptions about how YBCO functions to explain these experimental findings. For example, their first model suggested that when a uniform region of superconductivity was turned off with light, it would transform into a uniform CDW region – which, of course, did not match with their data.
“The model that best fits our findings so far suggests that superconductivity is working as a defect within a wave pattern. This implies that superconductivity and charge density waves prefer to be grouped in a very precise, nanoscopic pattern “Coslovich elaborated. “At the length scale of the waves themselves, they are linked orders.”
Illuminating the future
Coslovich described the ability to turn off superconductivity with light pulses as a significant achievement, allowing observations on the time scale of less than a trillionth of a second, with significant advantages over earlier approaches.
“When you use other approaches, such as producing a high magnetic field, you have to wait a long time before performing measurements, which allows CDWs to reorder around disarray and other phenomena to occur in the sample,” he explained. “By using light, we were able to demonstrate that this is an intrinsic effect, a genuine link between superconductivity and charge density waves.”
Turner stated that the research team is eager to build on this critical finding. First, they want to investigate how the CDWs become more structured when superconductivity is turned off by light. They also intend to alter the laser’s wavelength or polarization in future LCLS studies in the hopes of using light to promote, rather than quench, the superconducting state, allowing them to easily switch it on and off.
“There is a general interest in trying to do this with light pulses on very fast time scales because it could lead to the development of superconducting, light-controlled devices for the next generation of electronics and computers,” said Coslovich. “Ultimately, this work can also help guide people who are trying to build room-temperature superconductors.”
