Scientists find that triggering superconductivity with a flash of light involves the same fundamental physics that is at work in the more stable states needed by devices, opening a new path to producing room temperature superconductivity.
Just as people can learn about themselves by stepping out of their comfort zone, researchers can learn about a system by giving it a jolt that makes it a bit unstable – scientists call this “unbalanced” – and observing what is happening. settles back into a more stable state.
In the case of a superconducting material known as yttrium barium copper oxide, or YBCO, experiments have shown that, under certain conditions, unbalancing it with a laser pulse allows it to superconduct – conduct the lossless electrical current – much closer to room temperature than the researchers expected. That could be a big deal, given that scientists have been studying room-temperature superconductors for more than three decades.
But do observations of this unstable state have any bearing on the operation of high-temperature superconductors in the real world, where applications such as power lines, maglev trains, particle accelerators and medical equipment demand that they are stable?
A study published in Scientists progress on February 9, 2022, suggests the answer is yes.
“People thought that while this kind of study was useful, it didn’t hold much promise for future applications,” said Jun-Sik Lee, a researcher at the Department of Energy’s SLAC National Accelerator Laboratory and chief of the international research team that conducted the study.
“But now we have shown that the fundamental physics of these unstable states is very similar to that of stable states. So this opens up huge opportunities, including the possibility that other materials can also be pushed into a transient superconducting state with light. It is an interesting state that we cannot otherwise see.
What does normal look like?
YBCO is a compound of copper oxide, or cuprate, a member of a family of materials that was discovered in 1986 to conduct electricity with zero resistance at temperatures much higher than scientists thought possible.
Like conventional superconductors, which had been discovered more than 70 years earlier, YBCO changes from a normal state to a superconducting state when cooled below a certain transition temperature. At this point, the electrons pair up and form a condensate – a kind of electron soup – which conducts electricity effortlessly. Scientists have a solid theory on how this happens in old-school superconductors, but there’s still no consensus on how it works in unconventional superconductors like YBCO.
One way to attack the problem is to study the normal state of YBCO, which is quite strange in itself. The normal state contains a number of complex and intertwined phases of matter, each with the potential to help or hinder the transition to superconductivity, which jostle for dominance and sometimes overlap. Moreover, in some of these phases, the electrons seem to recognize each other and act collectively, as if dragging each other.
It’s a real tangle, and the researchers hope that better understanding will help understand how and why these materials become superconductors at temperatures well above the predicted theoretical limit for conventional superconductors.
Exploring these fascinating normal states is difficult at the hot temperatures in which they occur, so scientists typically cool their YBCO samples to the point where they become superconducting, then turn off the superconductivity to restore the normal state.
Switching is usually done by exposing the material to a magnetic field. This is the preferred approach because it leaves the material in a stable configuration – the kind you would need to create a practical device.
Superconductivity can also be turned off with a pulse of light, Lee said. This creates a normal, somewhat unbalanced state – out of balance – where interesting things can happen, from a scientific point of view. But the fact that it is unstable has made scientists reluctant to assume that everything they learn there can also be applied to stable materials like those needed for practical applications.
Waves that stay in place
In this study, Lee and his collaborators compared the two switching approaches – magnetic fields and light pulses – focusing on how they affect a particular phase of matter known as charge density waves, or CDW, which appears in superconducting materials. CDWs are wave patterns of higher and lower electron density, but unlike ocean waves, they do not move.
Two-dimensional CDWs were discovered in 2012, and in 2015 Lee et al. discovered a new 3D type from CDW. Both types are intimately related to high temperature superconductivity and can serve as markers of the transition point where superconductivity turns on or off.
To compare what CDWs look like in YBCO when its superconductivity is turned off with light versus magnetism, the research team performed experiments on three X-ray light sources.
They first measured the properties of the unperturbed material, including its charge density waves, at the Stanford Synchrotron Radiation Light Source (SSRL) at SLAC.
Next, samples of the material were exposed to high magnetic fields at the SACLA synchrotron facility in Japan and to laser light from the free-electron X-ray laser at the Pohang Accelerator Laboratory (PAL-XFEL) in Korea, so that changes in their CDWs could be measured.
“These experiments showed that exposing the samples to magnetism or light generated similar 3D patterns of CDW,” said SLAC scientist and study co-author Sanghoon Song. Although it is still unclear how and why this occurs, the results demonstrate that the states induced by either approach have the same fundamental physics. And they suggest that laser light could be a good way to create and explore transient states that could be stabilized for practical applications – including, potentially, room temperature superconductivity.
Reference: “Characterization of the normal state photoinduced by the charge density wave in the YBa superconductor2Cu3O6.67” by Hoyoung Jang, Sanghoon Song, Takumi Kihara, Yijin Liu, Sang-Jun Lee, Sang-Youn Park, Minseok Kim, Hyeong-Do Kim, Giacomo Coslovich, Suguru Nakata, Yuya Kubota, Ichiro Inoue, Kenji Tamasaku, Makina Yabashi, Heemin Lee, Changyong Song, Hiroyuki Nojiri, Bernhard Keimer, Chi-Chang Kao and Jun-Sik Lee, February 9, 2022, Scientists progress.
Researchers from Pohang Accelerator Laboratory and Pohang University of Science and Technology in Korea; Tohoku University, RIKEN Super Photon ring-8 GeV". It is owned by RIKEN and located in Harima Science Garden City, Hyogo Prefecture, Japan.