Q&A: Jun-Sik Lee explains how new SSRL capabilities will probe the mysteries of superconductors
The upgrades to SSRL’s resonant soft X-ray scattering beamline could reveal the hidden physics in high-temperature superconductors.
High-temperature superconductors hold tremendous promise. Because they conduct electricity without any resistance at cold but still relatively practical temperatures, they could make our energy infrastructure vastly more efficient and usher in new technologies in medicine and transportation, among other fields.
But before we can get to those innovations, researchers need to better understand how these materials work, which is where Jun-Sik Lee, a senior scientist at the Department of Energy’s SLAC National Accelerator Laboratory comes in.
Lee has positioned himself at the forefront of an experimental method called resonant soft X-ray scattering (RSXS). That method, which combines X-ray scattering and spectroscopy techniques, helps researchers see patterns of electron behavior inside high-temperature superconductors and other materials.
Now, Lee said, SLAC’s Stanford Synchrotron Radiation Lightsource (SSRL) is on a path toward world-leading RSXS capabilities. With a suite of upgrades just unveiled at SSRL’s Beam Line 13-3, Lee answered some questions about high-temperature superconductors and how those upgrades could help researchers get to the bottom of how they work, potentially opening the door to a more energy-efficient future.
What kinds of scientific questions are you interested in?
At a high level, one of the most important scientific questions is to understand why copper oxides, or cuprates, exhibit such high superconducting transition temperatures. In some cases these are as high as 133 kelvins (-140 degrees Celsius or -220 degrees Fahrenheit). That is much higher than conventional superconductors, which only work at temperatures just a few degrees above absolute zero, or 0 kelvins.
Superconductivity
Superconductivity is the property of certain materials to conduct electricity without energy loss when they are cooled below a critical temperature.
Solving this mystery could revolutionize energy technologies by enabling zero-resistance conductivity at practical temperatures, leading to 100% energy efficiency. If we uncover the fundamental mechanism behind high-temperature superconductivity, it could completely transform modern energy grids and future technologies.
What are the challenges researchers face in achieving that goal?
Currently, we don’t have a clear understanding of the fundamental mechanism of high-temperature superconductivity. Despite decades of research with enormous efforts from both experimental and theoretical sides, its microscopic origin remains elusive. Many studies suggest that the key lies in understanding the complex interactions between different electronic phases, such as superconducting states and charge density waves.
To tackle this challenge, we need to probe correlations between many different electronic properties at the microscopic level. However, this is technically demanding because many quantum phenomena, including those involving superconductors, emerge at extremely low temperatures and exhibit subtle or fluctuating patterns in their behavior. From an X-ray perspective, simultaneously achieving high resolution, a wide range of angles on our sample, and low temperatures – the factors that get us the best data – is challenging.
What is RSXS, and how does it help you study superconductors?
RSXS is a powerful technique used for directly probing electronic order in complex materials. Imagine looking at a painting through a pair of glasses that only shows one color. RSXS works similarly, but instead of color, it can be tuned to specific atomic elements in a material. This allows scientists to study how electrons are arranged at a very small scale, helping to understand why some materials become superconducting.
In cuprates, we can tune the X-rays to a specific frequency which only the copper atoms absorb. As a result, we can directly explore those atoms’ motion across the superconducting temperature. This makes it possible to detect subtle changes in various electronic properties with high precision. For example, RSXS can reveal a charge density wave and other patterns that are not easily accessible through other techniques.
You’ve just published a paper on some of the upgrades you’ve made to SSRL’s setup at Beam Line 13-3. Can you tell us about those upgrades and why they’re important?
One key issue was that the setup had a base temperature of about 25 kelvins (-250 degrees Celsius, or about -417 degrees Fahrenheit). Even though high-temperature superconductors exhibit superconducting behavior at relatively higher temperatures, their fundamental properties are still associated with the state of the system at absolute zero temperature, -273 degrees Celsius or -460 degrees Fahrenheit. By cooling the sample to low temperatures as much as possible, we can isolate and study the intrinsic quantum mechanical behavior of the superconducting state without interference from thermal disturbances or other competing interactions.
During the height of the pandemic, SLAC staff engineer Cheng-Tai Kuo, lead scientist Makoto Hashimoto and senior scientist Donghui Lu initiated a series of upgrades, one of which lowered the base temperature to around 10 kelvins (-263 degrees Celsius or -441 degrees Fahrenheit). We have also improved the angular motion of the system, which helps us collect better data.
What’s next for RSXS at SLAC?
We are currently focusing on uncovering hidden physics in high-temperature superconductors, particularly the complex interplay between charge-density waves and superconductivity, as well as details of their behavior below the superconducting temperature. Understanding those interactions could significantly advance the field of superconductivity.
We are also considering integrating new and more extreme sample environments for the next-generation RSXS instrument at low temperatures. For example, we’re considering methods to apply mechanical strain and high magnetic fields, both of which can serve as tuning knobs to improve our understanding of superconducting mechanism. The low-temperature capabilities will also be leveraged to facilitate more complex sample environments, such as those involving magnetic fields and mechanical strain.
We also aim to develop ultrafast RSXS capabilities to study how quantum states change over time, which would bring a synergy with the new capability at LCLS-II. We believe that these technical developments will allow us to gain new insights into the nature of quantum phases and how they’re structured. The next step for RSXS development at Beam Line 13-3 will position SSRL as a world-leading facility for studying complex quantum materials and emergent electronic phenomena.
The research was supported by the DOE Office of Science and the DOE Office of Manufacturing and Energy Supply Chains. SSRL is a DOE Office of Science user facility.
Citation: Cheng-Tai Kuo et al., Review of Scientific Instruments, 6 June 2025 (10.1063/5.0257317)
For questions or comments, contact SLAC Strategic Communications & External Affairs at communications@slac.stanford.edu.
About SLAC
SLAC National Accelerator Laboratory explores how the universe works at the biggest, smallest and fastest scales and invents powerful tools used by researchers around the globe. As world leaders in ultrafast science and bold explorers of the physics of the universe, we forge new ground in understanding our origins and building a healthier and more sustainable future. Our discovery and innovation help develop new materials and chemical processes and open unprecedented views of the cosmos and life’s most delicate machinery. Building on more than 60 years of visionary research, we help shape the future by advancing areas such as quantum technology, scientific computing and the development of next-generation accelerators.
SLAC is operated by Stanford University for the U.S. Department of Energy’s Office of Science. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time.
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