A team led by scientists at the US Dept. of Energy (DOE)’s SLAC National Accelerator Laboratory has observed a new type of “charge density wave” after combining powerful magnetic pulses with some of the brightest X-rays on the planet. The resulting 3-D effect appears closely linked to high temperature superconductivity, though its coexistence with superconductivity is perplexing to researchers because it seems to conflict with the freely moving electron pairs that define superconductivity.
“This was totally unexpected, and also very exciting. This experiment has identified a new ingredient to consider in this field of study. Nobody had seen this 3-D picture before,” said Jun-Sik Lee, a SLAC staff scientist and one of the leaders of the experiment conducted at SLAC’s Linac Coherent Light Source (LCLS) X-ray laser. “This is an important step in understanding the physics of high temperature superconductors.”
The 3-D effect that scientists observed in the LCLS experiment, which occurs in a superconducting material known as YBCO (yttrium barium copper oxide), is a newly discovered type of charge density wave. This wave does not have the oscillating motion of a light wave or a sound wave. It describes a static, ordered arrangement of clumps of electrons in a superconducting material. A 2-D version of this wave was first seen in 2012 and has been extensively studied. The LCLS 3-D version appears stronger than the 2-D form but closely tied to both the 2-D behavior and the material’s superconductivity.
The experiment was several years in the making and required international expertise to prepare the specialized samples and construct a powerful customized magnet that produced magnetic pulses compressed to thousandths of a second. Each pulse was 10-20 times stronger than those from the magnets in a typical medical MRI machine.
Those short but intense magnetic pulses suppressed the superconductivity of the YBCO samples and provided a clearer view of the charge density wave effects. They were immediately followed at precisely timed intervals by ultrabright LCLS X-ray laser pulses that allowed scientists to measure the wave effects. “This experiment is a completely new way of using LCLS that opens up the door for a whole new class of future experiments,” said Mike Dunne, LCLS director.
Researchers conducted many preparatory experiments at SLAC’s Stanford Synchrotron Radiation Lightsource (SSRL) and Argonne National Laboratory’s Advanced Photon Source (APS). LCLS, SSRL and APS are DOE Office of Science User Facilities. Scientists from the Stanford Institute for Materials and Energy Sciences at SLAC (SIMES), SSRL and LCLS were a part of the study.
“I’ve been excited about this experiment for a long time,” said Steven Kivelson, a Stanford University physics professor who contributed to the study and has researched high temperature superconductors since 1987. Kivelson said the experiment sets very clear boundaries on the temperature and strength of the magnetic field at which the newly observed 3-D effect emerges. “There is nothing vague about this. You can now make a definitive statement: In this material a new phase exists.”
The experiment also adds weight to the growing evidence that charge density waves and superconductivity “can be thought of as two sides of the same coin,” he added.
It is also clear, however, that YBCO is incredibly complex, and a more complete map of all of its properties is required to reach any conclusions about what matters most to its superconductivity, said Simon Gerber of SIMES and Hoyoung Jang of SSRL, the lead authors of the study.
Follow-up experiments are needed to provide a detailed visualization of the 3-D effect and to learn whether the effect is universal across all types of high temperature superconductors, said SLAC staff scientist and SIMES investigator Wei-Sheng Lee, who co-led the study with Jun-Sik Lee of SSRL and Diling Zhu of LCLS. “The properties of this material are much richer than we thought,” Lee said.
“We continue to make new and surprising observations as we develop new experimental tools,” Zhu added.