New dimension to high-temperature superconductivity discovered

An unprecedented blend of intense magnetic and X-ray laser pulses produces surprising 3-D effect
November 5, 2015

In this artistic rendering, a magnetic pulse (right) and X-ray laser light (left) converge on a high-temperature superconductor to study the behavior of its electrons (credit: SLAC National Accelerator Laboratory)

The dream to push the operating temperature for superconductors to room temperature — leading to future advances in computing, electronics and power grid technologies — has just become more real.

A team led by scientists at the Department of Energy’s SLAC National Accelerator Laboratory has combined powerful magnetic pulses with some of the brightest X-rays on the planet, discovering a surprising 3-D arrangement of a material’s electrons that appears closely linked to high-temperature superconductivity.

The scientists say this unexpected twist marks an important milestone in the 30-year journey to better understand how materials known as high-temperature superconductors conduct electricity with no resistance at temperatures hundreds of degrees Fahrenheit above those of conventional metal superconductors (but still hundreds of degrees below freezing). The study was published today (Nov. 5) in Science.

There are already many uses for standard low-temperature superconducting technology, from MRI machines that diagnose brain tumors to a prototype levitating train, the CERN particle collider that enabled the Nobel Prize-winning discovery of the Higgs boson, and ultrasensitive detectors used to hunt for dark matter — the invisible constituent believed to make up most of the mass of the universe.

‘Totally unexpected’ physics

“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.  (A planned upgrade to the LCLS, known as LCLS-II, will include a superconducting particle accelerator.) “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. Its coexistence with superconductivity is perplexing to researchers because it seems to conflict with the freely moving electron pairs that define superconductivity.

The 2-D version of this wave was first seen in 2012 and has been studied extensively. The LCLS experiment revealed a separate 3-D version that appears stronger than the 2-D form and 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.

A powerful blend of magnetism and light

This custom-made magnet was used in an experiment at SLAC’s Linac Coherent Light Source X-ray laser to study an effect known as a charge density wave. (credit: Jun-Sik Lee)

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 ultra-bright LCLS X-ray laser pulses, which 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.*

“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,” he said. “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.

A view of the X-ray Correlation Spectroscopy experimental station at SLAC’s Linac Coherent Light Source (LCLS) X-ray laser. This station was used for an experiment studying an effect in a superconducting material. (credit: SLAC National Accelerator Laboratory)

A more complete map

But it is also clear 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.

* Researchers conducted many preparatory experiments at SLAC’s Stanford Synchrotron Radiation Lightsource (SSRL), which also produces X-rays for research. LCLS and SSRL are DOE Office of Science User Facilities. Scientists from SIMES, the Stanford Institute for Materials and Energy Sciences at SLAC, and SSRL and LCLS were a part of the study.

Abstract of Three-Dimensional Charge Density Wave Order in YBa2Cu3O6.67 at High Magnetic Fields

Charge density wave (CDW) correlations have been shown to universally exist in cuprate superconductors. However, their nature at high fields inferred from nuclear magnetic resonance is distinct from that measured by x-ray scattering at zero and low fields. Here we combine a pulsed magnet with an x-ray free electron laser to characterize the CDW in YBa2Cu3O6.67 via x-ray scattering in fields up to 28 Tesla. While the zero-field CDW order, which develops below T ~ 150 K, is essentially two-dimensional, at lower temperature and beyond 15 Tesla, another three-dimensionally ordered CDW emerges. The field-induced CDW appears around the zero-field superconducting transition temperature; in contrast, the incommensurate in-plane ordering vector is field-independent. This implies that the two forms of CDW and high-temperature superconductivity are intimately linked.