The new wave

Print edition : January 22, 2016

A view of the experimental set-up at SLAC's Linac Coherent Light Source (LCLS) X-ray laser. Photo: SLAC National Accelerator Laboratory

A TEAM led by scientists at the United States Department of Energy’s SLAC National Accelerator Laboratory combined powerful magnetic pulses with some of the brightest X-rays to discover a surprising 3-D arrangement of a material’s electrons that appears closely linked to the phenomenon of high-temperature superconductivity. Japanese and Canadian researchers were also part of the study.

This unexpected twist marks an important milestone in the 30-year journey to understand how materials known as high-temperature superconductors conduct electricity with no resistance at temperatures hundreds of degrees above those of conventional metal superconductors but still hundreds of degrees below freezing. The study was published in “Science”.

It charts a new course for fully mapping the behaviours of electrons in these exotic materials under different conditions. Researchers have an ultimate goal to aid the design and development of new superconductors that work at warmer temperatures, even room temperature, which could lead to advances in computing, electronics and power grid technologies.

“This was totally unexpected, and also very exciting. This experiment has identified a new ingredient to consider in this field of study,” says Jun-Sik Lee, one of the authors of the experiment conducted at SLAC’s Linac Coherent Light Source (LCLS) X-ray laser.

The 3-D effect that scientists observed, 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 experiment required international expertise to prepare the specialised samples and construct a powerful customised 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, or magnetic resonance imaging, 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, which allowed scientists to measure the wave effects.

“The experiment sets very clear boundaries on the temperature and strength of the magnetic field at which the newly observed 3-D effect emerges....You can now make a definitive statement: In this material a new phase exists,” says Steven Kivelson, a Stanford University physics professor.

The experiment adds to the growing evidence that charge density waves and superconductivity “can be thought of as two sides of the same coin,” he added.

A more complete map of all of the properties of this complex material YBCO is required to reach any conclusions about which is more important for superconductivity, say Simon Gerber of Stanford Institute for Materials and Energy Sciences (SIMES) and Hoyoung Jang of Stanford Synchrotron Radiation Lightsource (SSRL), the lead authors of the study.

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