29.08.2012

Deep X-ray gaze reveals: superconductors are more complex than expected

Mysteriously disappearing charge stripes

Ceramic superconductors are more complex than expected. This was shown by the investigation of so-called lanthanum cuprates with the X-ray sources DORIS III in Hamburg and BESSY II at Helmholtz-Zentrum Berlin HZB. The electrical structures which form in this material may be quite different near the surface than in the bulk. This information is important to understand the complex processes in these materials, helping to build new superconductors with tailor-made properties. However, it may requires to add a series of investigations, as the international team of HZB scientist Christian Schüßler-Langeheine report in the scientific journal "Nature Communications".

Superconductors are materials which lose their electrical resistance below a characteristic transition temperature, thus conducting an electrical current absolutely resistance-free. This property is very useful for many applications. However, since the transition temperature of classical superconductors is close to absolute zero (minus 273.15 centigrade), it requires helium for cooling. Helium is expensive and liquefaction is sophisticated; for this reason, this method is generally restricted to research facilities as the world’s largest particle accelerator LHC at CERN in Geneva or the European XFEL X-ray laser in Hamburg which is currently under construction.

An attractive perspective, however, is provided by so-called high-temperature superconductors (HTS); some of them losing their electrical resistance at the temperature of liquid nitrogen (barely minus 200 centigrade). Liquid nitrogen, often also called liquid air, is much cheaper and easier to produce than liquid helium. Nevertheless, these ceramic superconductors, discovered in 1986, are often brittle and difficult to process. Moreover, the processes that make this material superconductive are complex and still not resolved in detail. With a complete understanding of the physical processes in these materials, scientists hope to build tailor-made high-temperature superconductors which possibly provide resistance-free conductivity even at room temperature.

An important material class for high-temperature superconductors are the so-called cuprates, these are copper compounds with copper anions. The "mother" of these HTS is a cuprate containing the rare earth lanthanum, the so-called lanthanum cuprate. From these compounds it is possible to produce superconductors with different properties by doping it with different metals as for example barium or strontium. In 1986, a lanthanum barium cuprate was the first compound that was identified as a high-temperature superconductor.

Even today, the details of the high-temperature superconductor mechanism are not completely understood. However, investigations with X-ray radiation and with neutrons have shown that doping lanthanum cuprates with metals leads to the formation of a magnetic and electrical stripe structure. Electrically charged, non-magnetic copper ions form regular stripes. The scientists assume that these stripes may suppress superconductivity.  Although seen in several other high-temperature superconductors, the electrical stripe structure was so far not verified with lanthanum strontium cuprates.

At the X-ray source BESSY II, the team of Schüßler-Langeheine managed to spot the charge stripes in lanthanum strontium cuprate at last. However, a more in-depth investigation at the DESY X-ray source DORIS III showed the structure disappearing in the lower levels of the material. The reason for this is not yet clear, but the physicists hope that the information from the lanthanum strontium cuprate will help to interpret the general significance of charge stripes for superconductivity.

The X-ray analysis also demonstrates that the most commonly used investigation method surprisingly does not always show the complete image. "A lot of information on cuprates is based on surface investigations," Schüßler-Langheine explains. "Obviously, the surface properties can differ from bulk properties."  Additional investigations will show how much the properties of other superconducting cuprates vary between surface and volume.

 

Original paper:Charge stripe order near the surface of 12 percent-doped La2-xSrxCuO4; Wu et al.; "Nature Communications"; DOI 10.1038/ncomms2019