18.10.2012

X-rays reveal surprising behaviour of superconductors

Ceramic superconductors don’t behave as expected, as a detailed X-ray analysis at the DESY X-ray source DORIS shows. An international research team observed unexpected structural distortions in the material at very low temperatures. These distortions, known as charge density waves, have a big influence on superconductivity, i.e. the lossless transport of electric current. The scientists led by Johan Chang of the Swiss Federal Institute of Technology in Lausanne and the Paul Scherrer Institute present their results in the current issue of the journal “Nature Physics”. The analysis contributes to a better understanding of high-temperature superconductors, whose inner workings are in many aspects still mysterious more than 25 years after their discovery.

Charge density waves in the crystal pattern have impact on the behaviour of the superconductor.

Superconductors are materials that lose all electrical resistance below a characteristic temperature and then conduct electric current without any loss. This technology is interesting for a number of energy-intensive applications in industry and research. The world’s largest particle accelerator, the LHC near Geneva, for example, makes use of superconducting magnetic coils.

However, classic superconductors must be cooled to a few degrees above absolute zero (minus 273.15 degrees Celsius). This is only possible using the noble gas helium, a procedure that is complicated and expensive. In contrast, special ceramic compounds known as high-temperature superconductors often conduct electric current without loss already above the temperature of liquid nitrogen (minus 196 degrees Celsius). They can be cooled with liquefied air, which is much cheaper and easier.

The discovery of ceramic superconductors was honoured with the Nobel Prize in Physics in 1987. But what exactly is going on in the materials is still not fully understood. The more precisely researchers understand these materials, the better they can tailor their properties. This way it might some day become possible to construct a superconducting material that transports electric current without loss even at room temperature.

Using measurements at the synchrotron radiation source DORIS at DESY, the team led by Chang was now able to observe how a ceramic superconductor from the widespread YBCO group (named after the elements yttrium, barium, copper and oxygen, which make it up) changed into the resistance-free state. With decreasing temperature, the electrons, which are normally uniformly distributed throughout the material, rearranged themselves in periodic patterns. The lattice ions of the crystal shifted accordingly – creating a wavelike charge density, which at very low temperatures may be energetically more favourable than the uniform charge distribution.

“It was surprising to see a charge density wave in a material with such a high critical temperature as YBCO, as this phenomenon is more commonly known from non-superconductors,” explained DESY scientist Martin von Zimmermann, a member of the research team. The charge density waves affect the superconductivity in the material, its characteristic critical temperature for the transition into the resistance-free state decreases. If the superconductivity is suppressed by a magnetic field, the charge density waves increase, as the researchers observed with the help of a specially designed magnet from the University of Birmingham. With 17 Tesla this magnet is more than 300 000 times stronger than earth´s magnetic field. The scientists were also able to detect the distortion of the crystal lattice at much higher temperatures than in previous magnetic resonance studies. This discrepancy between the two studies is to be clarified in future measurements.

The charge density waves and superconductivity represent two competing configurations of the material, the scientists concluded. “Using the magnetic field, we were able to beautifully show the competing behaviour of charge density waves and superconductivity,” von Zimmermann explained. “At high field strengths, the charge density wave is large and superconductivity small, and vice versa at small fields.”

Further analyses will examine this competition in more detail and clarify how the interplay between the two configurations takes place. Building on this knowledge, it could one day be possible to construct better superconductors.

The study was carried out by researchers from PSI and DESY, the Swiss Federal Institute of Technology in Lausanne, the Universities of Bristol and Birmingham, the Danish Technical University, the University of British Columbia (Canada) and the Canadian Institute for Advanced Research.

Original Publication

Direct observation of competition between superconductivity and charge density wave order in YBa₂Cu₃O_6.67; Johan Chang et al.; Nature Physics (2012, online in advance); DOI: 10.1038/NPHYS2456

Sample of the high-temperature superconductor.