Self-doped superconductors

YBCO is one of the most well-known ceramic copper-based superconductors. Its critical temperature is -183 Celsius, but what if there were a way to raise that temperature to make these invaluable materials viable without cryogenic cooling? New clues regarding the properties of YBCO from Swedish and Swiss researchers could be the first step towards such a goal [M. Magnuson et al. (2014) Sci Rep, 4, 717; DOI: 10.1038 /srep07017].

Writing in the journal Science Reports, materials scientists from Linköping and Uppsala University and Chalmers University of Technology and colleagues at the Swiss Synchrotron Light Source describe their X-ray spectroscopic findings on YBCO (YBa2Cu3O7-x) Their findings are published in the Nature journal Science Reports and point to a better understanding of how superconductivity arises in these materials. They carried out X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) experiments on YBCO at room temperature and at a chilly -258 Celsius, a temperature much colder than the material's critical temperature.

YBCO contains two types of structural units: stacked "planes" of copper oxide, which are thought to carry the superconducting current and separate "chains" of copper oxide that lie in between these planes. The role of these chains has puzzled scientists since the discovery of YBCO in 1987. However, a hint lay in the fact that doping the chains with oxygen and so changing their length can alter the critical temperature. That said, most researchers assumed that the doping level of the material was solely determined by the structure of the chains at the time of synthesis.

Now the team from Sweden and Switzerland has demonstrated that the chains in YBCO react to cooling by supplying the copper oxide planes with positive charges (electron holes) through a self-doping mechanism. Model calculations incorporating the RIXS data revealed that this self-doping process is accompanied by changes in the copper-oxygen bonds that link the chains to the planes. The finding challenges the conventional wisdom regarding how YBCO becomes a superconductor wherein a constant doping level in the copper oxide planes is assumed. The team suggests that earlier temperature-dependent experiments may have to be re-evaluated, which could add to clues to solve the puzzle of high-temperature superconductivity.

The team is now working on a more detailed temperature-dependent study that they hope will show whether restructuring and redistribution of the occupation of orbitals occurs at the phase transition to superconductivity or if this is a change that happens at higher temperature in the so-called pseudogap region. If the latter, then the implication would be that the critical temperature might be nudged higher by manipulating the chemistry to this end.

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the popular science book "Deceived Wisdom".