New evidence for self-doping in cuprate superconductors

A comparison of XAS data from two different temperatures shows that cooling results in a redistribution of charges between the superconducting copper oxide planes and chains.

YBCO is a cuprate ceramic material that becomes superconducting when it is cooled down below -183°C. Since the resistance and the power losses are zero in the superconductors, they find many promising applications. Electromagnets in electric motors can be made smaller with stronger magnetic field that gives more power and lower energy consumption. Magnetic levitating trains reach higher speeds by minimizing friction between wheels and the rails.

While technical benefits of high-temperature superconductors are clear, the mechanism of how these materials become superconducting at a relatively high temperature remains one of the major unsolved problems of condensed matter physics. Now, the researchers made a discovery that may give a new hint to the puzzle of high-temperature superconductivity.

Both X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) were used to investigate properties of optimally doped YBCO thin films grown at Chalmers University of Technology.
Experiments were performed at both room temperature and at -258°C, which is far below superconducting transition temperature. YBCO is a special high-temperature superconductor that is built of stacked copper oxide planes separated by copper oxide chains. The planes are commonly believed to carry the superconducting current.

The chains’ role in YBCO has puzzled scientists ever since the discovery of this material in 1987. It has long been known that the chains are associated with the electronic doping (x in the chemical formula). Recently it has been discovered that the chains are functioning as the “charge reservoir” that can be used to tune the temperature of the superconducting transition.

Contrary to previous assumptions, the new experimental results show that the chains in YBCO also react to a cooling by supplying copper oxide planes with positive charges (holes), so-called self-doping (see picture). By combining RIXS and ionic model calculations, the researchers concluded that this happens due to a reconstruction of the copper and oxygen molecular orbitals linking the planes and the chains.

The groundbreaking information about YBCO self-doping challenges the traditional understanding of the mechanism of superconductivity in copper-based high-temperature superconductors based on assumption of constant doping level in the copper oxide planes.
The interpretation of a number of previous temperature-dependent experiments has to be reconsidered in view of new results, and mystery of high-temperature superconductivity is getting closer to solution.

The researchers plan to conduct a more detailed temperature-dependent study to determine if orbital reconstruction occurs during the superconducting transition or if it’s already happening at a higher temperature in the so-called pseudogap region.

Article: Self-doping processes between planes and chains in the metal-to-superconductor transition of YBa2Cu3O6.9; M. Magnuson, T. Schmitt, V.N. Strocov, J. Schlappa, A. S. Kalabukhov and L.-C. Duda; Scientific Reports 4, 717 (2014). DOI: 10.1038/srep07017, Published: 12 November 2014