Produced by scanning electron microscopy and EELS electron spectroscopy, this image shows the positions of the individual atoms in the artificial layer system. Superconducting regions comprise yttrium (blue) and copper (pink), while the ferromagnetic layers comprise manganese (green) and lanthanum (red). Image: MPI Stuttgart.
Produced by scanning electron microscopy and EELS electron spectroscopy, this image shows the positions of the individual atoms in the artificial layer system. Superconducting regions comprise yttrium (blue) and copper (pink), while the ferromagnetic layers comprise manganese (green) and lanthanum (red). Image: MPI Stuttgart.

By studying an artificial structure composed of alternating layers of ferromagnetic and superconducting materials, German physicist have discovered that charge density waves induced by the interfaces extend deeply into the superconducting regions, indicating new ways to manipulate superconductivity. The results are published in a paper in Nature Materials.

High-temperature superconductors were discovered 30 years ago, comprising a class of ceramic metal oxide materials that can pass electrical current without energy losses at comparatively high temperatures. In yttrium barium copper oxide (YBaCuO), for example, the transition temperature for superconductivity is 92K (-181°C), allowing liquid nitrogen to be used as a coolant for reaching the superconducting phase.

Since the discovery of these materials, however, the microscopic mechanism responsible for their high-temperature superconductivity has remained a matter of debate. To try to resolve this debate, a team of physicists lead by Bernhard Keimer at the Max Planck Institute (MPI) for Solid State Research and Eugen Weschke at the Helmholtz-Zentrum Berlin für Materialien und Energie (HZB) investigated an artificial layer system composed of alternating nanolayers of YBaCuO and a ferromagnetic material. The thicknesses of the YBaCuO layers varied between 10nm and 50nm.

As the interfaces between the layers often determine the properties of such heterostructures, the physicists were particularly interested in investigating their role in this layer system. During his PhD work using resonant x-ray diffraction at HZB, Alex Frano detected tiny collective modulations of valence electrons around Cu atoms in the YBaCuO layer. Data analysis revealed that the resulting charge density wave does not remain located close to the interface but extends across the whole layer. "This finding is quite a surprise, as previous studies revealed a strong tendency of superconductivity to suppress the formation of charge density waves," explains Frano.

"Engineering artificial interfaces in heterostructures of ferromagnetic and superconducting layers allowed us to stabilize charge density waves even in the presence of superconductivity: YBaCuO remains superconducting, while the charges arrange in a periodic structure," explains Weschke. "Exploring the details of this coexistence on a microscopic scale is a challenging task for future experiments." This finding could potentially pave the way to controlling the superconducting state itself.

This story is adapted from material from the Helmholtz-Zentrum Berlin für Materialien und Energie, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.