Jun 23 2016
A team of physicists at BESSY II analyzed an artificial structure comprising of alternating layers of ferromagnetic and superconducting materials. The team observed that the charge density waves stimulated by the interfaces were found to deeply extend into the superconducting regions, suggesting new methods to control superconductivity. The findings are featured in Nature Materials.
Scanning electron microscopy in combination with EELS electron spectroscopy permits to visualise atomic positions of the individual atoms in the heterostructure: Superconducting regions of YBaCuO are identified by yttrium (blue) and copper (pink), the ferromagnetic layers by manganese (green) and lanthanum (red). (Photo credit: Courtesy MPI Stuttgart)
The discovery of high-Tc superconductors took place 30 years ago. A specific class of ceramic metal oxide materials had the potential to move through electrical current without losing energy. In comparison to the standard superconductors that need to be cooled almost to absolute zero, this particular feature is already present at comparably high temperatures. The transition temperature is 92 Kelvin (minus 181 degrees centigrade) in prototypical yttrium barium copper oxide (YBaCuO). Due to this, the liquid nitrogen acts as a coolant to reach the superconducting phase. The detection of high-temperature superconductivity has led to a search for applications, which are currently being used. However, the microscopic mechanism of high-Tc superconductivity is still a debatable matter.
Superconducting and feromagnetic thin layers
A group of scientists headed by Prof. Bernhard Keimer, MPI for Solid State Research, and Dr. Eugen Weschke, HZB, have recently examined an artificial layer system comprising alternating nanolayers of YBaCuO and a ferromagnetic material. The thicknesses of the YBaCuO layers differed from 10 nm and 50 nm.
Tiny collective modulations of valence electrons observed
Since interfaces frequently decide the properties of such heterostructures, physicists were principally fascinated in their role for the current system. During Alex Frano’s PhD study using resonant X-ray diffraction at BESSY II, he was able to spot miniature united modulations of valence electrons in the region of Cu atoms in the YBaCuO layer. Data analysis showed that the ensuing charge density wave did not stay close to the interface but spread 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.
Charge density wave is stabilized
Engineering artificial interfaces in heterostructures of ferromagnetic and superconducting layers allowed to stabilize charge density waves even in the presence of superconductivity: YBaCuO remains superconducting, while the charges arrange in a periodic structure exploring the details of this coexistence on a microscopic scale is a challenging task for future experiments.
Dr Eugen Weschke, HZB
A highly exciting standpoint of the current results is making it possible for controlling the superconducting condition itself.