Nanocomposites and nanomaterials

Percolation effects in high-temperature superconductor/half-metallic ferromagnet nanocomposites
Krivoruchko V.N., Tarenkov V.Yu.
Donetsk Institute for Physics and Engineering, the NAS of Ukraine. Prospect Nauki, 46, Kyiv-03028, Ukraine. 
E-mail: krivoruc@gmail.com

The system we study is random nanocomposites of a half-metallic ferromagnet and a high-temperature superconductor with different volume fraction and a large geometrical disparity: nanoparticles (~20–30nm) of La0.7Sr0.3MnO3 (LSMO) and particles (~5–10µm) of Bi2Sr2Ca2Cu3O6+x (Bi2223). Transport characteristics of the nanocomposites and, in particular, a superconducting percolation transition arising through contacts between components of different physical properties have been investigated. It was found that the classical percolation theory is strongly affected by both a large geometrical disparity between components and by the proximity effect. 
In the case of a composite system of single-valued noninteracting particles a percolation transition is observed already at 20% of the volume concentration of one component [1]. Thus, in our case, already 20% of Bi2223 should be enough to observe a superconducting transition of the composite. In fact, we have observed an opposite situation. Already about 20% of LSMO is enough to breakdown the superconducting networks. A superconducting transition of the nanocomposites with Bi2223 volume concentration above 80% becomes substantially wider and a network of weakly-coupled contacts through ferromagnetic inclusions (3D Josephson matter) is formed. We attributed this behavior to the presence of triplet superconducting correlations in the nanocomposite, which arise due to proximity effect. We argue that the constituent particle geometrical size and indirect interaction via proximity effect play a crucial role in achieving the necessary conditions enabling the percolation superconducting transition in the nanocomposites under consideration. The unusual transport properties of this type of nanocomposites bear much fundamental interest and make their promise for applications, e.g. as novel functional materials for microwave radiation detection.
The work was supported by the grant of the State Fund for Fundamental Research of the Ukraine (project No: F76/35-2018).  

1.	Bunde A., Dieterich W., Percolation in Composites // J. Electroceramics. – 2000. –5, N 2. – Р. 81-92.