Трехуровневая сверхпроводимость в YBCO, допирован- ном наночастицами NiO

We report the existence of three superconducting subsystems in samples of high-temperature superconductor YBa2Cu3O7δ doped by NiO nanoparticles and then subjected to 18-hour annealing. These subsystems exhibit different superconducting transition temperatures and respond differently to magnetic fields. While two-level superconductivity formed by grain and intergranular boundary subsystems is known in polycrystalline superconductors, our material reveals an additional subsystem formed by surface layers of grains with reduced critical temperature and enhanced sensitivity to magnetic field