  Magnetic levitation based on the flux pinning nature of type II
superconductors has the merit of self-stability, making it appealing for
applications such as high speed bearings, maglev trains, space generators, etc.
However, such levitation systems physically rely on the superconductor
pre-capturing magnetic flux (i.e. field cooling process) before establishing
the levitation state which is nonadjustable afterwards. Moreover, practical
type II superconductors in the levitation system inevitably suffer from various
sources of energy losses, leading to continuous levitation force decay. These
intrinsic drawbacks make superconducting maglev inflexible and impractical for
long term operation. Here we propose and demonstrate a new form of
superconducting maglev which is tunable and with self-stability. The maglev
system uses a closed-loop type II superconducting coil to lock flux of a
magnet, establishing self-stable levitation between the two objects. A flux
pump is used to modulate the total magnetic flux of the coil without breaking
its superconductivity, thus flexibly tuning levitation force and height
meanwhile maintaining self-stability. For the first time, we experimentally
demonstrate a self-stable type II superconducting maglev system which is able
to: counteract long term levitation force decay, adjust levitation force and
equilibrium position, and establish levitation under zero field cooling
condition. These breakthroughs may bridge the gap between demonstrations and
practical applications of type II superconducting maglevs.
