  The electric losses in a bulk or film superconductor exposed to a parallel
radio-frequency magnetic field may have three origins: In homogeneous
vortex-free superconductors losses proportional to the frequency squared
originate from the oscillating normal-conducting component of the charge
carriers which is always present at temperatures $T>0$. With increasing field
amplitude the induced supercurrents approach the depairing current at which
superconductivity breaks down. And finally, if magnetic vortices can penetrate
the superconductor they typically cause large losses since they move driven by
the AC supercurrent.
