VORTEX RING EXCITATIONS IN LAYERED SUPERCONDUCTORS

A special kind of thermal excitations can be found in the vicinity of T(c) in a layered superconducting structure. The excitations are vortex rings positioned in the layer with weaker superconductivity. For parameters corresponding to high-T(c) superconductors these excitations should consist of two coupled vortex rings with opposite helicities. Fixed in position by pinning forces and frozen, these excitations contribute to randomization of the superconducting state at low temperatures. Estimates are made for the density of states within the gap region and characteristic length of the random structure caused by the double ring excitations.