Superluminal geometrical resonances observed in Bi2Sr2CaCu2O8+x intrinsic Josephson junctions

We study Fiske steps in small Bi2Sr2CaCu2O8+x mesa structures, containing only a few stacked intrinsic Josephson junctions. Careful alignment of magnetic field prevents penetration of Abrikosov vortices and facilitates observation of a large variety of high-quality geometrical resonances, including superluminal with velocities larger than the slowest velocity of electromagnetic waves. A small number of junctions limits the number of resonant modes and allows accurate identification of modes and velocities. It is shown that superluminal geometrical resonances can be excited by subluminal fluxon motion and that flux flow itself becomes superluminal at high magnetic fields. We argue that observation of high-quality superluminal geometrical resonances is crucial for realization of the coherent flux-flow oscillator in the terahertz frequency range.