Josephson vortex motion in stacks of intrinsic Josephson junctions in Bi2Sr2CaCu2O8+x

Abstract

Abstract We have investigated the motion of Josephson vortices in stacks of intrinsic Josephson junctions in Bi 2 Sr 2 CaCu 2 O 8+ x (BSCCO). The stacks were realized as mesa structures on top of BSCCO single crystals. In external magnetic fields up to 5 T, oriented parallel to the layers, the current–voltage ( I – V ) characteristics exhibit a field dependent branch. By microwave emission measurements we show that this branch is due to a collective motion of Josephson vortices. For magnetic fields larger than 17 kOe we find a pronounced flux-flow step in the I – V characteristic that corresponds to a Josephson frequency of 1.5 THz. In this regime, we observe an intensive non–Josephson emission signal. We propose that this signal is due to Cherenkov radiation emitted from Josephson vortices moving faster than the lowest mode velocity of the stack. Numerical simulations using the coupled sine-Gordon model demonstrate downconversion of Cherenkov radiation due to the mixing of different modes.