We confirmed the existence of the collective transverse plasma modes excited
by the motion of the Josephson vortex lattice in stacked intrinsic Josephson
junctions of Bi2Sr2CaCu2O8+x by observing the multiple
subbranches in the Josephson-vortex-flow current-voltage characteristics. We
also observed the symptom of the microwave emission from the resonance between
the Josephson vortex lattice and the collective transverse plasma modes, which
provides the possibility of developing Josephson-vortex-flow electromagnetic
oscillators.Comment: 4 pages, 3 figure