Collective motion of Josephson vortices in intrinsic Josephson junctions in Bi2Sr2CaCu2O8+y

We report on the experimental observation of moving Josephson vortices in mesa structures patterned on the surface of Bi2Sr2CaCu2O8+y Single crystals. Mesas form stacks of typically 100 intrinsic Josephson junctions. In magnetic fields parallel to the superconducting copper oxide layers, a flux-flow branch develops on the current-voltage (I-V) characteristic. We investigate this branch in magnetic fields Hup to 15 kOe for junctions with lateral dimensions ranging from 20 x 20 mu m(2) to 500 x 20 mu m(2). Investigations show that the voltage of the branch scales with 1/H. For mesas of the same height its slope is inversely proportional to the junction area showing that the flux-flow resistance is independent of the particular junction length. Microwave emission is sensitive to the direction of fluxon motion. This shows that the flux-flow branch in the I-V characteristic is caused by the collective motion of vortices. We compare our data to numerical simulations based on the coupled sine-Gordon equations for strongly coupled, stacked Josephson junctions. We show that the observed branch can be understood as a flux-flow state caused by vortices moving with the lowest collective-mode velocity of the system. The value derived for the Swihart velocity is in good agreement with recent measurements of the c-axis Josephson plasma frequency.