  We study theoretically and experimentally influence of pancake vortices on
motion of the Josephson vortex lattice in layered high-temperature
superconductors. Mobility of the Josephson vortices in layered superconductors
is strongly suppressed by small amount of pancake-vortex stacks. Moving
Josephson vortex lattice forces oscillating zigzag deformation of the
pancake-vortex stacks contributing to damping. The salient feature of this
contribution is its nonmonotonic dependence on the lattice velocity and the
corresponding voltage. Maximum pancake effect is realized when the Josephson
frequency matches the relaxation frequency of the stacks. The pancake-vortex
damping is strongly suppressed by thermal fluctuations of the pancake vortices.
This theoretical picture was qualitatively confirmed by experiments on two
mesas prepared out of Bi_2Sr_2CaCu_2O_{8+d} whiskers. We found that the
Josephson-vortex flux-flow voltage is very sensitive to small c-axis magnetic
field. The pancake-vortex contribution to the current indeed nonmonotonically
depends on voltage and decreases with increasing temperature and in-plane
magnetic field. We also found that irradiation with heavy ions has no
noticeable direct influence on motion of the Josephson vortices but
dramatically reduces the pancake-vortex contribution to the damping of the
Josephson vortex lattice at low temperatures.
