  We have investigated the current-voltage characteristics (IVCs) of intrinsic
Josephson junctions (IJJs). Recently, it is predicted that the longitudinal
plasma wave can be excited by the parametric resonance in IJJs. Such an
excitation induces a singularity called as breakpoint region around switch back
region in the IVC. We have succeeded in the observation of the breakpoint
region in the IVC of the mesa with 5 IJJs at 4.2 K. Furthermore, it is found
that the temperature dependence of the breakpoint current is in agreement with
the theoretical prediction. This suggests that the wave number of the excited
plasma wave varies with temperature.
