Shapiro Step Response of Intrinsic Josephson Junctions With High Critical Currents of ${({\rm Bi}_{1-{\rm x}}{\rm Pb}_{\rm x})}_{2}{\rm Sr}_{2}{\rm CaCu}_{2}{\rm O}_{\rm y}$

We have studied the response of intrinsic Josephson junctions (IJJs) in cross-shaped samples of (Bi
_1-x
Pb
_x
)
₂
Sr
₂
CaCu
₂
O
_y
(x=0.15) with high critical currents
Ic
at 4.2 K to injection of microwave with frequencies
f
_rf
of 2-20 GHz. At the early stage of measurements Josephson vortex flow is induced in the IJJs by supplying high currents to them. After that, by injection of microwave power
P
to them clear constant voltage steps are successfully observed on their current-voltage characteristics, although their plasma frequency
f
_pl
is much higher than
f
_rf
and they are not resistively shunted. The constant voltage steps appear so as to satisfy the Josephson frequency-voltage relation and behave like Shapiro steps depending on
P
. Such behavior of steps is well reproduced by numerical simulations on Shapiro step response of JJs with shunt resistivity which is equal to the Josephson-vortex flow resistivity under microwave injection. Consequently, the observed constant voltage steps may be Shapiro steps out of the IJJs with the Josephson-vortex flow resistivity.