Sliding conduction by the quasi one-dimensional charge-ordered state in Sr${}_{14-x}$Ca${}_x$Cu${}_{24}$O${}_{41}$

Nonlinear conduction (NLC) of the two-leg spin ladder, Sr${}_{14-x}$Ca${}_x$Cu${}_{24}$O${}_{41}$, was investigated for the $x$=0, 1 and 12 materials . Although insulating materials ($x$=0 and 1) exibited the NLC both in the ladder- and rung directions, the NLC in the ladder direction of the $x$=0-material was found to be very special. We considered this to be due to the sliding motion of the charge ordered state, which was responsible for the resonance at microwave frequencies. We discussed possible candidates for the charge ordered state responsible for the NLC, including Wigner crystal in quasi one dimension (4$k_F$-CDW).