Dynamical transport properties of NbSe${}_3$ with simultaneous sliding of both charge density waves

Measurements of the nonlinear conductivity in NbSe${}_3$ when the high-T and the low-T charge-density waves (CDWs) are simultaneously sliding have been performed. It is shown that the threshold electric field, $E_{t1}$, for depinning the high-T CDW increases 4 - 5 times at the temperature at which the low-T CDW is formed, indicating the strong pinning effect resulting from the interaction between both CDWs. Under application of a radio-frequency (rf) field, Shapiro steps are never observed simultaneously for both CDWs. At the electric field less then one for high-T CDW sliding only Sapiro steps for low-T CDW were observed, and at higher field only Shapiro steps for high-T CDW exist.