Observation of dynamic coupling between the Q1 and Q2 charge-density waves in NbSe3

We report synchrotron based high-resolution x-ray scattering and in situ electronic transport measurements of the Q₁ and Q₂ charge-density waves (CDW's) in NbSe₃ as a function of applied electric field in the low-temperature ``switching'' regime. Detailed analysis of the line shape of the x-ray satellite peaks demonstrates that the Q₂ CDW changes state when it depins at E_T but does not exhibit any abrupt structural change at E^*_T, where the collective CDW current abruptly increases. In contrast, the Q₁ CDW does not exhibit structural changes at E_T but abruptly changes state at E^*_T to a structure very similar to the structure of the sliding Q₁ at high temperatures. These data demonstrate coupling between the Q₂ and Q₁ CDW's.