Sliding charge-density waves as rough growth fronts

Using high-resolution x-ray scattering techniques, we have measured the transverse static structure factor of the sliding charge-density wave (CDW) in NbSe₃. For temperatures between 70 and 120 K and for applied currents up to 40 times the threshold current for sliding, the scattering peak for the sliding CDW is significantly broader than that for the pinned CDW, indicating that the sliding state is less correlated than the pinned state. Using scaling analysis, we show that the CDW phase roughness exponent {α} rises from 0.60±{}0.01 in the pinned state to 0.80±{}0.01 in the sliding state, indicating that the phase fronts of the sliding CDW are significantly rougher than those of the pinned CDW.