Extended condensed matter systems driven over quenched random disorder exhibit diverse phenomena, including nonequilibrium phase transitions and history dependence. Examples of such systems include vortex arrays in type-II superconductors and charge-density waves (CDWs) in anisotropic metals. Using several synchrotron-based x-ray techniques, we have studied the structure of driven CDWs in the quasi one-dimensional metal NbSe3. The data clearly demonstrate that models based on the dissipative dynamics of driven elastic media are unable to describe the CDW problem accurately[1-4]. Two new descriptions capture some of the relevant physics. The first considers a viscoelastic medium[5], in which elastic couplings are replaced by couplings that are non-local in time. The second is based on a modified Swift-Hohenberg equation[6], which includes both amplitude and phase fluctuations of the condensate.
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