  We investigated low-temperature dynamics in a charge density wave (CDW)
conductor NbSe3, a widely studied representative of a class of systems of
driven periodic media with quenched disorder and relevant to a wider group of
systems exhibiting collective transport behaviors. To date, theoretical efforts
have not converged to produce a consistent description of the rich dynamics
observed in these systems, especially in the low temperature regime. We
developed modern sample preparation techniques and used frequency- and
time-domain transport measurements below the second characteristic Peierls CDW
transition to investigate the regime of temporally-ordered collective creep in
NbSe3 samples in the low temperature regime between 15 K and 32 K. By measuring
the frequency of coherent oscillations between two characteristic threshold
fields, ET and ET*, we show that in nine high-quality samples, pure, Ta-, or
Ti-doped, the current-field relation for the collective transport in this
regime closely follows a modified Anderson-Kim form across five orders of
magnitude with thermally- and field-activated behavior above ET for a range of
temperatures. This study, combined with our transport relaxation measurements,
provides relevant length, energy, and time scales that set the dynamics in this
regime and reveals that the collective dynamics, governed by large length and
energy scales, must be reconciled with microscopic local dynamics, with
barriers at orders of magnitude smaller scales. The interplay between the
collective and local mechanisms set the dynamics that is responsible for
extremely slow (creep-like) collective, yet temporally-ordered behavior.
Combined with the existing work, our results paint a consistent picture of a
transport phase diagram for CDWs, and density-wave systems in general, and
provide essential ingredients for a much-needed correct theoretical description
of these systems.
