Spatially resolved study of charge-density-wave strain inNbSe3:Evidence for a finite threshold for creep

We have used spatially resolved measurements of the low-field charge-density-wave (CDW) response in ${\mathrm{NbSe}}_3$ to obtain the CDW strain profile with single-wavelength resolution. Below $\sim 35\hspace{0.5em}\mathrm{K},$ the single-particle (linear) resistance R shows significant hysteresis with electric field E for E below a threshold field $E_T.$ The size of the local hysteresis loop $\Delta R\left(x\right)\mathrm{}$ and thus of the CDW strain $\epsilon \mathrm{}\left(x\right)\mathrm{}$ varies linearly with position to within 20 $\mu \mathrm{m}$ of the current contacts. Our results confirm that slip boundary conditions at the current contacts are responsible for the single-particle resistance hysteresis, but not for the “switching” observed at a larger field $E_T^{*}.$ Although CDW motion is slow and creeplike between $E_T$ and $E_T^{*},$ extremely slow hysteresis relaxation below $E_T$ implies that the creep rate changes by at least several orders of magnitude at $E_T.$ This threshold behavior is inconsistent with existing predictions for thermal creep, and highlights the highly unusual character of CDW dynamics at low temperatures.