Electro-optic search for critical divergence of the charge-density-wave diffusion constant at the onset of depinning

We have used electroreflectance measurements to study charge-density-wave (CDW) polarization dynamics at voltages near the CDW depinning onset (Von) in the quasi-one-dimensional conductor K0.3 Mo O3 ("blue bronze"). For low voltages, where the phase-slip rate is low, it is expected that the polarization relaxation time should be inversely proportional to the CDW diffusion constant, which is expected to diverge at Von. At T=78 K, we observe saturation of the relaxation time at low voltages, suggesting that we are in the low phase-slip limit and allowing us to estimate the noncritical value of the CDW diffusion constant D (∞) ∼0.02 cm2 s, consistent with the measured phason velocity. At other temperatures, the relaxation time increases with decreasing voltage even at the lowest voltages we could measure. In no case do we observe the expected "critical speeding up," setting an upper limit on the critical region of V Von -1critical <0.06.