Experimental studies of the mechanism of the electro-optical effect in blue bronze

We had previously shown that the infrared transmission in blue bronze (Formula presented) is modulated by an applied electric field, and showed that this change was related to deformations of the charge-density wave (CDW) [M. E. Itkis and J. W. Brill, Phys. Rev. Lett. 72, 2049 (1994)]. Since the CDW is also deformed by temperature changes, we expected the infrared (IR) transmission to also exhibit thermal hysteresis, and have observed this. Comparison of the magnitude and temperature dependences of the thermal hysteresis in the dc conductance and IR transmission indicate that the changes in the latter are not primarily due to changes in quasiparticle absorption. Comparisons of the electromodulated and temperature difference IR and Raman spectra suggest that the electromodulated changes result from quasiparticle screening of phonons, and we estimate that application of a field near the threshold for CDW depinning changes the quasiparticle density by a few percent near the current contacts.