| I investigated the electromodulated infrared transmission in blue bronze (K0.3MoO3) when in the charge-density-wave (CDW) state. Blue bronze undergoes a Peierls transition into a semiconducting state at low temperatures. Its infrared transmission is affected by the application of small electric fields, which polarize and depin the CDW. This electro-optical effect has the potential to elucidate the mechanism of current injection into the CDW at current contacts.; The spatial dependence and field dependence of the electromodulated transmission were measured using a “scanning transmission microscope,” revealing the relative change in transmission, Δτ/τ, to vary as the expected CDW phase gradient. In particular, we found that Δτ/τ varied linearly with position near the center of the sample, reflecting the bulk CDW polarization, but that there were larger changes near (≈100 μm) the contacts, associated with the extra CDW deformations necessary to drive the phase slip processes needed for current conversion.; At low temperatures (T < 110 K), Δτ/τ is thermally activated, suggesting that it is caused by changes in the density of quasiparticles excited across the CDW gap, which screen deformations of the CDW. We compared the temperature dependence of Δτ/τ to that of hysteresis in the low-field conductivity and the infrared transmission (which we discovered in the course of these studies). Our results indicate that polarized deformations are more susceptible to phase slip than to uniform deformations.; We also studied the Δτ/τ spectra to understand the possible role of phonons in the electro-optic effect. Initially, use of a relatively low intensity broad band source limited these investigations to spectral changes associated with bulk polarization and to relatively high transmission regions. However, once we introduced tunable diode lasers to our system, we observed several types of changes in phonon spectra. The application of an electric field predominantly modified by energy, linewidth, or oscillator strength of the observed phonons. We also searched for differences in the spectra associated with bulk and contact CDW deformations. The contact and bulk spectra were proportional, thus implying an upper limit on soliton cross-section/lifetime. |
