Electromodulated infrared transmission spectra of blue bronze

The infrared transmission of the quasi-one dimensional charge-density-wave (CDW) conductor blue bronze (K_0.3MoO₃) is affected by polarization of the CDW, and therefore by application of a voltage near or above the threshold for CDW depinning. In this paper, we compare the spectra associated with the relative change in transmission (Δτ/τ) taken for different temperatures and oscillating voltages. We find that the phonon spectrum is affected by CDW polarization; the linewidths or frequencies of most phonons change by ≈ 0.01 cm^-1. However, no new intragap states that can be associated with current injection are observed; i.e. the spectra associated with polarization of the CDW in the crystal bulk is identical to that associated with CDW current injection near the contacts. Our results indicate that, for light polarized perpendicular to the conducting chains, the density (n), cross-section (σ), and bandwidth (Γ) of intragap states are related by: n σ/Γ < 10^-10 (Å cm^-1)^-1. For expected values of the cross-section and bandwidth, this implies that the intragap states can be optically excited for a time less than 0.3 μs.