Electromodulated infrared transmission spectrum near the CDW gap edge in blue bronze

We compare the changes in the transmission (τ) spectra of blue bronze caused by changing temperature (ΔτT) and by polarizing the charge-density-wave (CDW) with an electric field near threshold (ΔτV), for photon energies between 800 and 1200 cm−1. Unlike at lower energies, the two spectra differ qualitatively in this spectral region, near the edge of the CDW gap. While temperature mainly affects phonon damping, CDW polarization shifts phonon energies. Both Δτv/τ and ΔτT/τ are expected to change sign for energies above the gap, but this occurs much more gradually (as a function of energy) in the electromodulated spectrum.