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

We compare the changes in the transmission (tau) spectra of blue bronze cau sed by changing temperature (Delta tau(T)) and by polarizing the charge-den sity-wave (CDW) with an electric field near threshold (Delta tau(V)), for p hoton energies between 800 and 1200 cm(-1). Unlike at lower energies, the t wo spectra differ qualitatively in this spectral region, near the edge of t he CDW gap. While temperature mainly affects phonon damping, CDW polarizati on shifts phonon energies. Both Delta tau(V)/tau and Delta tau(T)/tau are e xpected to change sign for energies above the gap, but this occurs much mor e gradually las a function of energy) in the electromodulated spectrum.