Optical investigation of the charge-density-wave phase transitions in NbSe3

We have measured the optical reflectivity R(v) of the quasi-one-dimensional conductor NbSe3 from the far infrared up to the ultraviolet between 10 and 300 K using light polarized along and normal to the chain axis. We find a depletion of the optical conductivity with decreasing temperature for both polarizations in the midto far-infrared region. This leads to a redistribution of spectral weight from low to high energies due to partial gapping of the Fermi surface below the charge-density-wave transitions at 145 K and 59 K. We deduce the bulk magnitudes of the charge-density wave gaps and discuss the scattering of ungapped free charge carriers and the role of fluctuation effects.