Far-infrared electrodynamic response of (TMTSF)₂ClO₄ in the normal and superconducting states

The far-infrared reflectance of (TMTSF)₂ClO₄ has been measured at temperatures between 0.45 and 26 K in the 10-70-cm^-1 frequency range for light polarized along the a-axis conducting chains. Slow cooling of the sample permits the anions to order near 24 K and a superconducting state to develop below 1.2 K while fast-cooling yields an insulating ground state. The thermoreflectance was found to be temperature dependent below the anion ordering temperature for the slow-cooling case only. A low-frequency edge in the absolute reflectance and a peak near 30 cm^-1, attributed to a phonon coupled, to the electronic background, are found to exhibit a strong temperature dependence in the relaxed state. The Kramers-Kronig-derived optical conductivity indicates that the 30-cm^-1 mode becomes highly asymmetric below the ordering temperature. On entering the superconducting state it is observed to shift upward in frequency as would be the expected effect on the real part of the self-energy of a phonon coupled to the electronic system of a frequency greater than the superconducting gap.