Infrared optical properties of the 10-K organic superconductor (BEDT-TTF)2[Cu(NCS)2] [where (BEDT-TTF) is bis(ethylenedithio)tetrathiafulvalene]

Low-temperature polarized bolometric absorption measurements have been performed on the ${\mathit{T}}_{\mathit{c}}$=10.4 K organic superconductor κ-(BEDT-TTF${)}_2$[Cu(NCS${)}_2$], where BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene. The ratio of the absorption at 5.3 K to that at 10.5 K from 10 to 40 ${\mathrm{cm}}^{\mathrm{-}1}$ showed no evidence of a conventional BCS gap. Polarized-reflectivity measurements at temperatures between 10 and 295 K are also reported for both the protonated and deuterated compounds, between 200 and 8000 ${\mathrm{cm}}^{\mathrm{-}1}$. The 10-K spectra were calibrated by a technique of simultaneously measuring the reflectivity R and the absorptance, 1-R. The resulting conductivities display vibrational features superimposed on an electronic background. This background shifts from mid-infrared interband transitions, when the dc conductivity is low, to far-infrared intraband transitions as the dc conductivity increases. The vibrations have been assigned to a mixture of normally inactive ${\mathit{a}}_{\mathit{g}}$ modes and normally active ${\mathit{b}}_{2\mathit{u}}$ modes of the BEDT-TTF molecule. A few of both types couple strongly to the charge carriers. One in particular has a very temperature-dependent frequency due to the changing intensity of the mid-infrared band.