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 T(c) = 10.4 K organic superconductor kappa-(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 cm-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 cm-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 a(g) modes and normally active b2u 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.