The infrared optical properties of the α-phase organic superconductor, α-(BEDT-TTF)2(NH4Hg(NCS)4)

Abstract

The title compound is the only known organic superconductor (Tc = 0.8 K) with an α-phase crystal structure, in which the conducting plane contains single BEDT-TT F molecules with alternating orientation, rather than the pairs of molecules found in the higher Tc k-phase compounds. We have measured the polarized reflectivity of protonated and deuterated (BEDT-TTF)2(NH4)Hg(NCS)4., from 80 cm-1 to 8000 cm-1, at temperatures between 6.5 K and 300 K. The overall shape of these spectra are discussed. The optical conductivity obtained after a Kramers-Kronig analysis shows no maximum in the mid-infrared as known from the compounds of the κ-phase due to the lack of dimerization of the BEDT-TTF molecules. This causes an accentuation of the far-infrared intraband conductivity. The vibrational features are assigned and discussed.