Infrared conductivity and electron–molecular-vibration coupling in the organic superconductor di[bis(ethylenedithio)tetrathiafulvalene] bis(isothiocyanato)cuprate(I), κ-(BEDT-TTF)2 [Cu(NCS)2]: Protonated and deuterated salts

Abstract

Polarized reflectance spectra of the organic superconductors protonated and deuterated κ-(BEDT-TTF${)}_2$[Cu(NCS${)}_2$] (H and D salts) [BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene] were measured over the range from 500 to 28 000 ${\mathrm{cm}}^{\mathrm{-}1}$ at room temperature with light polarizations parallel to the crystallographic b and c axes which lie on the two-dimensional conducting plane. Polarized reflectance spectra of the organic superconductor β-(BEDT-TTF${)}_2$ ${\mathrm{I}}_3$ and the organic metal β’ ’-(BEDT-TTF${)}_2$ ${\mathrm{AuBr}}_2$ were also measured in order to discuss the influence of different molecular arrangements and hydrogen-anion contacts on the electronic and vibrational properties of these salts. Frequency-dependent conductivities were calculated by a Kramers-Kronig transformation. By comparison of the infrared conductivity spectra of the H and D salts, the vibrational transitions induced by electron–molecular-vibration (EMV) coupling were clearly distinguished from the carbon-hydrogen bending modes of the BEDT-TTF moiety. A Drude-Lorentz dielectric function was used to evaluate the optical transport parameters and an excitation frequency of the charge-transfer (CT) band superimposed on a plasma-edge-like dispersion which was observed for each compound. The EMV-coupling energies are semiquantitatively estimated to be ca. 70 meV for both the H and D salts from the frequencies of the EMV coupling transition and the CT band in terms of the dimer charge-oscillation model. By use of the coupling energy, various parameters describing the superconducting state were evaluated and discussed on the basis of the BCS theory in a weak-coupling limit. Finally, the magnitudes of hydrogen-anion interaction were estimated from the frequency shifts of the C-H bending modes of the BEDT-TTF moiety.