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)

Abstract

Polarized reflectance spectra of the organic superconductors protonated and deuterated~-(BEDT-TTF)2[Cu(NCS)2] (H and D salts) [BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene] were measured over the range from 500 to 28000 cm ' 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 P-(BEDT-TTF)2I3 and the organic metal P"-(BEDT-TTF)2AuBr2 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 transforrnation. By comparison of the infrared conductivity spectra of the H and D salts, the vibrational transitions induced by electronmolecular-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.

Optical Properties of Deuterated Organic Conductor (BEDT-TTF)2[Hg(SCN)2Br]

“…There are much more pronounced differences in the vibrational structure: the intense bands at 1164 and 1295 cm À1 (E k b) À1 (E k c). Similar changes after deuteration have been observed in the spectra of superconductors [12,13,23]. A sharp vibrational band is observed in the conductivity spectra of the H 8 -salt at 877 cm À1 for E k b and at 881 cm À1 for E k c, which exhibits a shift upon deuteration of +5 and +7 cm À1 , respectively.…”

“…The deuteration effect has been examined in detail on j-(BEDT-TTF) 2 Cu(NCS) 2 [10,12,13,22] and j-(BEDT-TTF) 2 Cu[N(CN) 2 ]Br [23 to 25] by infrared and Raman spectroscopies in order to study the electron±phonon interaction and to assign the EMV features in the infrared spectra. These investigations are important for understanding the EMV coupling effect on superconductivity in organic materials.…”

Vibrational Spectra of β″-Type BEDT-TTF Salts: Relationship between Conducting Property, Time-Averaged Site Charge and Inter-Molecular Distance

“…As shown in (ET) 2 AuBr 2 exhibits a single peak. The IR exhibits metallic behavior even at room temperature [65]. The frequency of the identical to that at ρ = 0.5.…”

Bandwidth-controlled Mott transition inκ−(BEDT−TTF)2Cu[N(C<

“…Albeit it seems obvious -in particular when cooling down -that two contributions add up for this band, the explanations proposed over the years took into account only one single process. It was suggested that the mid-infrared peak is due to charge transfer inside the dimer 15,16,17,37,46 or due to transitions between the Hubbard bands formed by the correlated conduction electrons.…”