Optical investigations of intermolecular electronic interactions and “free” charge carriers in quasi-two-dimensional organic conductors and superconductors of the group κ-(BEDT-TTF)2Cu[N(CN)2]Br x Cl1 − x

Abstract

The reflectance spectra and optical conductivity spectra of quasi-two-dimensional organic conductor crystals of the group of compounds κ-(BEDT-TTF)2Cu[N(CN)2]Br x Cl1 − x with x = 0, 0.40, 0.73, 0.85, and 0.90 in the spectral region from 6 meV to 0.74 eV at temperatures T = 90-20 K have been quantitatively analyzed in the framework of a combined semiempirical model including the cluster (tetramer) theory for strongly correlated electrons coupling with intramolecular vibrations and the Drude theory of free electrons with the aim of studying intermolecular electronic interactions and their influence on the formation of the ground state in these crystals. It has been established that the parameters characterizing the charge transfer between molecules in dimers and tetramers and the vibronic coupling constants are almost identical for the compounds under investigation. A large value of the effective Coulomb repulsion (U/t = 1.3–1.5) indicates that strong electron correlations occur in the compounds both with a metal-insulator phase transition (x = 0, 0.40) and with a metal-superconductor phase transition (x = 0.85, 0.90). The conclusion has been drawn that, at x = 0, electron correlations favor an antiferromagnetic spin ordering and a metal-insulator phase transition, whereas for superconductors (x = 0.85, 0.90), the metal-insulator phase transition is hindered as a result of the structural disorder due to the difference in orientations of the terminal CH2 groups of the BEDT-TTF molecule. It has been shown that quasi-free charge carriers interact with electrons localized on the clusters and do not interact with intramolecular vibrations. It has been noted that the oscillator strength of the observed electronic transitions in the initial metallic band (N eff = 0.38–0.31 per dimer) is considerably less than the corresponding value for free (noninteracting) charge carriers (N = 1). This suggests that the Coulomb correlations and vibronic coupling play a decisive role in the kinetic phenomena observed in the molecular conductors and superconductors under investigation.