Electronic interactions in the organic conductors (TMTSF)2X (X=ClO4 and PF6) and (TMTTF)2X (X=Br and PF6) from their infrared spectra

Probing the metal-insulator phase transition in the (DMEDO-EBDT)2PF6 single crystal by optical measurements

The temperature and polarization dependence of the optical reflectivity  spectra of a quasi-one-dimensional 1/4-filled band system, (DMEDO-EBDT)(2)PF(6), have been investigated. We observed clear anisotropy in the electronic structures corresponding to the anisotropic transport properties. The appearance of a charge gap (E(g) > 0.1 eV) and transfer of the spectral weight accompanied by the metal-insulator phase transition were clearly observed. In addition, a split of the intramolecular vibrational modes was observed, which strongly suggested the existence of charge disproportionation in the low temperature phase. We also observed a photoinduced reflectivity change, which implied the occurrence of a photoinduced phase transition from the low temperature insulating phase to the high temperature metallic phase.

Dimensionality-driven insulator-to-metal transition in the bechgaard salts

Optical experiments were conducted on a series of organic linear chain conductors with different values of the interchain single-electron transfer integral tb, which quantifies the degree of anisotropy. Electron-electron interactions together with Umklapp scattering resulted in a correlation gap and an insulating state for small tb. An insulator-to-metal transition was observed when tb exceeded a critical value, on the order of the correlation gap Egap. The absence of a plasma edge on the insulator side of the transition for polarization perpendicular to the chains suggests that the electrons are confined to the chains. The optical features of the metallic state, when contrasted with the magnetic properties, are suggestive of spin-charge separation.