The dimensionality-driven metal-insulator transition in organic Bechgaard salts

Optical experiments were conducted on a series of organic linear chain conductors (i.e., (TMTTF)2X with X = PF6 and Br, and (TMTSF)2X with X = ClO4, PF6, AsF6) with different values of the interchain single electron transfer integral tb, which quantifies the degree of anisotropy. We have investigated the dynamics of the charge excitation spectrum in the frequency range from 1 to 105 cm-1. 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. This optical work shows that an insulator (TMTTF) to metal (TMTSF) transition can be associated with electron deconfinement. Moreover, by contrasting optical with magnetic properties we found indications which are suggestive of metal with spin-charge separation.