Comprehensive transport study of anisotropy and ordering phenomena in quasi-one-dimensional (TMTTF)2X salts (X = PF6,AsF6,SbF6,BF4,ClO4, ReO4)

The temperature-dependent dc resistivity of the quasi-one-dimensional organic salts (TMTTF)(2)X (X = PF6, AsF6, SbF6, BF4, ClO4, ReO4) [tetramethyltetrathiafulvalene (TMTTF)] has been measured in all three crystal directions in order to investigate anisotropy, localization effects, and charge- (CO) and anion-ordering (AO) phenomena at low temperatures. For all compounds and directions, we extract the transport mechanisms in different regimes. The data are thoroughly analyzed, related to structural properties, and extensively discussed in view of previous studies and latest theories. The most important observation is the opening of a gap in the density of states below the CO temperature T-CO that influences the transport in all three directions. The effect becomes stronger as the distance between the anions and the sulfur atoms decreases. The coupling between the anions and the TMTTF stack also has a strict influence on the anomaly in the resistivity at the AO T-AO. At very low temperatures, the activation law is replaced by three-dimensional variable-range hopping.