Magnetoresistance of the organic conducting tetramethyltetraselenafulvalene salts (TMTSF)2ClO4and (TMTSF)2PF6: Search for the coherent-diffusive transition or localization effects with increasing temperature

In an attempt to detect the changeover from coherent to diffusive behavior in the transverse conductivity or phonon-induced localization, we have extended previous magnetoresistance measurements on single crystals of the organic conducting tetramethyltetraselenafulvalene salts (TMTSF${)}_2$ ${\mathrm{ClO}}_4$ and (TMTSF${)}_2$ ${\mathrm{PF}}_6$ up to 200 and 125 K, respectively. For the geometry studied, namely current along the lowest conductivity axis and field along the intermediate axis, the magnetoresistance remains positive and of the same magnitude as expected from band theory. Surprisingly, although the transverse mean free paths become very small at higher temperatures and the transverse resistivity saturates, we did not detect any substantial changes in magnetoresistance corresponding to a change in the conductivity mechanism.