Magnetoresistance study of the effect of disorder on the organic superconductor bis-tetramethyltetraselenafulvalenium perchlorate

We report a study of the magnetoresistance of the organic superconductor bis-tetramethyltetraselenafulvalenium perchlorate [(TMTSF${)}_2$ ${\mathrm{ClO}}_4$] with disorder introduced by irradiation (c=0.06 and 0.12 % defects per mole) and by alloying, i.e., (TMTSF${)}_2$(${\mathrm{ClO}}_4$) ${}_{1\mathrm{-}\mathrm{x}}{}^{}{}_{}{}^{}$(${\mathrm{ReO}}_4$ ${)}_{\mathrm{x}}$ (0.03≤x≤0.07). The magnetoresistance was measured for current flow along ${\mathrm{c}}^{\mathrm{*}}$, perpendicular to the highly conducting a axis with H∥b’; for fields up to 6 T from 4.2 to 30 K. Kohler’s rule is obeyed for the alloys and the results can be understood in terms of standard theory for an open Fermi surface. The strong perturbations caused by irradiation-induced defects lead to deviations from Kohler’s rule which are attributed to a change in the effective tight-binding overlap integral along ${\mathrm{c}}^{\mathrm{*}}$. Above approximately 8 K all crystals have a $T^2$ term in the resistivity. For the alloys this term is independent of the defect concentration, i.e., the residual resistivity, and it may arise from electron-electron scattering.