Role of anion ordering in the coexistence of spin-density-wave and superconductivity in (TMTSF)$_{2}$ClO$_{4}$

Using various transport and magnetotransport probes we study the coexistence of spin-density wave and superconductor states in (TMTSF)$_2$ClO$_4$ at various degrees of ClO$_4$ anions ordering. In the two-phase complex state when both superconductivity and spin-density wave are observed in transport, we find prehistory effects, enhancement of the superconducting critical field, and strong spatial anisotropy of the superconducting state. These features are inconsistent with the conventional model of structural inhomogeneities produced by anion ordering transition. We reveal instead that superconductor and spin-density wave regions overlap on the temperature – dimerization gap $V$ phase diagram, where $V$ is varied by anion ordering. The effect of anion ordering on (TMTSF)$_2$ClO$_4$ properties is thus analogous to that of pressure on (TMTSF)$_2$X (X$\,=\,$PF$_6$ or AsF$_6$), thereby unifying general picture of the coexistence of superconductivity and spin-density wave in these compounds.