Role of anion ordering in the coexistence of spin-density-wave and superconductivity in (TMTSF)_2ClO_4

摘要

Using various transport and magnetotransport probes, we study the coexistence of spin-density wave and superconductor states in (TMTSF)_2ClO_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)_2ClO_4 properties is thus analogous to that of pressure on (TMTSF)_2X (X = PF_6 or AsF_6), thereby unifying general picture of the coexistence of superconductivity and spin-density wave in these compounds.