Increase of Superconducting Correlation due to Dimensionality Change in Quasi-One-Dimensional Conductors

Response functions for spin-density-wave (SDW) and d-wave singlet superconductivity ($d$SC) in quasi-one-dimensional (Q1D) electron systems are calculated by a renormalization group technique. It is shown that the response functions for both SDW, $\chi_\rs$, and $d$SC, $\chi_d$, are enhanced by interchain hopping, $t_\perp$, i.e., by quasi-one dimensionality. When the Fermi surface deviates from perfect nesting, $\chi_\rs$ saturates below the energy scale of imperfectness of the nesting, while $\chi_d$ is hardly affected. Consequently, the superconducting correlation increases even away from the SDW phase. This gives a possible interpretation of the recent experimental results of Q1D organic conductor (TMTTF)$_2$SbF$_6$, where $T_{\rm c}$ increases even away from the SDW phase.