Suppression of superconductivity by nonmagnetic disorder in organic superconductor κ-(BEDT-TTF)₂Cu(NCS)₂

The suppression of superconductivity by nonmagnetic disorder is investigated systematically in the organic superconductor κ-(BEDT-TTF) ₂Cu(NCS)₂. We introduce a nonmagnetic disorder arising from molecule substitution in part with deuterated BEDT-TTF or BMDT-TTF for BEDT-TTF molecules and molecular defects introduced by X-ray irradiation. A quantitative evaluation of the scattering time τ_dHvA is carried out by de Haas-van Alphen (dHvA) effect measurement. A large reduction in T _c with a linear dependence on 1=τ_dHvA is found in the small-disorder region below 1=τ_dHvA≃1 × 10 ¹² s^-1 in both the BMDT-TTF molecule-substituted and X-ray-irradiated samples. The observed linear relation between T_c and 1=τ_dHvA is in agreement with the Abrikosov-Gorkov (AG) formula, at least in the smalldisorder region. This observation is reasonably consistent with the unconventional superconductivity proposed thus far for the present organic superconductor. A deviation from the AG formula, however, is observed in the large-disorder region above 1=τ_dHvA≃1 × 10 ¹² s^-1, which reproduces the previous transport study [J. G. Analytis et al.: Phys. Rev. Lett. 96 (2006) 177002]. We present some interpretations of this deviation from the viewpoints of superconductivity and the inherent difficulties in the evaluation of scattering time.