Anisotropic three-dimensional superconductivity of the incommensurate organic superconductor (MDT-ST)(I₃)_0.417

Superconducting properties are investigated for the incommensurate ambient pressure organic superconductor (MDT-ST)(I₃)_0.417 [MDT-ST: 5H-2-(1,3-dithiol-2-ylidene)-1,3-diselena-4,6-dithiapentalene, T_c = 4.3 K]. The upper critical fields show anisotropic three-dimensional angle dependence in contrast to the highly two-dimensional one. Although the coherence lengths are highly anisotropic ξ_∥c:ξ_∥b: ξ_∥a ∼ 1:8:11, the interlayer coherence length, ξ_∥c, is much longer than the thickness of the conducting sheet. These results indicate that the present compound is a highly anisotropic three-dimensional superconductor among the layered organic superconductors. The superconducting transition widths in the conducting plane exhibit anomalous decreases with decreasing temperature, suggesting that the flux-lines are pinned by infinite anion chains.