Enhanced Tc in a dual-layered molecular superconductor

We have revisited the structural and electronic properties of the filamentary organic superconductor (BEDT-TTF)2Ag(CF3)4(TCE), first synthesized in 1994. Detailed structural investigations reveal that the BEDT-TTF molecules are arranged in two distinctly different packing motifs, κ and α', which alternate from layer to layer. This molecule-based superconductor with dual BEDT-TTF packing motifs has a Tc five times higher than that of its polymorph that contains only κ-type packing. Using the established empirical correlations between the bond lengths (C-S and C=S) and the oxidation state of the BEDT-TTF molecule, we have found that there is a uniform charge distribution in the κ-layers (corresponding to an oxidation state of +0.5 for all BEDT-TTF molecules), whereas in the α'-layer, half of the molecules are nearly fully oxidized to +1, while the other half are close to neutral, resulting in a charge-ordered neutral layer. These findings indicate that this material can be considered as a promising candidate for a distinctly two-dimensional superconductor.