New directions in the synthesis and exploration of novel organic superconductors: Inverse isotope effect in the organic superconductor kappa(L)-(ET)(2)Ag(CF3)(4)(1-bromo-1,2-dichloroethane) [ET equals bis(ethylenedithio)tetrathiafulvalene]

We report the first study of the effect of isotopic substitution on the superconducting transition temperature in the kappa-(ET)(2)M(CF3)(4)(1,1,2-trihaloethane) [ET - bis(ethylenedithio)tetrathiafulvalene; M = Cu, Ag, and Au] family of organic superconductors, where M(CF3)(4)(-) is a large, discrete (non-polymeric) anion. This work also represents the first single crystal determinations of T-c in this series of superconductors. Substitution of the eight hydrogen atoms of the ET electron-donor molecule by deuterium causes the T-c of kappa(L)-(ET)(2)Ag(CF3)(4)(1-bromo-1,2-dichloroethane) to increase from 2.90 +/- 0.04 K to 3.11 +/- 0.04 K (here kappa(L) signifies a phase with a lower T-c, between 2 and 6 K, and kappa(H) is a different phase with higher T-c, between 9 and 12 K). Thus, this is the first example which demonstrates that the inverse isotope effect previously observed in kappa-(ET)(2)Cu(SCN)(2) (this salt contains a ''self-assembled'' polymeric anion and was the first organic superconductor with T-c > 10 K) is also present in a kappa-phase ET-based superconductor containing a discrete anion. This finding is significant for the future development of organic superconductors with novel physical properties, because entirely new directions for the synthesis of unique superconducting systems are now possible with the use of large discrete anions which can be synthesized in a rational and repeatable fashion (as opposed to randomly synthesized self-assembled polymeric anions).