Abstract
The organic electron-donor molecule ET [bis(ethylenedithio)tetrathiafulvalene] was synthesized twice by the identical route: (a) with natural (12C) carbon isotopic distribution; (b) with 95% 13C, in the central C-C double bond position. The superconducting transition temperatures (in magnetic fields of 0.0–0.4 T) of single crystals prepared by both routes of ET synthesis were found, by the use of ac susceptibility measurements, to be identical within the precision of the measurements (∼0.1K or 1%) for the ambient-pressure superconductors κ-(ET)2Cu(NCS)2 κ-(ET)2Cu[N(CN)2]Br. The absence of an effect of such isotopic replacement on Tc indicates that the general C= stretching vibration (a high-energy optical mode at ∼1500 cm−1) is not the dominant contributor to the superconducting pairing mechanism in these systems.
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