THE CENTRAL BOND C-13=C-13 ISOTOPE EFFECT FOR SUPERCONDUCTIVITY IN THE HIGH-T(C) BETA-ASTERISK-(ET)2I3 PHASE AND ITS IMPLICATIONS REGARDING THE SUPERCONDUCTING PAIRING MECHANISM IN TTF-BASED ORGANIC SUPERCONDUCTORS

We report a new study, based on magnetization measurements, of the isotope effect for C-13 substitution in the electronically active central double-bonded carbon atoms (C=C) of the TTF moiety of ET [bis(ethylenedithio)tetrathiafulvalene] in beta*-(ET)2I3 [also denoted as beta(H)-(ET)2I3], which is the crystallographically ordered form of beta-(ET)2I3 produced by application of pressure. A recent report by Merzhanov et al. [C. R. Acad. Sci., Paris, 314, 563 (1992)] has shown that this same C-13 substitution leads to a ''giant'' isotope effect (DELTAT(c) = -0.6 K) in resistive determinations of T(c) (approximately 8 K) for beta*-(ET)2I3, which lends experimental support to a theory by Yamaji [Solid State Commun., 61, 413. (1987)] for salts of TTF-analogue donor molecules that includes optical phonons of symmetric intramolecular vibrations as a source of mediation for superconducting pairing of charge-carriers. In contrast to this result, our study shows the absence of an isotope effect within a precision of +/- 0.1 K (approximately 1%), indicating that the central atom C=C stretching motion (approximately 1500 cm-1) of ET cannot be a dominant mechanism for phonon exchange. Our results are consistent with those of a very recent study of the same isotope effect for kappa-(ET)2Cu[N(CN)2]Br and kappa-(ET)2Cu(NCS)2 [Inorg. Chem., 32, 3356 (1992)].