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

We report a new study, based on magnetization measurements, of the iso tope effect for C-13 substitution in the electronically active central double-bonded carbon atoms (C=C) of the TTF moiety of ET [bis(ethylen edithio)tetrathiafulvalene] in beta-(ET)2I3 [also denoted as beta(H)- (ET)2I3], which is the crystallographically ordered form of beta-(ET)2 I3 produced by application of pressure. A recent report by Merzhanov e t al. [C. R. Acad. Sci., Paris, 314, 563 (1992)] has shown that this s ame C-13 substitution leads to a ''giant'' isotope effect (DELTAT(c) = -0.6 K) in resistive determinations of T(c) (approximately 8 K) for b eta-(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 vi brations as a source of mediation for superconducting pairing of charg e-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 sa me isotope effect for kappa-(ET)2Cu[N(CN)2]Br and kappa-(ET)2Cu(NCS)2 [Inorg. Chem., 32, 3356 (1992)].