Central bond (sup 13)C(double bond)(sup 13)C isotope effect for superconductivity in the high-T(sub c) (beta)*-(ET)(sub 2)I(sub 3) phase and its implications regarding the superconducting pairing mechanism in TTF-based organic superconductors

摘要

We report a new study, based on magnetization measurements, of the isotope effect for (sup 13)C substitution in the electronically active central double-bonded carbon atoms (C=C) of the TTF moiety of ET (bis(ethylenedithio)tetrathiafulvalene) in (beta)*-(ET)(sub 2)I(sub 3) which is the crystallographically ordered form of (beta)-(ET)(sub 2)I(sub 3) 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 (sup C) substitution leads to a ''giant'' isotope effect ((Delta)(Tc) = (minus) 0.6 K) in resistive determinations of (Tc) ((approximately) 8 K) for (beta)-(ET)(sub 2)I(sub 3), which lends experimental support to a theory by Yamaji (Solid State Common., 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 (plus minus) 0.1 K.((approximately) l%), indicating that the central atom C=C stretching motion ((approximately) 1500 cm(sup (minus)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)(sub 2)Cu(N(CN)(sub 2))Br and (kappa)-(ET)(sub 2)Cu(NCS)(sub 2) (Inorg. Chem., 32, 3356 (1992)).