IN SEARCH OF AN ISOTOPE EFFECT ON T(C) IN KAPPA-(ET)2X SUPERCONDUCTORS WITH S-34 ISOTOPIC-SUBSTITUTION IN THE 8 SULFUR-ATOMS OF BIS(ETHYLENEDITHIO)TETRATHIAFULVALENE (ET)

We describe the first studies of the superconducting isotope effect wi th S-34 substitution for all eight sulfur atoms of the ET molecule in kappa-(ET)2Cu[N(CN)2]Br (T(c)=11.6 K) and kappa-(ET)2Cu(NCS)2 (T(c)=9. 6 K). The frequencies of two symmetric normal mal mode vibrations asso ciated with the C-S stretching motions (approximately 500 cm-1) of ET decrease by about 2.7% with this substitution. Furthermore, the mass o f ET is increased by about 16 amu. A small shift, DELTAT(c)=-0.08+/-0. 07 K, indicates that the C-S stretching motions of ET do not provide a dominant exchange mechanism for Cooper pairing. The results rule out the possibility of a conventional BCS isotope effect with ET as the re levant mass entity, but they are consistent with a small BCS mass effe ct with alpha=0.17+/-0.15 for kappa-(ET)2Cu[N(CN)2]Br.