DETERMINATION OF A MASS ISOTOPE EFFECT ON T-C IN AN ELECTRON-DONOR-BASED ORGANIC SUPERCONDUCTOR, KAPPA-(ET)(2)CU(NCS)(2), WHERE ET REPRESENTS BIS(ETHYLENEDITHIO)TETRATHIAFULVALENE

We describe the first determination of a genuine mass isotope effect o n T-c arising from the isotopic substitution of atoms in the ET [bis(e thylenedithio)tetrathiafulvalene] molecule of an electron-donor-based organic superconductor, kappa(ET)(2)Cu(NCS)(2) (T-c = 9.6 K, inductive onset). The isotopic substitution in the ET molecule involves concurr ent replacement of the four carbon atoms in the terminal 1,2-ethanediy l groups with C-13 and all eight sulfur atoms with S-34. This substitu tion, C-13(4)S-34(8), increases the mass of the ET molecule by 20 amu, which is a 5% increase in the normal mass. With the use of AC suscept ibility measurements on a large sampling of single crystals, including both undeuterated and fully deuterated salts, we obtain Delta T-c = - 0.12+/-0.05 K for this isotope effect. Assuming a BCS-like mass effect with ET as the relevant mass entity (M), this effect gives alpha = 0. 26+/-0.11 for T-c proportional to M(-alpha). Additionally, our experim ents with fully deuterated kappa-(ET)(2)Cu(NCS)(2) (H-2 replacing eigh t hydrogen atoms in ET) confirm the occurrence of a large inverse isot ope effect for the deuteration, and yield the first definitive value f or this effect, Delta T-c = +0.28+/-0.06 K. The isotope effect for C-1 3(4)S-34(8) substitution, however, is unaffected by deuteration. Contr ary to an earlier report by others, we find a zero isotope effect with in +/-0.06 K for C-13(4) substitution alone.