The central bond 13C=13C isotope effect for superconductivity in high-Tc .beta.*-(ET)2I3 and its implications regarding the superconducting pairing mechanism

Abstract

The authors report a new study, based on dc magnetization measurements, of the superconducting isotope effect for [sup 13]C-substitution in the central C=C atoms of ET [bis(ethylenedithio)tetrathiafulvalene] in [beta]*-(ET)[sub 2]I[sub 3] [or [beta][sub H]-(ET)[sub 2]I[sub 3]], the crystallographically ordered form of [beta]-(ET)[sub 2]I[sub 3] produced by pressure. The large shift reported by Merzhanov et al., [Delta][Tc] = -0.6 K, if correct, would clearly indicate that the central C=C stretching motion in the TTF moiety is a dominant exchange mechanism for superconducting pairing, because the BCS isotope effect for acoustic vibrations of the ET molecule as a whole predicts a shift, [Delta][Tc] = [Tc][sup 12]C[(386/384)[sup [minus]1/2]-1] = -0.02 K, where [Tc]([sup 12]C) refers to the salt with natural isotopes. In contrast, our study shows the absence of an isotope effect within [+-]0.1 K, indicating that the C=C stretching motion of ET is not a dominant exchange mechanism. These results are consistent with those of our very recent study of the same [sup 13]C isotope effect for the highest [Tc] ambient pressure organic superconductors, viz., [kappa]-(ET)[sub 2]Cu[N-(CN)[sub 2]]Br and [kappa]-(ET)[sub 2]Cu(NCS)[sub 2]. 22 refs., 1 fig.