ISOTOPE EFFECT IN THE ORGANIC SUPERCONDUCTOR BETA-H-(BEDT-ITF)2I3 WHERE BEDT-TTF IS BIS (ETHYLENEDITHIOTETRATHIAFULVALENE)

We have performed the simultaneous investigation of the isotope effect on the superconducting transition and on the Raman spectra in the organic superconductor beta(H)-(BEDT-TTF)2I3 (T(c) = 8 K). For this purpose, we substitute C-13 for C-12 on the carbon sites of the central double bond of BEDT-TTF molecule. The isotope shifts measured by Raman experiments can be fairly well explained by standard molecular dynamics. However, the critical temperature is lowered by 0.2 K in the C-13 enriched material. We analyse the possible sources of this remarkable downward shift which leads to an isotope coefficient higher than the BCS value. The extended-Huckel calculations of the density of states for the two HOMO bands of beta(H)-(BEDT-TTF)2I3 do show that, within the framework of a weak coupling theory, its sizeable variation on the scale of omega(D) cannot account for the observed isotope effect. On the other hand, we discuss how inelastic electronic scattering observed in resistivity measurements just above T(c) can lead through a pair breaking mechanism to a sizeable increase of the isotope coefficient.