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Isotope effect in the organic superconductor βH-(BEDT-TTF)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 β H -(BEDT-TTF) 2 I 3 (T c =8 K). For this purpose, we substitute 13 C for 12 C 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 13 C 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 β H -(BEDT-TTF) 2 I 3 do show that, within the framework of a weak coupling theory, its sizeable variation on the scale of ω D cannot account for the observed isotope coefficient 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