In search of an isotope effect on Tc in κ-(ET)2X superconductors with 34S isotopic substitution in the eight sulfur atoms of bis (ethylenedithio) tetrathiafulvalene (ET)

We describe the first studies of the superconducting isotope effect with 34 S substitution for all eight sulfur atoms of the ET molecule in κ-(ET) 2 Cu[N(CN) 2 ]Br( T c =11.6 K ) and κ-(ET) 2 Cu(NCS) 2 ( T c =9.6 K ). The frequencies of two symmetric normal mode vibrations associated with the C-S stretching motions (∼500cm −1 ) of ET decrease by about 2.7% with this substitution. Furthermore, the mass of ET is increased by about 16 amu. A small shift, ΔT c =-0.08±0.07 K , indicates that the C-S stretching motions of ET do not provide a dominant exchange mechanism for Copper pairing. The results rule out the possibility of a conventional BCS isotope effect with ET as the relevant mass entity, but they are consistent with a small BCS mass effect with α =0.17±0.15 for κ -( ET ) 2 Cu [ N ( CN ) 2 ] Br .