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

Abstract

We describe the first studies of the superconducting isotope effect with 34S substitution for all eight sulfur atoms of the ET molecule in κ-(ET) 2Cu[N(CN) 2]Br( T c =11.6 K) and κ-(ET) 2Cu(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.